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Italy12246 Posts
I've wanted to write a blog about astrophysics for a while, but for some reason i've gotten around doing it only now.
Today is actually a very important day for us astronomers and astrophysicsts: it's the 25th birthday of the Hubble Space Telescope, which is one of the most awesome things humanity has ever built. 25 years after its launch, 30 years after its construction, and 38 years after its original proposal, it is still one of the most powerful and versatile telescopes we have; even new, state of the art ground-based observatories like VLT and Keck have a hard time matching its performance.
Infrared image of the same star cluster taken with the Hubble telescope, and with the ground-based Very Large Telescope, one of the best observatories ever built. Note how Hubble's resolution is better, even though the telescope itself is considerably smaller and more simple than VLT.
In this blog i'll illustrate a few basic astrophysical notions, along with some pretty pictures taken by Hubble. Most of the pictures are taken either from Hubble's webiste (hubblesite.org), or from the commemorative slides found on Nasa's website.
HST has gone through a messed up mirror that for a bit massively decreased its performance, 4 servicing missions with Space Shuttles that changed every instrument on board, a couple of broken gyroscopes that eventually got fixed, and a few failures on most instruments mounted on it. Despite this, the telescope is still able to operate at near 100% efficiency (which is pretty much unheard of for such an old satellite), and over its lifespan the data gathered has produced hundreds of scientific papers, a number unmatched by any other instrument minus the ground telescopes of the ESO observatory.
This is a picture of the galaxy M100 right after Hubble's launch, and after the first maintainance mission. Hubble's main mirror is slightly deformed - the curvature is off by 0,0022 millimeters, resulting in massive distortions. The first maintainance mission added correptive optics to fix this flaw, resulting in much better resolution.
One of the most important concepts in astrophysics is that, because the speed of light is finite, looking at objects farther away means we are actually looking back through time: light takes about 8 minutes to travel from the Sun to earth, about 4 years from the closest star to the Sun, 2 and a half million years from the closest massive galaxy. The farther away objects - which are also the faintest, whose light has a harder time reaching us - are also the oldest. The ability to see faint objects then becomes extremely important when one is trying to study the evolution of the universe.
The EM Spectrum and the atmosphere
Currently, the only way for astrophysicists to study any celestial body is to analyze the light it emits. However, simply being on Earth strongly limits the light we actually can see: our atmosphere only allows through a tiny portion of the entire spectrum, called the optical or visible range, as well as some radio waves. Anything more energetic, like ultraviolet light, x and gamma rays, or less energetic, like infrared, microwaves and some radio waves, are absorbed by our atmosphere; the only way to see a source emitting X raysthen is to have a satellite outside of the atmosphere, dedicated to that particular interval of the electromagnetic spectrum.
The atmosphere lets visibile and radio light pass, but any other kind of radiation is blocked, meaning we need satellites to see Ultraviolet or Infrared light for example. This is one of the reasons why space observatories are so useful.
The atmosphere is problematic for a second reason, called atmospheric seeing. When we observe the stars from the ground, they appear to twinkle; this isn't caused by the stars themselves, but by any kind of turbolence present in the air, which distorts the original image. This distortion strongly limits the resolution of any telescopes on the ground; it can be corrected to a certain extent, but it's a very expensive procedure that still doesn't produce perfectly still images. This is the second reason why putting a telescope like Hubble in space makes perfect sense, even though it is designed to see light from the (near) ultraviolet to the near infrared, which would mostly be accessible from the ground.
This is a scientific image of two stars orbiting each other, before and after removing the atmosphere's turbolence; the difference is massive. Hubble, being in space, doesn't need this correction.
The Solar System
Hubble allows for very precise imaging and studies of planets in the Solar System; because it's so accurate, only a spacecraft actually visiting a planet can take higher quality pictures. The main advantage of using Hubble instead of a space probe to study a planet is that it's possible to look at each planet periodically, to study its changes and gain a unique insight in the object's geology or metereology. The following pictures show respectively the evolution of Jupiter's Great Red Spot, a massive storm that has lasted at least 150 years and covers an area comparable to the size of the Earth, and polar auroras on Jupiter and Saturn much like those that happen on Earth, showing that both these planets possess strong magnetic fields. The interaction of these magnetic fields with any charged particle present in space is what causes auroras, just like on Earth.
Stars and Nebulae
Some of the most known and prettiest images taken by Hubble are those of nebulae, like the Crab and Cat's Eye nebulae or the gas clouds known as the Pillars of Creation. The term nebula is actually really generic; until about the 1900's it meant any observable object that didn't have a spherical shape, like a star or a planet, but instead looked kind of like a cloud (nebula means cloud in latin). Since then we have learnt that those fuzzy patches of diffused light can be broadly classified in several types:
1) Star formation regions. These are massive clouds of hydrogen gas; because of gravitational instabilities, sometimes this hydrogen collapses on itself, heating up to the point of igniting nuclear reactions within it and forming a new, bright, hot blue star. This happens over and over in a star formation region; indeed, identifying these clouds tells an astronomer wether a certain galaxy, or a region of a galaxy, is actively forming new stars or is not.
One of the most known star forming gas clouds in the Milky Way Galaxy, called the Pillars of Creation. The first two images are in visible light, and show the gas clouds. The third one is taken in infrared light, which isn't observed by the clouds and allows to see the stars behind them.
2) Planetary nebulae. These clouds are created as a star evolves. During its lifetime, it will expel most of its envelope, until only a small core of the original star (called a white dwarf) remains. This is also how our own Sun eventually will evolve, about 6 billion years from now.
The Cat's Eye nebula is what remains of a star very simliar to our Sun at the end of its evolution: the envelope has been expelled, and all that remains is a small white dwarf star, the white dot at the center.
The Carina nebula contains the envelopes of several stars, including some between 10 and 100 times bigger than our Sun. Instead of turning into a white dwarf, they eventually will explode in massive supernovas, leaving behind a neutron star or black hole.
3) Supernova remnant. Supernova remnants are also created by a star's evolution, but their history is much more traumatic. When a very massive star reaches the end of its life, rather than "peacefully" losing its envelope and leaving a tiny remnant behind, it explodes in a massive explosion, called a supernova, possibly leaving a smallish, but very compact object, either a black hole or neutron star (basically a massive clump of neutrons). The matter that doesnt form the central object is ejected at massive velocities and forms a cloud-like structure around the original place where the star was.
The Crab nebula is what remains of the explosion that actually gave supernovas their name. The light of the explosion reached Earth in 1054, and its sudden appearance in the sky was recorded by astronomers all over the world, who thought they were seeing an incredibly bright new star, hence the name supernova (super new in latin).
4) Dark Nebula. This kind of nebula isn't made of simple elements like the others, but of tiny grains of more complex molecules, which astrophysicists simply refer to as "dust". Dust has the particular property of absorbing most visible light, thus obscuring any object behind it at these wavelenghts. In order to penetrate a dust ring it's necessary to observe at some other wavelength.
Hubble has also given significant contributions to exoplanet research - looking for planets around stars far away from ours. This is usually done in two ways: either one blocks out the star's light with some filter, and tries to catch a glimmer of light reflected by any eventual planets that might be present, or tries to catch the "footprint" of the planet as it orbits in front of the star and absorbs part of its light.
In this image, the central star is blocked out; the tiny dot the arrow points to is a planet reflecting the star's light. As the planet has orbited the star over the years, its position has changed slightly
Galaxies
Galaxies come roughly in two groups - ellipticals and spirals. As the name suggests, spirals are your typical, pretty galaxy that seems to have a central luminous "bulge", and several spiral arms that envelope the bulge. Ellipticals on the other hand are usually a bit less exciting - they appear as spherical or near spherical clumps of stars, almost like a spiral's central bulge without any arms.
These two kinds of galaxies are massively different from each other. Spirals tend to be filled with gas and star forming regions, leading to very young stellar populations that emit mostly blue and ultraviolet light. Ellipticals on the other hand show very little trace of gas, having already converted most of it in stars, or lost it through some other process. Their stars are very old, which means they mostly emit red and infrared light. This is why the are classified as "red and dead" galaxies. Spectroscopy shows that while a spiral galaxy rotates in a coherent way around its central axis, ellipticals don't have any notable "group" rotation.
The Whirlpool Galaxy, whose technical name is M51, is a prototypical spyral galaxy. It is interacting with a "companion" galaxy, and eventually the two will merge and for a single galaxy.PGC-6240, known as the Rose Galaxy, and also the "twins" NGC 4038/NGC 4039, aka the antenane galaxies are also a spiral galaxies undergoing a merger.M104, called the Sombrero galaxy, is an elliptical galaxy, but it is surrounded by a ring of dust which absorbs visible light.M87 is a massive elliptical galaxy; there is no trace of the massive gas clouds present in spiral galaxies.
Cosmology
Cosmology is the study of the Universe on large scales - rather than focusing on individual objects like stars, clouds or galaxies, cosmologists study the structure of the entire Universe, its "shape", the way matter is distributed in it, its age, how it's expanding, and so on. In order to study exactly how matter is distributed in the universe, of course, we need to know the distance of the objects we see, which isn't nearly as simple as it sounds. Imagine you are seeing a faint dot of light in the sky; how can you tell wether it's a very close, not weak source of light, or really far, but powerful enough to reach us? For most astrophysical objects, it is impossible.
A special exception to this rule is a category of objects called "standard candles". These are objects whose inherent luminosity is known or easily computed, regardless of distance. Once we identify several of these objects it's then very easy to tell which ones are closer or farther away from us: the brightest will be closer, and viceversa. The tricky thing is that this process applies to very, very few phenomena and objects.
The most famous of these are particular stars called Cepheids. Cepheids are massive, pulsing stars whose luminosity varies over very regular periods of a few days or weeks. The length of the period is related to the star's absolute luminosity: the longer the period, the brighter the star. In fact, it's thanks to this kind of stars that astronomer Edwin Hubble in the 1920's first discovered that the universe was expanding: far away galaxies are also running away from us faster, following a very simple linear law: v=H * d, where v is the velocity, H is a constant, d is the distance. The Hubble telescope among its other great contributions to cosmology has helped in making more accurate measurements of the distance of Cepheid stars as well as other standard candles, improving the precision with which the constant H is known.
Hubble has also been able to take pictures of the farthest galaxies ever seen, in an observation survey called the Hubble Deep Field (and later, Ultra and Extreme Deep Fields). The telescope pointed at what seemed to be a mostly empty area of space, watching the same tiny area of about one thirteenth-million of the total area of the sky, over several months, searching for the faintest objects we had ever seen. The result was discovering roughly 10000 galaxies, some of which had formed as little as 450 million years after the Big Bang
Part of the Hubble Ultra Deep Field. The two objects that seem to form a "cross" of light are very, very faint stars, while all the others are galaxies.
This survey among others has helped our understanding of the so-called Cosmic Web. Matter tends to clump up, forming stars, which then form galaxies, which tend to clump up in galaxy clusters of thousands of galaxies, leaving immense voids between them. Clusters in turn also clump up in massive structures called superclusters,4. If you could zoom out and see how superclusters are distributed, you would see something that looks like a web, or a sponge: superclusters are tendrils of relatively high mass density, with massive voids in between them:
The Cosmic Web. The red and yellow areas are high density superclusters, while the darker ones are the voids between them.
I hope this was a good and educational read for anyone that made it all the way to the end! If you have any questions on anything, or if you would like to know more details, feel free to let me know!
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Italy12246 Posts
Sure, about what? There's is so much stuff to write about it really is impossible to include everything without writing a book, even without showing any math
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I never quite understood what happens when a star collapses to a black hole. I can imagine how that works with neutron stars that the electrons merge with protons and that the neutrons cannot collapse any further. What happens to all the particles when a black hole forms?
Also, those deep field images are the most amazing photos ever taken. Like every little dot is one whole galaxy. It's completely, utterly unimaginable. Also, I had to google how many galaxies there are in the universe and now my mind has burst. What to do?
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On April 25 2015 01:39 Teoita wrote: Sure, about what? There's is so much stuff to write about it really is impossible to include everything without writing a book, even without showing any math i'd like something about inflation. maybe something about slow roll
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Italy12246 Posts
What actually happens is unknown. It's even possible that quarks might stop the gravitational collapse and form a quark star, like neutrons can do when a neutron star forms, but that idea is waaay out there.
As to what actually happens to matter past the event horizon during and after the collapse, we don't know yet. We know that there's basically always a point when nothing can stop gravity, and the entire core collapses on itself, and we know the properties (or at least, some fo them) of a black hole once it's formed, but other than that we don't have very clear ideas.
Someone likely has solved relativity's field equations for the entire gravitational collapse, but even those do not work at small scales where you'd need to reconcile gravity with quantum mechanics to know what is going on exactly, and that's not really my field so i can't say anything more specific.
The TLDR is as far as we know, somehow shit doesn't stop collapsing until the entire stellar core is concentrated in a single point. At that point however, every physical law we know actually doesn't work, so really we don't know
edit: also
Also, those deep field images are the most amazing photos ever taken. Like every little dot is one whole galaxy. It's completely, utterly unimaginable. Also, I had to google how many galaxies there are in the universe and now my mind has burst. What to do?
Yeah i have a poster of the Ultra Deep Field in my room, it's just great. And that's just a tiny little dot in the sky that isn't supposed to have much in it, that's the great part. And yeah the universe is...really overwhelming. I fucking love it
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Italy12246 Posts
Regarding inflation: i made a post about cosmology and inflation in some thread that got closed a while ago:
Essentially, the most accepted cosmological model for the behavior of the universe in large scales, called the lambda-cold dark matter model, predicts among other things that the universe MUST have had to go through a period of extremely accelerated expansion in its early stages.
The reason for this is the existance of three very basic problems im lambda-cdm models that do not include inflation. These are called the flatness problem, the horizon event problem, and the magnetic monopole problem. In order:
1) Flatness: we know from empirical observation that the structure of space-time is flat. Ideally, it could be any kind of 4-dimensional surface with positive, negative or null curvature (just to picture things, in 3 dimensions a sphere has positive curvature while a plane is flat). Not only is the universe flat, but it can be shown easily that the further back in time you go, the more flat the universe gets. This is seriously bizarre; why the hell would the universe be absolutely perfectly flat at its start, when it can assume any possible value for its curvature while still following the same exact physical rules? It's an amazing coincidence, and it's extremely unlikely to just so happen by chance.
2) Horizon distance: this is even wierder. Basically, most photons existing in the universe are part of what is called CMB, or cosmic microwave background. These photons were emitted together billions of years ago, when the universe was still extremely hot. In fact, it was so hot that photons kept interacting with matter, keeping atoms from forming, bouncing from one nucleus to the other instead of being free to go their own way. As soon as the universe cooled enough, the photons suddenly stopped interacting with matter so strongly, and were free to go their own way. This radiation permeates the entire universe. By studying the CMB you can easily see that it was emitted at the same exact temperature at every point in the sky. On paper this makes sense, but if you look at things more carefully, you realize that light (and therefore, information) at the time of the CMB emission did not have time to travel through the entire sky as we see it. In fact, you can divide the sky in roughly 20000 patches of equal size; each of those would be able to transmit information within itself, but not to its neighbours. If these patches can not transmit any information to each other (ie, they can not reach the same temperature), why the fuck are they at that temperature?
3) Magnetic monopoles: this comes from more complex quantum mechanics stuff so i won't go in detail about it. Essentially, some quantum mechanics models predict that magnetic monopoles should exist, and their presence should be easily detectable. So why do we not see them? Is it possible that the universe evolved in a way that made them disappear?
Inflation theories solve all these problems. Inflation essentially states that there was a period in the early history of the universe, during which the expansion of the universe was INSANELY fast. This solves the 3 problems of traditional lambda cdm models: 1) Even if you start with a very curved universe, if you stretch it immensly it ends up being very very flat. 2) If the universe expands extremely fast for a while, then before it expanded it was extremely tiny, to the point where it could exchange information with every part of it, reaching the same temperature easily. 3) Slightly more complex, but ideally even if magnetic monopoles exist, the expansions "stretches" them so much that they become insanely rare, almost non existant.
How here's the problem: well shit if everything becomes perfectly uniform, how come the universe we see isn't uniform? Why do we see stars, globular clusters, galaxies, galaxy clusters and superclusters? So far, you'd think that inflation would make everything perfectly homogeneous.
However, what does happen is that when you first start with your tiny, small universe before inflation, it isn't perfectly homogeneous because of quantum mechanics. At a very very tiny scale the universe isn't empty: particles are constantly destroyed and created, according to Heisenberg's uncertainty principle: E*t>h, where E is the energy, t is the time, h is planck's constant. As long as a pair of particles of energy E exist for a time shorter than t, in order to respect the uncertainty principle, they are free to be born out of nothing, and return to nothing shortly after. This phenomenon is called vacuum energy. The way it ties into inflation is simply that these fluctuations are blown to huge sizes when infllation starts, and are what forms the large scales structures like galaxy superclusters that we see today.
Now, regarding slow/fast roll specifically, i'm not sure how to explain it without math so i will just assume you know roughly how a scalar field works. This is how we imagine the inflation scalar field looks like:
Imagine a ball sliding on a plane shaped like that potential; initially it will move very slowly, as the potential is kind of flat. This is called the slow roll condition. As it moves, it picks up more and more speed until it reaches a minimum, which is the fast roll condition. Inflation works just like that. On the left side of the curve, where it's relatively flat, the field isn't "pushing" the universe to expand very quickly, meaning the universe is "slowly" rolling towards the minimum. As inflation continues however the curve becomes more and more steep, and the universe keeps accelerating its expansion until it reaches the minimum of the potential.
I hope that made sense.
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So what kind of telescope do I need to attach my camera to in order to take some sweet ass stellar photos?
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Italy12246 Posts
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Is there a general sense of "up" and "down" in space? For example, on Earth, if I stand on my hands, blood will come down and I will feel the pressure in my head so I'm clearly aware than I am upside down. If I was in space, will I have any sense of up and down?
I ask because I'm looking at the gas cloud pictures you posted (pasted one of them below), and it looks like the top area is most dense, and as you go down, it gets less dense, like remnants of the gas are falling. Are there clouds where the formation is reversed? As in the clouds are dense on the bottom and as you go up in the formation there is less density.
+ Show Spoiler +
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What's next for telescopes? Crazy that we're using the same telescope for 25 years, and it's been top of the line.
Any new larger telescopes to put in orbit, or an asteroid, etc? Anyway, the pictures were really pretty, thanks the write up.
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Italy12246 Posts
Well up and down is defined by the gravitational field being present. If you were floating in orbit on the space station, (or ina gas cloud really) you couldn't tell up from down since nothing is pulling you. In taking that picture Hubble most likely just rotated on itself to make the pillars look straight and "right".
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Italy12246 Posts
On April 25 2015 02:52 FiWiFaKi wrote: What's next for telescopes? Crazy that we're using the same telescope for 25 years, and it's been top of the line.
Any new larger telescopes to put in orbit, or an asteroid, etc? Anyway, the pictures were really pretty, thanks the write up.
Regarding space telescopes, the next two bigass missions are the James Webb Space Telescope and, hopefully, ATLAST.
JWST is kind of Hubble's successor, but it's mainly an infrared mission so it's not quite the same. It's been delayed for a while, and hopefully it will be launched in 2018, and hopefully it will start gathering data around 2019-2020. It won't be orbiting Earth like Hubble does, so it will take a while after launch before it can actually be used. In those couple of years it should actually work together with HST, which is really cool. For comparison, Hubble's main mirror is two meters, Webb's is 6.5.
ATLAST (the name is actually a pun on how long it took to decide the exact band the telescope is supposed to work on) is Hubble's true successor as far as the part of the em spectrum it will study, but it's just a proposal now that will hopefully be built in the next decade or two. It will also go far away from Earth, on a similar orbit to JWST. People are expecting it to have an 8 to 16 meter mirror, which is absolutely unheard of for a space telescope.
Regarding ground telescopes, while it's true that HST still has the best resolution, the last generation observatories like Subaru, VLT and Keck are close to matching that performance, and the next one should be able to surpass it. The technology used to correct the deformation introduced by the atmosphere (which is pretty much the biggest limiting factor for ground observatory) has improved immensly, allowing us to build even bigger telescopes. For example, VLT has 4 mirrors of 8.2 meters each; the next generation is expected to have between 25 and 40 meters of aperture.
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What's the current state of astrophysical research? What was the last big finding? How big are hopes for new findings and how much is still unsolved?
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Italy12246 Posts
As with all science, for every answer we find, we come up with 10 more things we dont know. We really dont know shit, and this goes for pretty much everything, not just astrophysics. It's really exciting.
I guess the biggest last finding is the discovery of the acceleration of the expansion of the universe in the early 2000's, and along with that all the observational confirmations for the lambda cdm model in these past decade.
There's MASSIVE things we haven't discovered yet, and that we will hopefully figure out in the next decades. The ESA mission Lisa Pathfinder should be the first to detect gravitational waves from astrophysical sources, exoplanet research is skyrocketing, the AMS particle detector on the international space station just recently detected what could be the signature of dark matter particles annihilating each other. These are just the flashy things, there's plenty more don't know.
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Teo are you an astrophysicist?
That's what I'm trying to get into (finally going through school now), working as an architect right now. I didn't know that you were so interested in space. This is my favorite blog EVER lol, I love finding stuff that I might have just written in my sleep Fight Club style.
Happy birthday hubble!
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Italy12246 Posts
Yeah i am, i'm getting my master's and i want to pursue it as a career
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cool to see such dedication to outreach!!. my wife has a phd in astrophysics (galactic dynamics) and i am a "half-assed astronomer" myself (astroparticle physics.7th year grad student...) so if you ever feel we can be of service, drop me a message
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The most interesting thing to me in all of physics (okay i guess this is kinda only tangentially related but still) is why there is no good quantum theory of gravity
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On April 25 2015 05:04 Teoita wrote: Yeah i am, i'm getting my master's and i want to pursue it as a career
Who hires an astrophysicist?
I mean, what you know is great, but is there anyone in the public sector that desires this kind of knowledge about the universe? If all of the sudden everyone in the world forgot how everything functions beyond our solar system, I don't think much would change for anyone's life (minus the astrophysicist, pardon me, I'm a mechanical engineer). Anyway, seems mostly the government bodies of NASA or large EU programs, and universities, etc.
Anyway, thanks for answering my question. My real interests lie in the questions we don't have answers for yet, it's just the classical big bang theory is very unsatisfying to me, and I have a desire to understand what was before, what initiated the big bang, why was it at that point in time exactly, etc etc.
The other thing you mentioned in another reply is you gave the rolling ball example, to determine where the lowest potential would be. But if we make the assumption that there is nothing in the universe outside of what was created during the big bang, we can look at our 3(or 4 however you look it) fundamental forces.
Gravity: Okay, so this force should result in everything attracting back together Strong Interaction: Isn't this only the interaction between quarks, and as such, we never really experience it in the observable universe? All the effects we see are the residual strong force, which is a result to quarks not lying on the exact same point in space, and thus a nucleus can exist? Weak Interaction: Electromagnetism: Aren't things mostly charge neutral once you get big enough, don't see how this force should be at all significant in the grand scheme of the universe, only microscopically? Weak Interaction: Know little about this (like everyone else it seems), but I know it's a super short range force associated with changing quarks to different flavors? So the only way to make everything work is assume that our universe is made up of 70% and 25% energy and mass respectively, and say we have no way of detecting it with anything, it does not have any interactions with anything "real" (otherwise we could recognize it's there), and yet it's still able to somehow speed of the expansion of the universe? Just confusing as to what the current theory is. And also, gravity acts even when thermodynamic equilibrium has been reached, so isn't the location of lowest potential somewhere close to a compaction of the universe?
My other question was about the loss of information principle, or whatever it's referred to. Saying that if we knew the exact property of every single property in the universe, we could predict the exact future or past of everything. Is this possible, is information conserved? Because the two things that came up were the Heisenberg's uncertainty principle, and the information loss in a black hole... Anyway, just was wanting your input on that, kind of related to determinism and being able to trace origins back before big bang, etc.
And lastly, my other question was benefits do you see from astrophysics coming in the future? It seems like where all the interesting stuff has been occurring recently, are the particle accelerators. Most of everything that occurs in the solar system can be explained quite well to a layman in the subject like me.
Feel free to pick just little tidbits you wish to answer, if none at all. Thanks!
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Italy12246 Posts
On April 25 2015 05:56 Sn0_Man wrote: The most interesting thing to me in all of physics (okay i guess this is kinda only tangentially related but still) is why there is no good quantum theory of gravity
Mostly a combination of a) the math to write it either doesnt exist, or isn't well understood b) it's such a clusterfuck that every theory of quantized gravity hasn't produced any experimental prediction that can be verified with current technology
On April 25 2015 06:02 FiWiFaKi wrote:Show nested quote +On April 25 2015 05:04 Teoita wrote: Yeah i am, i'm getting my master's and i want to pursue it as a career Who hires an astrophysicist? I mean, what you know is great, but is there anyone in the public sector that desires this kind of knowledge about the universe? If all of the sudden everyone in the world forgot how everything functions beyond our solar system, I don't think much would change for anyone's life (minus the astrophysicist, pardon me, I'm a mechanical engineer). Anyway, seems mostly the government bodies of NASA or large EU programs, and universities, etc.
I have pretty much the same preparation as a physicist as far as problem solving goes, so if i didn't want to do research i would have the same opportunities. For research, you pretty much work at a university, if you're good there's good jobs. It's a very hard field for sure, but i'm confident i can make it.
Besides, while immediate knowledge may not help, technological progress is pushed by basic research like astrophysics. In fact, the CCD's in digital cameras and phones for example are a byproduct of the need for better relevators for optical photons in telescopes, just to make an example.
Regarding the four basic forces, you are confusing a basic force with a scalar field. A scalar field that may or may not be quantized is simply an interaction between two objects, that behaves in a particular mathematical way. It doesn't have to be one of the basic forces, all inflation is saying is "look if in the universe there was a potential of some kind with those properties, then that can produce inflation". What that potential actually is - in quantum mechanical term, what an eventual quantum of that field, called inflaton in this case - or what its properties are, is an even more advanced topic; i attended a seminar about the theoretical side of it when BICEP2 announced they had detected proof of inflation and frankly i didn't understand a word after the first couple of slides.
To this day, the only quantized scalar field outside of the four basic forces that has been shown to exist through experiments is the Higgs field, along with the Higgs boson, but it's as far as theoretical physics goes it's a very convenient way to describe a certain phenomenon. I personally deal with lots of scalar fields when i study the gravitational field inside a galaxy - that's just a particular kind of gravitational field, without any fancy quantums or whatever necessary. The "language" used is still that of a scalar field.
Regarding "the way to make the universe work", there's actually not just one.
The most accepted is the lambda cdm model i discussed earlier - which is essentially applied general relativity, in which you have about 70% of the mass-energy density of the universe in dark energy (which we have no idea wtf it is), roughtly 30% in mass, and 25% of that is dark matter which is only slightly less mysterious, and only a minuscle fraction is in radiation.
Some people instead believe the correct way to look at things is to modify gravity completely, adding on a few pieces to general relativity, but i personally dislike that explanation because it starts from the premise of "wtf we need to find some way to make dark energy and dark matter disappear since we dont know wtf it is", and it ends up being overly complex, and not explaining data very well at that.
I'm not sure what you mean by "loss of information principle", but mathematically speaking, solving what is called the N-body problem (ie, an arbitrary number of objects interacting through gravity) isn't possible (to our knowledge) without some underlying assumptions and semplifications, and even then you need to use a really, really powerful calculator to do that; but i guess yeah, if you have a system of N particles and you know how they interact (which is a massive assmption) you can pretty much simulate its evolution, if you have enough computing power and/or enough time to let the computer do the math. That part is actually really important in studying how galaxies evolve for example, where you might for example simulate the evolution of a system consisting of a central massive black hole, a billion stars, and a few million solar masses worth of gas, to see exactly what happens to it, and then compare that for example with the properties of the galaxies in the Hubble Ultra Deep Field to see if they match at a certain time.
On anything more theoretical than that i'm really not sure since it isn't my field, sorry for example, i don't really know how the uncertainty principle interacts with things like Hawking's radiation or black hole's entropy.
Other than just figuring out exactly what our Universe is like and how it works on a grand scale, which is no small feat, astrophysics like any other kind of basic research provides massive technological developments. People have this wierd way of looking at scientific research in a vacuum, but it's really unpredictable to say exactly what will come out of it, since well, we haven't discovered it yet. Yet, without Fourier analysis we woulnd't have jpeg files, without Hilbert spaces and quantum mechanics we wouldn't understand semiconductors, meaning we could kiss microprocessors goodbye, without general relativity we woulnd't have GPS and so on.
To be fair, as Feynman once said "physics is like sex, sure it has practical uses but that really isn't why we do it". I want to do research because it just makes me feel happy, i feel like it's what i was born to do. The first time i walked into a telescope's control room i felt like i was home.
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Normal, non-quantum gravity is so simple and intuitive though
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I haven't done integration in 5 years tho
I also don't recognize upside-down delta or O with an I through it.
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Italy12246 Posts
Ah also, non-quantum gravity also inclues General Relativity. Believe me when i say that shit isn't simple and it's certainly not intuitive
Upside down delta V is the gradient operator in that case. O with an I through it is the greek letter phi and in this case it represents the gravitational potential. So yeah, math
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Lots of interesting stuff here, I'm really enjoying reading what you have to say. I took some Astro/Physics electives and I was really tempted to go into it as well (which I didn't).
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Also I featured your blogs. Blue staff member and makes great blogs, how were you not already featured
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Italy12246 Posts
Dunno i don't really make blogs that often
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Germany25642 Posts
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How have galaxy clusters and superclusters been confirmed? Is it just a matter of putting all observable galaxies on a 3D grid and looking at the interactions? Or is the concept of clusters based on unproven theoretical assumptions?
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Some more factual comments since I got hold of an actual keyboard:
- as a formerly theoretical physicist, I am still quite reserved with respect to inflation. It has so much "elegance reasoning" in it that if I took some of the common arguments for supersymmetry and exchanged a few words, you won't even notice that I am not giving you the standart reasoning for inflation. And you probably know how well it went with supersymmetry until know. But that's more of a personal point - anything that just supposedly happened once in a very distant past and is not possible to recreate is very difficult to "prove" at the level of rigor of experimental physics.
- the quantum theory of gravity is an interesting problem that puzzles people for decades. It is obvious and well-known that the most straightforward approach is pointless - you can't just take flat (or any for that matter) spacetime as the free theory and built a spin-2 graviton perturbation theory around it - for many reasons: because the free/interacting dichotomy doesn't make any sense in GR (first and foremost because it is not invariant to transformations), because there are no renormalisable interactions of spin-2 particles and also because of such fun technical details such as that the hamiltonian of GR is identically zero for physical solutions. So you have to think of something else. One widely beloved approach is the strings, but I was really amazed when I got some lectures on loop quantum gravity, because that's incomparably more straightforward (but also with many issues). A fantastic insight was when the professor has shown us that when we are forced to abandon any notion of "background metric", we are getting rid of all geometry and everything reduces to just combinatorics. The idea that the whole universe is based on counting combinations of triangles is simply wonderful.
- I would like to reinforce your idea that there is job market for you. If you like doing research, I can't imagine you not getting a good job, particularly if you are willing to move a little around Europe. I know from my US friends that the situation overseas may sometimes seem dire, but at least in the EU, astrophysics is now in a pretty big surge and there are positions of various levels - everything depending on your skill and financial demands. However a word of warning that you probably already know: the part when you actually do research may sometimes not be the majority of your time, because of paperwork needed to get funding - and that's annoying for many people and even drives them away.
Anyway, I have read some of your texts (not all sadly) in this blog and you are pretty good in expressing ideas, so you have a good disposition to teach and write, skills that will make job finding easier and the job potentially more rewarding (teaching is awesome, I can atest to that).
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United States24342 Posts
Thank you for writing up this summary and compiling a beautiful group of photographs.
Something you didn't talk about but I find fascinating is gamma ray bursts.
edit: As you can see, Hubble and I basically share a birthday!
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I thought quark stars are fairly likely. We haven't observed any black holes of less than about 5 Solar Masses, and the Chandresekhar Limit is something like 3 Solar Masses for a neutron star.
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On April 25 2015 01:39 Teoita wrote: Sure, about what? There's is so much stuff to write about it really is impossible to include everything without writing a book, even without showing any math Could you talk elaborate a bit on the "dust" and the dark nebulae, please?
Edit: also this blog was so good I read the whole thing, and almost all the comments, before going and looking at the new post in SFW Random pics that make you laugh.
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On April 25 2015 10:45 Just_a_Moth wrote:Show nested quote +On April 25 2015 01:39 Teoita wrote: Sure, about what? There's is so much stuff to write about it really is impossible to include everything without writing a book, even without showing any math Could you talk elaborate a bit on the "dust" and the dark nebulae, please? Edit: also this blog was so good I read the whole thing, and almost all the comments, before going and looking at the new post in SFW Random pics that make you laugh. I hope you don't mind if I take a crack at this.The dust is fairly complex when it comes to composition. You have grains of carbon (think graphite), silicates, and some organic compounds (including ethanol :D). These grains and molecules range in size from several microns to several tenths of a nanometer and contribute the majority of the opacity of these clouds, but they make up a fraction of the mass of the cloud. Hydrogen is ~70% of the mass of the interstellar medium, with Helium making up most of the rest.
Here's a link to the journal article that first discovered alcohol in the interstellar medium. It has a few good puns toward the beginning and end of the article.link
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I watch "Into the Universe" all.the.time.. It's fascinating, I would love if you posted more about this!
Curious, what do you make of the so-called "Supervoid"?
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On April 25 2015 11:50 Mordanis wrote:Show nested quote +On April 25 2015 10:45 Just_a_Moth wrote:On April 25 2015 01:39 Teoita wrote: Sure, about what? There's is so much stuff to write about it really is impossible to include everything without writing a book, even without showing any math Could you talk elaborate a bit on the "dust" and the dark nebulae, please? Edit: also this blog was so good I read the whole thing, and almost all the comments, before going and looking at the new post in SFW Random pics that make you laugh. I hope you don't mind if I take a crack at this.The dust is fairly complex when it comes to composition. You have grains of carbon (think graphite), silicates, and some organic compounds (including ethanol :D). These grains and molecules range in size from several microns to several tenths of a nanometer and contribute the majority of the opacity of these clouds, but they make up a fraction of the mass of the cloud. Hydrogen is ~70% of the mass of the interstellar medium, with Helium making up most of the rest. Here's a link to the journal article that first discovered alcohol in the interstellar medium. It has a few good puns toward the beginning and end of the article. link Thanks man.
Just wondering if you know more than the wrote, cause you said it's fairly complex, if so I'd be happy to hear about it if you're willing to type it out.
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I actually find this stuff really interesting and almost wish I could go back in time to study it at uni in lieu of what I took . I wanted to be an astrophysicist when I was little but didn't want to put in the time and effort (more or less). Finances and some other things were also part of the reasoning though.
Cool stuff and pretty pictures though. Also, I'd still like to take you up on a visit the next time that I come to Italy, whenever that may be .
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On April 25 2015 13:23 Just_a_Moth wrote:Show nested quote +On April 25 2015 11:50 Mordanis wrote:On April 25 2015 10:45 Just_a_Moth wrote:On April 25 2015 01:39 Teoita wrote: Sure, about what? There's is so much stuff to write about it really is impossible to include everything without writing a book, even without showing any math Could you talk elaborate a bit on the "dust" and the dark nebulae, please? Edit: also this blog was so good I read the whole thing, and almost all the comments, before going and looking at the new post in SFW Random pics that make you laugh. I hope you don't mind if I take a crack at this.The dust is fairly complex when it comes to composition. You have grains of carbon (think graphite), silicates, and some organic compounds (including ethanol :D). These grains and molecules range in size from several microns to several tenths of a nanometer and contribute the majority of the opacity of these clouds, but they make up a fraction of the mass of the cloud. Hydrogen is ~70% of the mass of the interstellar medium, with Helium making up most of the rest. Here's a link to the journal article that first discovered alcohol in the interstellar medium. It has a few good puns toward the beginning and end of the article. link Thanks man. Just wondering if you know more than the wrote, cause you said it's fairly complex, if so I'd be happy to hear about it if you're willing to type it out. lol "fairly complex" is a physics excuse. I really don't know much more than what I wrote. I just checked one of my books and found out this though. There isn't much we can do to detect these clouds. There are a few things we can tell though. Photons tend to interact with things that are roughly the same as their wavelength. When we look at these clouds, they absorb background light in wide swaths of the spectrum, so we can tell that they range in size greatly, from a few tenths of a nanometer to a few microns. We can tell because all the way from hard UV to far infrared is blocked out.
A few other clues we have come from some other absorption lines. We see fairly specific lines that correspond to the amount of energy needed to interact with bonds between silicon and oxygen and bonds between Carbon atoms in polycylic aromatic hydrocarbons (PAHs). One thing I read in my book is that the PAHs are sort of daisy chained together to form really big molecules similar to graphite. Another interesting thing I found is that, if there is water around these dust grains, they can get a kind of icy mantle around their rocky core. Probably not applicable to dark nebulae, it's cool to think of these little dust grains as tiny little Enceladuses. These observations are what suggest that the dust is like extremely fine sand (the silicates) and extremely fine graphite (the PAHs). On Earth we would call the dust a colloid or silt, and there may even be some very fine sand. To give you an idea of the size of particle, some of the particles are smaller than the globs of fat in homogenized milk. Basically, this stuff is not like much anything we have on Earth.
Finally, let's talk about density. Dark Nebulae typically have a number density (this is the number of particles in a given volume rather than mass per volume) of about 100 - 300 particles per cubic centimeter. Using a classic physics trick, we can say that roughly one of those will be a dust particle. The average separation between these tiny dust grains is much greater than the separation between dust particles on a fairly clear day on Earth. The reason these clouds block out so much light is because they are big. They are generally on the order of about 100 light years across. Just as a point of comparison, the Sun is in a fairly lowly populated region of the galaxy and is ~4 light years from its nearest neighbor. Within 50 light years on either side of the Sun are hundreds of stars.
Honestly though, the best I can do from here is give you links: Wikipedia entry for Cosmic Dust Wikipedia entry for ISM Wikipedia PAH Wikipedia on Grain Size -- Sort of tangential but kind of interesting if you reaally want to know about the kind of sizes of particles Enceladus: The best or worst place to play hockey
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I can't even express how happy it makes me that this blog/thread exists on Team Liquid. THIS is why I love all you people.
Teo, it'll be a few years yet before I'm employed as an astrophysicist, but maybe I'll see you around at a conference or something someday. I'll make sure I always wear my TL pin on my suits
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This blog was really interesting and informative, thanks a lot for doing it (:
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Italy12246 Posts
Thanks to everyone for the kind comments Of course, more people adding to the discussion is always nice!
On April 25 2015 08:48 Mordanis wrote: I thought quark stars are fairly likely. We haven't observed any black holes of less than about 5 Solar Masses, and the Chandresekhar Limit is something like 3 Solar Masses for a neutron star.
That's true, but computing the degeneracy pressure for quarks hasn't been fully done yet to my knowledge, so who knows if it can actually support a collapsed core or not. For now it's a cool idea but it doesn't have solid observation or a complete theoretical background backing it up.
On April 25 2015 08:11 fancyClown wrote: How have galaxy clusters and superclusters been confirmed? Is it just a matter of putting all observable galaxies on a 3D grid and looking at the interactions? Or is the concept of clusters based on unproven theoretical assumptions?
They have been observed directly through galaxy surveys (observation campaigns that target thousands of galaxies) and, in the case of clusters, they can be spotted both in Xrays (the space between galaxies in a cluster is filled with gas that emits at those frequencies), as well as leave a "trace" in the CMB. When a CMB photon "hits" an (energetic) electron in a cluster, it can grab part of its energy, essentially disappearing from the CMB and leaving behind a cold spot in it.
On April 25 2015 12:13 Ctone23 wrote:I watch "Into the Universe" all.the.time.. It's fascinating, I would love if you posted more about this! Curious, what do you make of the so-called "Supervoid"?
Eh Stephen Hawking knows cosmology a bit better than i do
That void is really wierd; it's reasonably simple to calculate how big structures like superclusters or super voids can be, as it's essentially tied to how much matter/radiation/dark matter/dark energy the Universe is actually made of. Anything bigger than that to our knowledge shouldn't exist, so yeah, i don't have an explanation. It's just another hint that we have a long way to go!
On April 25 2015 16:12 Kommatiazo wrote:I can't even express how happy it makes me that this blog/thread exists on Team Liquid. THIS is why I love all you people. Teo, it'll be a few years yet before I'm employed as an astrophysicist, but maybe I'll see you around at a conference or something someday. I'll make sure I always wear my TL pin on my suits
Deal!
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United Kingdom35817 Posts
Fantastic blog and awe-inspiring pictures, thanks.
Astrophysics/cosmology/whatever tends to be the kinda area I get lost in when I accidentally look at wikipedia and end up with a zillion tabs open.
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cool icon man, nice blog too
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I love people loving space. For me, it is just too big.
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On April 25 2015 22:40 Liquid`Nazgul wrote: I love people loving space. For me, it is just too big.
That's why you just focus on one planet or body or something
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Thanks for this awesome post
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color me impressed teo, in all our talks this never came up lol
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Two questions: -How come stars don't burn out faster? Do they really have a big enough supply of flammable material (hydrogen?) that they can burn for billions of years or can they somehow replenish themselves? If so, how?
-I once heard/read/saw that gravity is one of the three or four basic 'forces' or 'natural occurences' that helped shape the universe the way it is today (gravity allowing matter to clump up and form stars/planets rather than remaining the huge shapeless gascloud it was at the beginning right?). If true, what are the other forces?
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Italy12246 Posts
1) Yeah, some stars can keep burning for billions of years. Once they are done with one phase of burning - say, converting hydrogen in helium - they can procede to the next phase, in this case converting helium in carbon. Generally, the more massive the star, the more elements it can burn (the bigger once can produce iron), but the quicker they burn out. The only way for a star to replenish itself is to be in a binary system, with two stars orbiting each other. In these systems sometimes one of the two stars will start stealing matter from the other, and adding that to its own mass. This impacts stellar evolution a bit, but it doesn't change how long stars are expected to burn that signifincanlty.
2) Electromagnetism, the weak force (which is responsible for some nuclear decays, called the beta decay) and the strong force, which is what binds protons and neutrons in nuclei.
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God I love space, just learned some entry stuff in physics class
Quasars.. as far as I know they're some of the brightest and most luminous objects we know of(even outshining host galaxies?), and the energy comes from mass being sucked into a nearby supermassive black hole. But how are quasars formed, and are they observable from earth with entry level telescopes?
5 stars
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On April 26 2015 00:09 Kommatiazo wrote:Show nested quote +On April 25 2015 22:40 Liquid`Nazgul wrote: I love people loving space. For me, it is just too big. That's why you just focus on one planet or body or something
In Astronomy 1 at uni the lecturer spent 15 minutes describing different types of lunar craters, their classification by size and morphology and explaining their formation in detail. I love astronomy but I doubt I could care about something so specific as much. And I have to assume she was barely scratching the surface (no pun intended) of lunar science.
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Italy12246 Posts
On April 26 2015 05:19 Aocowns wrote:God I love space, just learned some entry stuff in physics class Quasars.. as far as I know they're some of the brightest and most luminous objects we know of(even outshining host galaxies?), and the energy comes from mass being sucked into a nearby supermassive black hole. But how are quasars formed, and are they observable from earth with entry level telescopes? 5 stars
None knows how quasars really are formed, if i knew the answer to that i'd probably win a noble prize. They are a particular kind of black hole accreting matter as you said; they are about 100 times brighter than your average, active (meaning "sucking matter") supermassive black hole. None knows for sure how those black holes form, or why some seem to accrete so much more than others, and why this seems to happen more often going further back in time.
The brightest quasar has an apparent magnitude of 12.9, while depending on where you observe from (assuming you are close to a urban area) your limiting magnitide will be 4-8ish so unfortunately no, you can't see them with an amateur's telescope (higher magnitude means faint objects, it's a pretty silly system tbh). To be fair, without a badass telescope like Hubble all you would see is a luminous dot identical to any star.
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nebulae are some of the the most beautiful things I have seen (in pictures of course ^^)
great blog Teo, all hail Hubble
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On April 26 2015 04:11 Teoita wrote: 1) Yeah, some stars can keep burning for billions of years. Once they are done with one phase of burning - say, converting hydrogen in helium - they can procede to the next phase, in this case converting helium in carbon. Generally, the more massive the star, the more elements it can burn (the bigger once can produce iron), but the quicker they burn out. The only way for a star to replenish itself is to be in a binary system, with two stars orbiting each other. In these systems sometimes one of the two stars will start stealing matter from the other, and adding that to its own mass. This impacts stellar evolution a bit, but it doesn't change how long stars are expected to burn that signifincanlty.
2) Electromagnetism, the weak force (which is responsible for some nuclear decays, called the beta decay) and the strong force, which is what binds protons and neutrons in nuclei. correct me if I'm wrong but aren't some of the larger (by atomic number) elements found on Earth created by dying suns? I believe I heard that in some documentary I watched
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Italy12246 Posts
Yeah, stars can form up to iron, and then the heavier elements are produces during very final moments of life when massive stars go supernova
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Our sun is around halfway through it's life cycle right? Or was it not quite there yet. Either way it's a bit far off supernova unfortunately
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Italy12246 Posts
It is, and it won't go supernova anyway because it's too small.
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Calls the sun small, makes my life seem so small
Jk
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thanks! very interesting and educational blog and nice linked images and websites!
one of the pillars does look like a cock though
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Great blog, Teoita. Nice to see science getting its due attention on TL :D
Maybe I should write a similar blog for particle physics/quantum mechanics. That's my field, after all. Would be a nice accompaniment to this blog - the large and the small! Plus there do seem to be a lot of misconceptions about QM floating around this thread
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Italy12246 Posts
That would be awesome
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Just wanted to say this was very intresting to read. Ty!
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As a fellow Master's student currently writing a thesis in Astrophysics, excellent blog.
Very cool!
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Italy12246 Posts
Thanks What's your thesis on? I have a couple of exams to go and will start mine this fall, i'm thinking of doing something about AGN since i've always found that fascinating, but i'm not 100% sure yet.
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It is on dust formation in AGB stars. Nothing really ground breaking, but extremely interesting. The molecule I am looking at only has one data point in experiments though so its very very theoretical treatment hehe.
AGN is fascinating as well, but I guess that applies to everything in the field. So much of it is amazing. Good luck on your exams, those last few are the most tense! Are you currently studying in Italy?
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Italy12246 Posts
Yeah, there's a university in Milan called Bicocca that has a Master's program that is 100% astrophysics so i'm doing that. Lots of universities have like 3-4 astrophysics exams that try rush to explain everything from stellar astrophysics to cosmology, plus a bunch of generic classes, and to be fair i was like fuck that shit why would i even do a half arsed particle physics exam because i have to, while also half arsing it in astrophysics.
Once i graduate i'll look for a phd around Europe. Unfortunately Italy is a god awful country for scientific research, the funds just aren't there and researchers are underpaid and treated like shit compared to the rest of the world, so if you want to go make a career out of it you are almost forced to move abroad. We have some of the best universities as far as actually teaching us stuff goes, but after that it's just downhill. The upside is, italian scientists tend to be really appreciated and really good because the selection is so tough, only the best and most committed actually make it.
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A good read, Teo, even if I was familiar with quite a lot of what was said. Best of luck your masters degree and getting into the astrophysics biz. Looking forward to more of these blogs :D
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Interesting. I am currently studying in Berlin, Germany. I do my thesis for my main institution at another university that specializes more in Astronomy/Astrophysics. Seems weird, but the universities here all have an agreement where they can allow students to attend classes, do theses, etc between them.
One of the most unfortunate things about Physics in general is that the students are always "rushed" to catch up with the fast developments of the field. Just so much information has come out since the developments of quantum mechanics, quantum field theory, experimental applications. The density is unbelievable.
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Thanks very much for this post. Was a bit of a trip down memory lane for me. I did my bachelor's in Physics, and I added quite a number of astronomy/astrophysics courses. For my undergrad thesis I made a statistical model of # of gamma ray bursts vs Flux for exploding primordial black holes as would be visible to one of the instruments on the fermi gamma telescope. The end result was that the fluxes were way too low, and that at the high flux end the log-log slope was too close to that of just natural expansion. Still it was a really fun project, and my advisor loved it because I made him all sorts of scripts to calculate useful stuff (redshift to proper distance and FLRW metric to calculate R/R0). It's all a touch blurry since I was working on this stuff like 8+ years ago.
I eventually ended up like EM a touch more so I went the electrical engineering route. I'm happy with what I do, but on occasion I wonder what it would have been like to go the astronomy route. I still have a copy of PJE Peebles' Physical Cosmology, I think I'll read through it slowly and try to digest it.
Anyway thanks again!
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United States24342 Posts
Wow I'm actually mildly surprised by the number of users with physics backgrounds this blog attracted.
Also, someone finally mentioned gamma ray bursts!
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Italy12246 Posts
On April 26 2015 22:51 revy wrote: Thanks very much for this post. Was a bit of a trip down memory lane for me. I did my bachelor's in Physics, and I added quite a number of astronomy/astrophysics courses. For my undergrad thesis I made a statistical model of # of gamma ray bursts vs Flux for exploding primordial black holes as would be visible to one of the instruments on the fermi gamma telescope. The end result was that the fluxes were way too low, and that at the high flux end the log-log slope was too close to that of just natural expansion. Still it was a really fun project, and my advisor loved it because I made him all sorts of scripts to calculate useful stuff (redshift to proper distance and FLRW metric to calculate R/R0). It's all a touch blurry since I was working on this stuff like 8+ years ago.
I eventually ended up like EM a touch more so I went the electrical engineering route. I'm happy with what I do, but on occasion I wonder what it would have been like to go the astronomy route. I still have a copy of PJE Peebles' Physical Cosmology, I think I'll read through it slowly and try to digest it.
Anyway thanks again!
You are welcome
I've done my bachelor thesis on Gamma Ray Bursts myself, although i focused on ultra long GRBs.
Out of curiosity, how does the primordial black hole evaporation tie with GRBs? All i know is that it's a suggested explanation, but most people prefer the collapsar models. I guess the gamma radiation is the tail end of Hawking's radiation? How would you explain the afterglow?
And yeah, i didn't mention GRB's in the OP since Hubble specifically hasn't contributed all that much to their study, other than occasionally studying afterglows
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On April 25 2015 02:29 Serejai wrote: So what kind of telescope do I need to attach my camera to in order to take some sweet ass stellar photos? As far as I am aware you'll need to take infrared photos from a telescope in space and massively photoshop them for color and contrast.
These pictures are great, but you should remember that they have been edited for the general public, not for scientific value.
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Italy12246 Posts
You dont need an IR camera, all you need is separate filters. Take one picture per filter of the same source and combine them digitally.
But yeah, scientific images aren't nearly as pretty.
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On April 26 2015 23:01 Teoita wrote:Show nested quote +On April 26 2015 22:51 revy wrote: Thanks very much for this post. Was a bit of a trip down memory lane for me. I did my bachelor's in Physics, and I added quite a number of astronomy/astrophysics courses. For my undergrad thesis I made a statistical model of # of gamma ray bursts vs Flux for exploding primordial black holes as would be visible to one of the instruments on the fermi gamma telescope. The end result was that the fluxes were way too low, and that at the high flux end the log-log slope was too close to that of just natural expansion. Still it was a really fun project, and my advisor loved it because I made him all sorts of scripts to calculate useful stuff (redshift to proper distance and FLRW metric to calculate R/R0). It's all a touch blurry since I was working on this stuff like 8+ years ago.
I eventually ended up like EM a touch more so I went the electrical engineering route. I'm happy with what I do, but on occasion I wonder what it would have been like to go the astronomy route. I still have a copy of PJE Peebles' Physical Cosmology, I think I'll read through it slowly and try to digest it.
Anyway thanks again! You are welcome I've done my bachelor thesis on Gamma Ray Bursts myself, although i focused on ultra long GRBs. Out of curiosity, how does the primordial black hole evaporation tie with GRBs? All i know is that it's a suggested explanation, but most people prefer the collapsar models. I guess the gamma radiation is the tail end of Hawking's radiation? How would you explain the afterglow? And yeah, i didn't mention GRB's in the OP since Hubble specifically hasn't contributed all that much to their study, other than occasionally studying afterglows I'm pretty sure the curve of energy release is all wrong. An evaporating black hole emits energy at an exponential rate. If you look at the curve of flux from a supernova it sort of increases, levels off, and then slowly falls. Black hole evaporation is very violent. Also, you wouldn't get a black hole to evaporate in a region that has enough material to form a remnant nebula, as it would gain mass faster than it loses mass. Edit: Also, all black holes should release about the same energy (equal to about the rest mass of the moon, ~6.6 x 10^39 J), and supernovae are more complex events, with type I and type II and their subtypes. Also, Supernovae emit a few orders of magnitude more energy than black holes evaporating.
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Italy12246 Posts
I'm not sure it's that simple though; GRB prompt emission lightcurves are really really variable (and violent), and they are not the same as the associated supernovas (which are only type 1b or 1c) that long GRBs show, which becomes visibile after the initial GRB. Also as the black hole evaporates its Eddington limit decreases, so i can see how sometimes it may not accrete enough matter to stop evaporating
Seeing as black hole evaporation has never actually been detected, and its full understanding requires quantizing gravity anyway, it sounds like you are oversimplifying things, but if they are supposed to release that little energy (why is that by the way? Do they all detonate at a similar point like type 1a supernovae?) then yeah, they are nowhere close what GRB's emit.
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On April 27 2015 01:31 spinesheath wrote:Show nested quote +On April 25 2015 02:29 Serejai wrote: So what kind of telescope do I need to attach my camera to in order to take some sweet ass stellar photos? As far as I am aware you'll need to take infrared photos from a telescope in space and massively photoshop them for color and contrast. These pictures are great, but you should remember that they have been edited for the general public, not for scientific value.
You can actually make pretty great images of the sky using just a DSLR camera and a good lens - the only "special" thing you need is a mount with a clock drive to compensate for the rotation of the Earth - and actually there is even the possibility to do all-sky images with very fast fish-eye lenses, those you can do just from a fixed tripod and at very dark locations, you can get really stunning images in even in this very simple way.
If you want to do semi-wide shots of the sky (think 50-100 mm lens), you can buy a mount for $100 and have a lot of fun (I sure did), if you want to use a real telescope and work with focal lengths around a meter (more is very rarely useful), then it gets more complex, because mounts that can track Earth rotation so precisely are terribly expensive and thus you need autoguiding - anyway, for $1000 you can already get a somewhat decent setup and feel like you have a small Hubble in your garden.
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If you opt for academics later on, I hope you have more positions over wherever you are than in France :D. Astrophysics is typically one of those subjects where many ultra good profiles will battle for very few positions.
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I actually had never heard of prompt GRBs. Anyways, Hawking Radiation is basically similar to blackbody radiation, and the temperature is given by T = h_bar * c^3 / ( 8 pi G M k) , where M is the mass, k is Boltzmann constant, h_bar is reduced Planck constant. Putting all the mks constants in, you have T = 1.2 x 10^23 * M^-1 . If the "effective temperature" is higher than the background temperature (About 3 K), than the black hole will radiate more than it absorbs as long as it is far away from sources of matter. I got the figure for energy from this. If you set 1.2 x 10^23 M^-1 = 3K, the answer is roughly the mass of the moon (off by about a factor of two, but ...), and all of the mass of the black hole is converted to photons.
All of the equations in here can be found here (and prettier :D), but there isn't much theoretical development. Wikipedia
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Italy12246 Posts
You misunderstood what i said; long GRBs (the ones associated with supernovae) emit light in two phases, called the prompt emission and the afterglow. Prompt emission is the actual gamma burst, which lasts from a few seconds to several minutes, and it's the most violent since it's mostly gamma rays. The light curve (ie flux emitted over time) of prompt emission is completely different from a supernova; if one is associated with the GRB, it can only be seen for very long times once the afterglow is fading.
Your calculation doesn't include the fact that the CMB radiation temperature actually increases with redshift as 1+z, and GRB's are seen at redshifts as high as 9 (meaning CMB temperature ends up being 30 K instead of 3), which doesnt help. You are also assuming there is no accretion but let's not go there
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On April 27 2015 05:22 Mordanis wrote:While thinking about this, I came across this article. Might be the coolest thing I've ever heard of. http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.107.111101Anyways, it looks like about 7 evaporation events have been observed as of 1995. Here's a link to an article. The article is behind a paywall, but you can look at the first two pages if you click "look inside". It seems like black hole evaporation describes some, but not all, GRBs.
The idea that some GRBs are PBH evaporation events is very, very fringe in the GRB community as far as I know. I have never heard it from anyone from the GRB group I losely work with even mentioned as a possibility.
Everyone really needs to keep in mind that however engrained in the popular culture, Hawking radiation is still purely theoretical.
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A black hole evaporation event has to be pretty faint.
How much mass does a black hole retain until the last second of its life? ~ 1000000 kg or so ? Hence 1000000 kg/s * c^2 is the largest luminosity it will ever reach over a full second.
It would have to be to be pretty close to be observable, and would certainly not be red-shifted by the expansion.
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Italy12246 Posts
On April 27 2015 05:36 opisska wrote:The idea that some GRBs are PBH evaporation events is very, very fringe in the GRB community as far as I know. I have never heard it from anyone from the GRB group I losely work with even mentioned as a possibility. Everyone really needs to keep in mind that however engrained in the popular culture, Hawking radiation is still purely theoretical.
Yep, most of the GRB community is pretty convinced by the Hypernova models. SN1998bw is the most important event supporting that interpretation, but there's plenty more. We have detected hundreds of GRBs, so explaning 7 of them is situational at best
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On April 27 2015 05:52 Maenander wrote: A black hole evaporation event has to be pretty faint.
How much mass does a black hole retain until the last second of its life? ~ 1000000 kg or so ? Hence 1000000 kg/s * c^2 is the largest luminosity it will ever reach over a full second.
It would have to be to be pretty close to be observable, and would certainly not be red-shifted by the expansion.
The article linked above specifically notes that while most conservative models predict lower burst output, there are feasible models where the final burst releases 10^9 to 10^14 grams of rest energy - the whole uncertainity comes about when the QCD effects kick in.
Anyway, thinking about it, the paper is from before any real optical counterparts with spectra were known, thus before the cosmological distibution of GRBs was established, making the point probably moot with today's knowledge.
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Also, fuck GRBs in general. Among other things, we are running a small robotic telescope in the southern hemisphere which is set up to do GRB follow-ups and the last big hit occured 3 years before I joined the team, so I don't get a high-profile GRB publication (there is one with "GRB" in title that I have written, but that merely describes the patient wait we are currently exhibiting).
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Italy12246 Posts
haha :D imagine how the people hoping to detect neutrinos from a supernova in the Milky Way must feel then...
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On April 27 2015 06:08 opisska wrote:Show nested quote +On April 27 2015 05:52 Maenander wrote: A black hole evaporation event has to be pretty faint.
How much mass does a black hole retain until the last second of its life? ~ 1000000 kg or so ? Hence 1000000 kg/s * c^2 is the largest luminosity it will ever reach over a full second.
It would have to be to be pretty close to be observable, and would certainly not be red-shifted by the expansion. The article linked above specifically notes that while most conservative models predict lower burst output, there are feasible models where the final burst releases 10^9 to 10^14 grams of rest energy - the whole uncertainity comes about when the QCD effects kick in. Anyway, thinking about it, the paper is from before any real optical counterparts with spectra were known, thus before the cosmological distibution of GRBs was established, making the point probably moot with today's knowledge. Yeah sorry, was just going through the wiki equations and not looking at the article. Even optimistic 10^14 grams is not that much, given the sun puts out ~ 4 x 10^12 g/s.
On April 27 2015 06:16 Teoita wrote: haha :D imagine how the people hoping to detect neutrinos from a supernova in the Milky Way must feel then...
How lucky were Kepler and Tycho? One supernova for each of them, both easily visible for the naked eye.
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On April 27 2015 06:18 Maenander wrote:Show nested quote +On April 27 2015 06:08 opisska wrote:On April 27 2015 05:52 Maenander wrote: A black hole evaporation event has to be pretty faint.
How much mass does a black hole retain until the last second of its life? ~ 1000000 kg or so ? Hence 1000000 kg/s * c^2 is the largest luminosity it will ever reach over a full second.
It would have to be to be pretty close to be observable, and would certainly not be red-shifted by the expansion. The article linked above specifically notes that while most conservative models predict lower burst output, there are feasible models where the final burst releases 10^9 to 10^14 grams of rest energy - the whole uncertainity comes about when the QCD effects kick in. Anyway, thinking about it, the paper is from before any real optical counterparts with spectra were known, thus before the cosmological distibution of GRBs was established, making the point probably moot with today's knowledge. Yeah sorry, was just going through the wiki equations and not looking at the article. Even optimistic 10^14 grams is not that much, given the sun puts out ~ 4 x 10^12 g/s.
Sure, the point was that these things could be pretty ubiquituous and thus the bursts would be happening rather close to us - which is, I believe, considered quite unlikely now.
On April 27 2015 06:16 Teoita wrote: haha :D imagine how the people hoping to detect neutrinos from a supernova in the Milky Way must feel then...
They are probably still dreaming about 1987
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Italy12246 Posts
There's a detector at the labs in Gran Sasso made specifically for that, that started functioning like one month after that supernova. There hasn't been one close enough since.
And yeah, if the black hole evaporation models predict they are that faint, there's no way they can be correct.
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On April 27 2015 06:28 Teoita wrote: There's a detector at the labs in Gran Sasso made specifically for that, that started functioning like one month after that supernova. There hasn't been one close enough since.
LOL I didn't know that, what a story!
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Italy12246 Posts
The funny part is it's still functional, they are all thinking "come on some of you fuckers blow up, you've taken enough time already!"
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Well, it's like waiting for some nucleus with a half life of a hundred years or so to pop.
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On April 27 2015 06:40 Maenander wrote: Well, it's like waiting for some nucleus with a half life of a hundred years or so to pop.
Of those you can just bring a couple of billion and you are golden
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Great read, thanks for posting
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First, amazing blog! I have a strong passion for Physics education so I'm always happy to see people explain difficult concepts in an approachable way.
On April 26 2015 18:34 eonrulz wrote:Great blog, Teoita. Nice to see science getting its due attention on TL :D Maybe I should write a similar blog for particle physics/quantum mechanics. That's my field, after all. Would be a nice accompaniment to this blog - the large and the small! Plus there do seem to be a lot of misconceptions about QM floating around this thread
I'm have a graduate degree in Experimental Nuclear!
We should put together a series of blogs for TL! :D
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Mute City2363 Posts
I took an astrophysics module as part of my maths course, but was too shit to think about continuing on that line though. It's really interesting stuff though; yet another part of applied maths I wish I was more competent at :[
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United States4883 Posts
The Hubble is a PoS! Get that thing outta orbit, it's not doing anything useful up there!
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28057 Posts
On April 27 2015 14:45 thecrazymunchkin wrote: I took an astrophysics module as part of my maths course, but was too shit to think about continuing on that line though. It's really interesting stuff though; yet another part of applied maths I wish I was more competent at :[ Yeah it's extremely daunting. I've taken some physics/astro classes as electives and I think I could have managed to fight my way to a degree in it, but it would have been really hard, and I would never compare to some of the talented people in those classes who just understood stuff like it was no big deal.
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Mute City2363 Posts
On April 27 2015 15:09 TheEmulator wrote:Show nested quote +On April 27 2015 14:45 thecrazymunchkin wrote: I took an astrophysics module as part of my maths course, but was too shit to think about continuing on that line though. It's really interesting stuff though; yet another part of applied maths I wish I was more competent at :[ Yeah it's extremely daunting. I've taken some physics/astro classes as electives and I think I could have managed to fight my way to a degree in it, but it would have been really hard, and I would never compare to some of the talented people in those classes who just understood stuff like it was no big deal.
My friend's on her PhD course right now; published a paper during her undergraduate degree whilst we were studying for finals like it was a regular thing to do :/
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Very nice job Teo it also makes me proud that you are italian ^^
Maybe one day I will do something similar concerning some areas of Philosophy you inspired me!
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Lorning
Belgica34430 Posts
Very nice read~~
I'm also planning to go for Astrophysics and I'm really excited about it
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im amazed about the colors, here i am always thinking that the universe is white/black like the old photos and then bam! the most amazing colors are there. thx for the blog!
i cant event ... the pillars ... holy shit... i dont even x.x
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Well, all the colours are photoshopped in, the images certainly aren't been shot 1:1 across the whole visible light spectrum but they are based on something I think. Translated to visible spectrum because... thats how humans can process it. As somebody mentioned, the images used for science don't look like that at all, but they aren't worth releasing to the public in general.
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Italy12246 Posts
They are usually a mix of several bands, which specific ones depend on the image:
http://hubblesite.org/gallery/behind_the_pictures/
Because sources tend to emit across a much bigger band than just the visible one, technically the universe has more colors than we can possibly perceive
Scientific images are black and white; the revelators used aren't actually sensible to color but simply count photons arriving (usually), so in order to get colors in an image you need to take a pic of the same source with different filters.
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Mute City2363 Posts
On April 28 2015 00:57 Lorning wrote: Very nice read~~
I'm also planning to go for Astrophysics and I'm really excited about it
Go for it! Or at least go for something maths / physicsy
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Wow I actually spurred a conversation, makes me smile.
It was an undergrad thesis, don't go looking too hard at it . Even at the time I did the work it was very fringe to think exploding PBHs could explain even a portion of short GRBs. Basically the idea is that the PBH light curve is something like exp(alpha*t) where alpha is the number of species the PBH can create at the event horizon. For the vast majority of it's life alpha is pretty constant, but once the PBH temperature gets to a point where it can feasibly produce quarks alpha step changes upward by an order of magnitude or something like that. This could cause a theoretically high luminosity for some vanishing amount of time.
I think I remember that PBHs that would be exploding today would have had an initial mass of ~10^15 g. I think during the last 5 seconds they had ~1 solar luminosity and maybe a few orders of magnitude larger than that for the last 1 second. Way too faint to be seen at extra-galactic distances as was the gist of what my thesis was looking at. Still, was a great problem to sink my teeth into as a student.
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Italy12246 Posts
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So much science. I love it. Thanks Teoita for the blog, and thanks to everyone for the discussion. It was a very interesting read ! If you have more, keep it coming mate <3
Now it's time to turn towards eonrulz's blog :D
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While I don't have the rigorous mathematical or scientific training to fully comprehend astrophysics your blog post definitely helped made it simpler. And of course, stargazing or looking at pictures of galaxies light years away is one of those things that should bring awe to the mind of anyone.
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I remember reading that the sun would keep burning for another 6Billion years, 20 years ago as a kid. For a second there I thought "How can there still be 6 billion left, after such a long time". Then i facepalmed.
Well, thats one of the fascinating things about space - how incomprehenseably big everything is. 6 billion is a number few people can really imagine (sure, as something on paper, but what it really means)
great post!
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