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  Click here to go to the first staff post in this thread.   Thread: Ask me about astronomy

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    Retired Staff Frank LeRenard's Avatar
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    Ask me about astronomy

    To blatantly steal Fay's idea, and because I'm terrible about communicating with the community here on a regular basis, ask me anything about astronomy.

    I'm a graduate student as well, roughly two years from my degree (so I'm almost an expert by definition). I focus on galaxies in my research, but have a healthy smattering of knowledge about most other fields in astronomy (planets, stellar astrophysics, cosmology, etc.), so I will do my best to answer whatever in as Neil Tyson-ish a fashion as I'm able.

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    Didn't try, Succeeded Fay V's Avatar



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    Saboteur!

    Okay so, what is dark matter?

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    Retired Staff Frank LeRenard's Avatar
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    Quote Originally Posted by Fay V View Post
    Saboteur!

    Okay so, what is dark matter?
    Starting with the easy ones, huh?
    Dark matter is a placeholder name for a phenomenon observed in the physics of galaxies and galaxy clusters that implies that there's some missing mass or something that mimics missing mass. For example, in disk galaxies like the Milky Way, you expect from Newton's gravity equation (or from general relativity, if you want to go that far) that systems of stars like galaxies should show something called differential rotation, which basically means that the speed at which things rotate around the center of mass of the system should drop the farther out you go. But the rotation speeds observed in almost all disk galaxies are pretty much flat with radius, meaning they don't taper off at all (some even increase with radius), which is not quite the opposite of what you'd expect but close enough. You can solve this problem by assuming there's a lot of mass in the system you're not seeing for whatever reason (maybe it's really dim stars, maybe it's gas that doesn't emit strongly in any wavelength, etc.); in other words, 'dark matter'.

    Now, the exotic kind of 'dark matter' you usually hear about was thought up because when you actually try to account for all the normal matter (given the physics of how the various kinds of matter emit light, given the light we detect at all wavelengths, given results from experiments attempting to find things like rogue black holes or invisible white dwarf stars or free-floating planets or whatever, and right down to theoretical calculations from first principles about the ratio of photons to atoms created in the Big Bang itself, which is a surprisingly easy calculation to do), you still fall distressingly short. So physicists like to invoke a non-luminous, only-interacts-via-gravity kind of exotic matter to solve this problem. Of course, you can also solve the problem by modifying how gravity works, but for whatever reason people don't like to try that as much as the particle side of things.
    Still, until the dark matter is actually discovered (no direct search for it has yielded any convincing results), it could either be missing matter or that we have gravity wrong; hence why I say it's a placeholder name.

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    Senior Manna's Avatar
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    what is gravity

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    Heretic! FlynnCoyote's Avatar


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    How many solar masses are required for a star to collapse beyond the point of a neutron star and become a black hole?

    What's the largest known star?
    * * *
    We'll find a reason, or else realize that we don't need one.

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    Retired Staff Frank LeRenard's Avatar
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    Quote Originally Posted by XoPachi View Post
    This reminds me that I should go check Term's thread about film.

    Anyway, I've heard that the Universe is ever expanding and that scientists believe that it will hit a cap and collapse in on itself at the same rate it is expanding. I'm not sure if I have that right or if that's your field of expertise, but if you can would you like to elaborate on that?
    This is the other 'dark' thing, dark energy. Edwin Hubble (the dapper man after whom the Hubble Space Telescope is named), using a certain kind of variable star as a "standard candle" (meaning all objects of its type have, theoretically, the same brightness; if you know the absolute brightness, you can compare that to the relative brightness to get distance, since things look dimmer the farther away they are) found that almost all galaxies seem to be moving away from us, and that the farther away from us a galaxy is the faster it's receding. This is how things would look if the space in which all of these galaxies are embedded (including us) was getting larger: see the raisin cake analogy. Incidentally, if you extrapolate the rate of expansion backwards, you can estimate the age of the universe.

    But then, some other folks who were following up on Hubble's work using a much brighter standard candle to get at much farther distances (something called Type Ia supernovae) found that not only is the universe expanding, but when you go really far out (hundreds of millions of light years), things start to recede even faster than you would expect from the simple raisin cake argument. What that means is that apparently the expansion rate is speeding up. And since there's no known force that would do that (it's basically anti-gravity), they call it 'dark energy', and even less is currently known about that than about 'dark matter'.

    This actually means that the universe will never re-collapse; it will keep expanding forever, faster and faster (and general relativity allows space itself to expand faster than the speed of light, so this has no limit).

    Quote Originally Posted by Manna View Post
    what is gravity
    Newton described a force that all things that have mass exert on each other, pulling each other toward their mutual center of mass, and he called it gravity. Einstein later on found that you can get back Newton's gravity by doing horribly complex geometry; if you bend space-time (which has four dimensions: x, y, z, and time), objects embedded in that space-time will act just like Newton's gravity law says they should, despite that no 'force' per se is being exerted (things just kind of fall around the curvatures of the space-time fabric... which is impossible to visualize because we can only picture things in 3 dimensions).

    Why things with mass bend space-time is still largely unknown, but that's gravity. If you want me to fully explain it, I would start having to talk about Ricci scalars and metric tensors and Cristoffel symbols and all that other fun stuff.

    Quote Originally Posted by FlynnCoyote View Post
    How many solar masses are required for a star to collapse beyond the point of a neutron star and become a black hole?

    What's the largest known star?
    That's a complicated question. The maximum mass the CORE of a star can have before it becomes a black hole is about 3 solar masses (a solar mass is the mass of the sun, so 3 times the mass of the sun). The mass of the star that hosts that core is a different story, because near the ends of their lives, really massive stars are so freaking bright that pressure from all the photons they're emitting (and yes, light does exert pressure) can literally tear the stellar atmosphere apart and throw it off into space (in the manner of a Wolf-Rayet star), which means the mass you end with is not the mass you start with. Also, when you get extremely massive (hundreds of solar masses), you create what's called a "pair instability supernova", which means that things are so hot in the star's core you actually start creating matter-antimatter pairs that then mutually annihilate. This adds such an energy kick to the supernova that it blows the entire star (core and all) apart, meaning you're not left with a black hole (you're left with nothing). So there's a bit of a sweet spot, where you get the most efficient black hole formation, which I think is around 20 solar masses (too low and you get a neutron star instead, too high and the star just blows itself apart).

    As for the largest known star, do you mean the most massive or largest as in how physically big it is (largest radius)?

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    Retired Staff Tiger's Avatar
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    What happens what matter reaches the event horizon of a black hole? Say a star comes to close to a black hole, what happens? Do we know what happens inside a black hole; and do white holes exist?

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    Heretic! FlynnCoyote's Avatar


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    I meant most massive. But why not answer both since you brought it up?
    * * *
    We'll find a reason, or else realize that we don't need one.

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    Retired Staff Frank LeRenard's Avatar
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    Quote Originally Posted by Tiger View Post
    What happens what matter reaches the event horizon of a black hole? Say a star comes to close to a black hole, what happens? Do we know what happens inside a black hole; and do white holes exist?
    So, the event horizon, with regard to black holes, is also the Schwartzchild radius, which is the radius at which the gravitational force from the black hole is strong enough that the escape velocity (how fast you have to be moving to leave the gravitational potential) exceeds the speed of light. It has the weird property that the more massive the black hole, the larger the event horizon radius. This is important because it affects the tidal forces; tidal forces are the stretching forces, the difference between the gravitational pull on the side of the orbiting object nearest the black hole and the side farthest away. Since the mass is all concentrated in the center of the black hole (a singularity), when you cross the event horizon of a more massive black hole, you're farther away from the mass inside and hence the tidal forces aren't as strong. So oddly enough, it's much easier to get inside the event horizon of a supermassive black hole than a black hole with, say, the mass of the sun. So as for what happens when a star gets near the event horizon, it depends on the mass of the black hole.

    That said, there is a type of object called an X-ray binary, which is just a binary star system visible strongly in X-rays (very energetic light, so can only come through very energetic processes). Some of those we think are stars being devoured by black holes, and what happens there is that the atmosphere of the star is stripped off of the star and flows into an accretion disk around the black hole. The matter is falling in so fast, it can't all get it right away (like dumping a gallon of water into a very small drain all at once). Some of it eventually gets in, some of it splashes out (usually this is the form of two jets; the story I hear about that is that no one knows why we always get two jets), and most of it is just so densely packed in that disk that it grates against itself and heats way up (just friction, like rubbing your hands together until they feel hot, but hot enough to generate X-rays).

    The black hole itself during this whole process just gains mass. The star loses mass, and eventually becomes just a husk of a star that continues to orbit the black hole. I'm sure under certain circumstances, the whole star can fall in, too, if the drag from the accreting material is strong enough to cause it to lose that much angular momentum; the slower your orbital speed, generically, the closer in you fall to the thing you're orbiting, and all those X-rays are orbital energy being converted into light. I'm not sure if this would generate any explosions or not.

    What goes on inside? No one knows!

    As for white holes, the answer is, theoretically yes, but in reality probably not. Theoretically yes because general relativity allows for such a thing, but probably not because they're supposed to be super goddamn bright, and in all our years of looking no one's ever seen one. So if they're out there, they're weirdly rare considering how plentiful black holes seem to be.

    Quote Originally Posted by FlynnCoyote View Post
    I meant most massive. But why not answer both since you brought it up?
    Okay, so... big caveat here. I just looked these up:
    Largest radius: UY Scuti at ~1700 solar radii (1700 times bigger than the sun)
    Largest mass: R136a1 at ~265 solar masses

    But let me add something to this: I'm going to guess that over short periods of time, these may both change. The largest radius one maybe not as quickly, but the largest mass one absolutely. I notice that star I listed there is one of those Wolf-Rayet stars I mentioned earlier, and it's in the Large Magellanic Cloud (a satellite galaxy of the Milky Way; southern hemisphere people know it well if they ever get a chance to look at the night sky down there), which means it's fairly distant. Unless a star is in a well-defined binary system, getting the mass requires building a model of the star in a computer and fitting that model to, say, the observed spectral lines (which tell you its composition, temperature, and a whole host of other things). The uncertainty in this process is equal to the uncertainty with which we know stellar physics, which I will boldly state is very very high (it's okay; the stellar guy in our department is the one who told me this). Even if you have an orbit, the uncertainty is still high since the orbit you're seeing is traced out on a flat plane (the plane of the sky) from our point of view, so you can't tell the difference between an orbit that's circular but tilted with respect to our line of sight, or one's that flat-on but elliptical. So getting masses of stars requires a whole heck of a lot of assumptions that may or may not be accurate (especially for stars this massive!).
    The radius may be more accurate, since sometimes you can actually physically just measure the radius of stars (like, with a ruler) using something called interferometry, as well as a whole host of other direct or semi-direct methods like measuring light curves. So I'd be more apt to believe that number than the mass number, but apparently (according the Wikipedia), this is the current state of things. Just know that there's bound to be lots of debate here, and that the teams who proposed these numbers may not come out on top.

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    Didn't try, Succeeded Fay V's Avatar



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    does physics tell us that photons are absent from time?

    So as you approach the speed of light time appears slower relative to you. In class today someone was making the argument that from the point of view of a photon there is no time, photons are out of time. Which seems weird. To us that makes sense, from our relative perspective time does not effect photons, but from the relative point of view of a photon wouldn't it still be effected by time?

    The claim was no, because when you hit the speed of light you divide by zero or get infinite, so math breaks. Personally I think this is a cope out, just because math breaks does not mean that the item itself is not effected by time, we do see photons can get from point A to point B. so wouldn't it have to have some effect from time? even if functionally from our perspective it is not effected by time.

    (this is not astronomy, but you know physics.)

 

 

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