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Forum » SpaceEngine » Science and Astronomy Discussions » Science and Astronomy Questions
Science and Astronomy Questions
WatsisnameDate: Sunday, 19.06.2016, 22:45 | Message # 526
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Quote Mosfet ()
That would be the Hertzprung-Russell diagram, if I remember well.


That's a part of it. smile An HR diagram is a plot of star luminosity vs. surface temperature, which shows that stars follow particular evolutionary pathways, rather than just having random properties.

One powerful use of it is in determining the ages of globular clusters. The idea is that since all the stars in a cluster formed at basically the same time, then as the cluster ages, more massive stars will evolve off the main sequence first. So as the cluster ages, the the main sequence turnoff propagates down the diagram from top left to bottom right.

But... how do we know the massive stars evolve off the main sequence first? How do we attach an age as a number to a star based on the HR diagram?

To do that, we need to be able to model how stars evolve. This comes from the study of stellar structure, using knowledge from astrophysics, thermodynamics, nuclear physics, and all kinds of stuff. Stellar model building is a huge part of computational astronomy.

Essentially there are four time-independent equations that must be satisfied in a stellar model:



It is effectively impossible to solve these equations together analytically. Instead we must use numerical methods, building up a model of a star which can then be simulated through time. It is from these models that we calculate stellar lifetimes based on initial mass and composition, and how their structure and exterior appearance change over time. HR diagrams then allow us to test the models against what we observe in nature to be sure that the models really work.

Sorry that this might be a more complicated answer to what seems like a simple question -- how do we determine the ages or lifetimes of stars. But the fact is that it's a lot of work, with no simple analytic formula that can be derived.

We can however come up with a general formula for main-sequence lifetimes that is inexact, but "close":
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/startime.html





 
AlekDate: Monday, 20.06.2016, 01:26 | Message # 527
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Quote spacer ()
but still no particle of gravity! its gonna be big challenge to find it


This is what many people said about gravitational waves or the twisting of spacetime by a rotating object--the effects are so weak many people believed that if we ever proved them it would be in some far-off century when we're super-advanced in knowledge and have insanely sensitive equipment that brought to the current day we wouldn't even understand--yet here we are having proved both correct. It's only a matter of time until we prove the graviton to either be real or an incorrect assumption, and it might not take as long as you might think.





Living among the stars, I find my way. I grow in strength through knowledge of the space I occupy, until I become the ruler of my own interstellar empire of sorts. Though The world was made for the day, I was made for the night, and thus, the universe itself is within my destiny.
 
WatsisnameDate: Monday, 20.06.2016, 02:23 | Message # 528
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I am wary of any claim in physics that involves describing things as "nothing". E.g. "particles can be created out of nothing", or "the Big Bang came out of nothing". It's not that they are wrong, it's just that they are sloppy! smile Either we aren't defining what "nothing" actually means in these contexts, or we don't know, or we are making assumptions based beyond the limits of our knowledge.

If I am sloppy with what I mean by "nothing", then all of the following are true:

1) I can store and pull energy out of nothing.
2) I can support my own weight in Earth's gravity by standing on nothing.
3) Nothing can rotate and drag things with it.
4) Nothing makes the universe expand faster.






 
midtskogenDate: Monday, 20.06.2016, 08:10 | Message # 529
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Mathematically it's easy to visualise something coming from "nothing" and turning back into "nothing" again, and the physics of our observable universe would have to deal with it. A particle appearing and disappearing could be visualised as something higher-dimensional briefly passing through our observable space. Perhaps even Big Bang could be described that way. But we generally don't observe things appearing and disappearing, certainly not at scales above atoms, so the universe doesn't seem to be full of high-dimensional stuff and us simply stuck in a few dimensions.




NIL DIFFICILE VOLENTI
 
spacerDate: Monday, 20.06.2016, 14:38 | Message # 530
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i heard that inside black holes. time just stops. its mean that we can never reach the center of the black hole?
because the time stops so its impossible.

also, i think we need to change the thread name to astronomy and science questions





"we began as wanderers, and we are wanderers still"
-carl sagan

-space engine photographer


Edited by spacer - Monday, 20.06.2016, 14:40
 
WatsisnameDate: Monday, 20.06.2016, 18:25 | Message # 531
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Time slows down in a gravitational field, relative to the rate of time farther away. So to someone outside the hole, you are frozen in time just above the horizon. (You are not visibly frozen there because the light is also redshifted to blackness). But to you, you fall through the horizon and meet the singularity, in a finite (actually quite short!) amount of time. Things that fall into a black hole do end up at the center, according to them, while to everyone else they form thin shells along the black hole's horizon.

This same time dilation occurs on Earth, too. Time passes more slowly at sea level than on a mountain top, or in orbit. GPS actually has to account for this to work correctly. smile





 
apenpaapDate: Monday, 20.06.2016, 18:34 | Message # 532
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That makes me wonder what you see of the rest of the universe if you look outward while you're falling in. Since infinite time passes to the rest of the universe when you cross the event horizon, would you see the history of the universe unfold really rapidly if you paid attention to it? And what would you see once you're beyond the event horizon?




I occasionally stream at http://www.twitch.tv/magistermystax. Sometimes SE, sometimes other games.
 
steeljaw354Date: Monday, 20.06.2016, 19:01 | Message # 533
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Do white holes exist, like where it spews stuff out instead of sucking it in?
 
spacerDate: Monday, 20.06.2016, 19:03 | Message # 534
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Quote steeljaw354 ()
Do white holes exist, like where it spews stuff out instead of sucking it in?

could be possible. that the opposite of the black holes. nothing can enter.
some theory said that the big bang was a white hole.





"we began as wanderers, and we are wanderers still"
-carl sagan

-space engine photographer


Edited by spacer - Monday, 20.06.2016, 19:03
 
midtskogenDate: Monday, 20.06.2016, 19:17 | Message # 535
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Since something falling into a black hole appears to slow down as seen from a distant observer, how come that the gravitational waves from black hole mergers appear to us as brief events and not slowed down?




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WatsisnameDate: Monday, 20.06.2016, 20:57 | Message # 536
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Quote apenpaap ()
That makes me wonder what you see of the rest of the universe if you look outward while you're falling in. Since infinite time passes to the rest of the universe when you cross the event horizon, would you see the history of the universe unfold really rapidly if you paid attention to it? And what would you see once you're beyond the event horizon?


For a non rotating black hole, the observer falling in does not see the sped up evolution of the universe. This seems at odds with the idea of time slowing down and stopping at the horizon. But to make sense of it, (or at least the best way that I know of to make sense of it), think of the motion of the light relative to you as you fall in. The light has to fall into the hole as well. Only so much of the light is able to catch up to you before you meet the singularity. (This description is a little bit hand-wavy -- the correct way to look at it is with a space-time diagram).

What does it look like to fall into a non-spinning black hole? Like this:


Not quite what you expect, is it? smile Even passing through the horizon is very counter-intuitive. Here's the same video with a map and clock. Note the clock is slowed down not because of time dilation (the person falling in does not notice their clock slowing down -- everything seems normal), but is instead slowed down in order to make the trip sensible. In reality, it would be so fast as to unfold in the blink of an eye.

Things are very different for a spinning black hole. I talk about some of that here and here.

Quote steeljaw354 ()
Do white holes exist, like where it spews stuff out instead of sucking it in?


Only in the mathematics, not in nature. General relativity allows such a space-time to exist, but there is no mass distribution that could produce it. It would have to be set up as an initial condition for the space-time.

Quote midtskogen ()
Since something falling into a black hole appears to slow down as seen from a distant observer, how come that the gravitational waves from black hole mergers appear to us as brief events and not slowed down?


They are! smile The time dilation just doesn't get severe until very close to the horizon. If you watch something fall into a black hole, you don't really notice an appreciable slowdown because the significant part of it happens so quickly right as its about to fall through. Simultaneously, the same effect that causes the time dilation is also redshifting and weakening the signal to invisibility. So instead of it looking like the object is slowing down and freezing near the horizon, it just vanishes from view and becomes indistinguishable from the horizon.

The gravitational waves also aren't emitted purely from the horizon, but from the bulk of the surrounding space-time as the black holes orbit and merge.

You could also say the black holes never actually merge for the external observer. But they get so close so quickly, and radiate away irregularities (by gravitational waves!) so quickly, that as far as any observer can possibly tell, they are merged into a single black hole.

Quote spacer ()
some theory said that the big bang was a white hole.


Nope. Wrong space-time metric. The Big Bang and expanding universe is described by the FLRW metric. A white hole is described by a time-reversed Schwarzschild metric. The former is that of the space-time produced by a uniform mass distribution. The latter is produced by a point mass (in this case negative mass for a white hole). The Big Bang is not a point. smile





 
AlekDate: Monday, 20.06.2016, 23:05 | Message # 537
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If singularities are points, is there any way to really determine their exact location without rounding errors? Or are they small enough to have quantum effects and only be a small cloud of probability?




Living among the stars, I find my way. I grow in strength through knowledge of the space I occupy, until I become the ruler of my own interstellar empire of sorts. Though The world was made for the day, I was made for the night, and thus, the universe itself is within my destiny.
 
FastFourierTransformDate: Tuesday, 21.06.2016, 09:48 | Message # 538
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Quote Watsisname ()
You could also say the black holes never actually merge for the external observer. But they get so close so quickly, and radiate away irregularities (by gravitational waves!) so quickly, that as far as any observer can possibly tell, they are merged into a single black hole.


This is interesting. Please tell us more about it. The merger doesen't complete for an external observer? Immagine them not orbiting each other but insted making a frontal collision. Would they seem to touch or not? With a big telescope from orbit we could peer into the region where time dilation is so noticiable that we would see the separation beetwen them?
 
steeljaw354Date: Tuesday, 21.06.2016, 10:06 | Message # 539
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How do we know what happens if nobody has experienced falling into a black hole?
 
midtskogenDate: Tuesday, 21.06.2016, 10:25 | Message # 540
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Quote steeljaw354 ()
How do we know what happens if nobody has experienced falling into a black hole?

By science.

We know that Earth has a solid inner core and a liquid outer core, but nobody has gone there, nor have we sent any probes there. We know mass of the sun but nobody has ever gone there to measure it. And so on.





NIL DIFFICILE VOLENTI
 
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