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Forum » SpaceEngine » Science and Astronomy Discussions » Science and Astronomy Questions
Science and Astronomy Questions
apenpaapDate: Friday, 05.09.2014, 12:31 | Message # 271
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75% is counting by mass, I believe. It's a lower percentage than simply counting by number of atoms because hydrogen is so light.




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WatsisnameDate: Friday, 05.09.2014, 21:59 | Message # 272
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Ah, that's right. Somehow I got to thinking relative chemical abundances were measured by atom, but those figures were not.

While important to say the right thing, it doesn't change the answer very much, and it actually brings us even closer to 1080.





 
Zaddy23Date: Tuesday, 09.09.2014, 22:39 | Message # 273
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What colour are brown dwarves? The name implies they are brown, SE implies they are reddish, and the internet says they are purple, so in all this hubbub I was wondering if SE's brown dwarves are only limited to a certain type for now or if the colour is wrong, or even if SE is right.




Along with fezes and bowties, brown dwarves are cool.
 
HarbingerDawnDate: Tuesday, 09.09.2014, 22:55 | Message # 274
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Brown is just a name. Their color depends on their temperature, unless they don't glow enough to have a large emissive component to their appearance, in which case it depends on composition. Just like stars and planets. Also, the internet most certainly does not say that they're purple.




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Zaddy23Date: Tuesday, 09.09.2014, 23:41 | Message # 275
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Ah, ok, I just saw a few articles saying that most of their emissions would be closer to the UV spectrum, creating a purple effect like this:


EDIT: Nevermind, just found out that's only 1 type of brown dwarf, the rest are in fact different colours:





Along with fezes and bowties, brown dwarves are cool.

Edited by Zaddy23 - Tuesday, 09.09.2014, 23:44
 
HarbingerDawnDate: Wednesday, 10.09.2014, 00:17 | Message # 276
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No, most of their emissions are in infrared, not UV. Also, even within one class of brown dwarf you can have different apparent colors depending on age, mass, and composition.




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WatsisnameDate: Wednesday, 10.09.2014, 03:57 | Message # 277
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Quote Zaddy23 ()
I just saw a few articles saying that most of their emissions would be closer to the UV spectrum


No, this is confusing their apparent color with their emission. As HarbingerDawn said, most brown dwarf emission is infrared, not UV. There is fundamental physics which relates the emission of an object to its temperature. Higher temperature --> shorter (bluer) peak emission wavelength. At the temperature range of brown dwarfs, peak emission is infrared. Even the much hotter Sun peaks in visible light. Peak emission in UV doesn't occur until you approach A type stars with temperatures >7500K.

Since our eyes are not sensitive to the brown dwarf's emission, the color that we would see them as depends more on what light they reflect from another light source. This depends on their composition just as with any other object. If there is no other light source, then they will appear invisible or a dull reddish color.





 
SalvoDate: Wednesday, 10.09.2014, 11:48 | Message # 278
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Quote Zaddy23 ()
purple effect

Purple color must be a consequence of composition of atmosphere an many other factors.

Just look at Saturn: it is yellow, but it has a "blueish" atmosphere.
If you think that Brow dwarfs are red, with a blue atmosphere and a few reflecting light, they might become "purple" as well. (I think, that's my supposition)





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FastFourierTransformDate: Wednesday, 10.09.2014, 12:29 | Message # 279
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Some brown dwarfs renders made by cgzebra1 and mcplanetearth



Edited by FastFourierTransform - Wednesday, 10.09.2014, 15:32
 
WatsisnameDate: Wednesday, 10.09.2014, 13:38 | Message # 280
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Those are some beautiful images. I would guess the 3rd row seems the best representation of their general appearance. The 4th row is probably depicting a blue aurora effect or something of that nature. It cannot be directly thermal.




 
Zaddy23Date: Wednesday, 08.10.2014, 00:12 | Message # 281
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Quote FastFourierTransform ()
Some brown dwarfs renders made by cgzebra1 and mcplanetearth

That helps alot, thanks.





Along with fezes and bowties, brown dwarves are cool.
 
n0b0dyDate: Friday, 07.11.2014, 15:52 | Message # 282
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Quote SpaceEngineer ()
6. Are there any lonely stars, star clusters, planets, nebulaes outside galaxies and in between them? (I ve been wondering about this for years )

Intergalactic globular clusters are already found. BTW, spherical dwarf galaxies are very similar to giant globular clusters - so maybe all globular clusters are ancient dwarf galaxies consumed by our galaxy (and other galaxies), that are prevented from full destruction because of its high density.

Intergalactic stars may form by ejection of a star in supernovae explosion in binary system or close approaching of a third star in a binary system. This processes gives a big velocity to a star, which allows it to fly away from the galaxy. Another process is a collision (merging) of galaxies - up to 50% of stars may be spreaded out in a huge expanding cloud of stars, but too dim to be observable by modern telescopes.

Intergalactic planets of course must exist too, formed by the same mechanism as intergalactic stars, but we have only one method to bind them - microsensing.


The above question has been answered .
It remains to be implemented in SE now smile
 
WatsisnameDate: Saturday, 08.11.2014, 09:44 | Message # 283
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I think that question was more completely and accurately answered by SpaceEngineer than by that link. The results reported there aren't even what we consider conclusive yet (more work needs to be done), and it deals with the universe at high redshift -- not the universe as it is now.

So as for implementing it? I'd say no.





 
WatsisnameDate: Wednesday, 12.11.2014, 07:34 | Message # 284
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Copied from page 48 of the Work Progress Thread:

Quote Adiabat
Hello!
I know that but my point is: the singularity that is supposed to live there is a zero dimension space region thus with zero volume. No matter the mass you consider, the result is the same, you get a zero divider leading to mathematical infinity.

The question is whether that infinity result should be considered as a physical object or not. I've always been told that it is a bad move to admit a physical counterpart for a mathematical infinity. It is showing instead that there is a problem with the theory or that, at least, we have reached its domain limits. And that's also why physicists always try to avoid that in their equations by using renormalization, for what I know. No matter the mass, you will have an infinite density with a zero volume.

So what? Why infinite gravity and density should give a different gravitational field in that case if you consider 10 or 1 billion solar masses? Without singularity I've no problem, it is clear for everyone. But with it, I don't get it. I don't see where is the physical sense to even bother considering any mass you want with a zero dimensional object. This is the same conceptual problem with zero dimensional particles and quantum physics Bell's theorem.

So, even if I'm seeing plenty of nice 3D pictures of black holes in magazines and TV or supposed indirect observations, I'm still dubious scientific community is all OK with that concept without any debates... There is no tools in general relativity or quantum mechanics to describe exactly what's happening when a star collapse, reach the event horizon and approach the singularity limit. What's happening when you approach that point? Is there one or more intermediate state(s)? Is matter "simply" rebounding as quantum loop gravity is suggesting because space as has a minimal Planck volume, avoiding singularity? Infinity is not a good answer. It is a mathematical warning. At least I hope it is not a good answer for some physicists because if not, I feel like science is in a sort of wonderland on that matter...

Well, I guess this is not the place to talk about that (my apologizes to harbinger). Let's hope we will soon get a precise picture of the supposed black hole at Cygnus X1 location, maybe with space interferometry. I will not bet on a black hole staying there...

Cheers!


Hi Adiabat,
This was a great post, and you deserve a proper answer/explanation. I’d like to begin by assuring you that every single relativist who studies black holes has asked these same kinds of questions before learning the ropes. Myself, included. smile

The next important note before diving in here is that treating particles as infinitesimal is a method of classical physics. It is an assumption, yes, but there is nothing wrong about the assumption for as long as you recognize what and where its limitations are. For very many applications, it doesn't hurt at all. For quantum mechanics, it obviously fails in certain respects, because particles also have wave-like characteristics, and these are important for various observations/behaviors. Hence, QM is not a classical theory (and Bell's theorem is still valid -- we just have to accept that quantum mechanics operates under a different logical structure than we might like).

As for General Relativity, it does not in any meaningful way impact the results for black hole formation or dynamics either. It only is important if you want to understand the immediate vicinity of the singularity itself.

So here’s the deal:

Gravitational singularities arise because our current best theory of gravitation (GR) is a classical theory. It treats space-time as continuous and infinitely differentiable, for instance. It does not consider quantization. Within the framework of GR, singularities are guaranteed to result from the collapse of a distribution of matter/energy within its own Schwarzschild radius. That is, once a distribution of mass has a gravitational field sufficiently strong to produce an event horizon around it, then further collapse cannot be avoided. This is shown through various singularity theorems, e.g. by Penrose.

The singularity even has a very precise definition in GR, beyond “a point of infinite density”. If you find a place on a space-time manifold where all geodesics (space-like, light-like, and null), and all time-like paths with bound acceleration (any allowed observer), end after finite proper time, and the manifold cannot be smoothly extended beyond that location, then you have found a singularity.

It may very well be that a future quantum theory of gravity will dispel singularities, and this is a large active area of ongoing theoretical research. However, it makes absolutely no difference to whether black holes exist in nature, or what they would look like, whether from outside or from the point of view of an observer falling into one. The expected size of the region where quantum mechanical effects would become important to the collapse is very, very small. A person falling into a black hole would be killed by tidal forces long before reaching it. Information about the “singularity”, (or whatever it may be), cannot be obtained either, because no allowed trajectories can come off of it. You would not be able to see the singularity even if you could put your nose right up to it. For all practical intents and purposes of describing black holes, the distinction between the classic singularity vs. some future description is not an important one.

Lastly, and again because a black hole is a guaranteed outcome of generalized gravitational collapse, there are many astrophysical phenomena that must produce them, e.g. massive stars when they die. You should expect to see them out there in the real world. Indeed, we see a very large number of objects that exhibit the characteristics we expect of a black hole, and are nearly impossible to explain in any other way. There is no good reason to think Cygnus X1, Sagittarius A*, or pretty much any other current black hole candidate, is anything other than a veritable black hole.

For additional information, and an excellent and rigorous discussion of black holes, gravitational collapse, and space-time dynamics in general, I can recommend nothing more highly (assuming you either have the necessary prerequisites or the willingness to devote the time to learning them) than the text Gravitation by Misner, Thorne, and Wheeler. Part VII of the text is completely dedicated to this subject and every single page is full of brilliant insights. smile

I hope that helped! Cheers.





 
SalvoDate: Thursday, 13.11.2014, 16:39 | Message # 285
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I just watched Interstellar and I have a few questions for you: if you stay on a stable orbit of 3 Schwarzschild radius from a Black Hole for something like an half of hour, how is the time affected? Is there a formula for it? Does that depends on the Black Hole's mass?

The film says 27 years are passed after they've been there, but I don't know if that value is rational or random...





The universe is not required to be in perfect harmony with human ambition.

CPU: Intel Core i7 4770 GPU: ASUS Radeon R9 270 RAM: 8 GBs

(still don't know why everyone is doing this...)
 
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