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Forum » SpaceEngine » Science and Astronomy Discussions » Science and Astronomy Questions
Science and Astronomy Questions
UnnamedDate: Friday, 25.04.2014, 02:31 | Message # 256
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There is even a 200,000km gas giant on SE.
And for some reason procedural brown dwarfs are smaller than gas giants.





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Edited by Unnamed - Friday, 25.04.2014, 02:32
 
HarbingerDawnDate: Friday, 25.04.2014, 03:58 | Message # 257
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Quote Unnamed ()
And for some reason procedural brown dwarfs are smaller than gas giants.

Their gravity compresses them to a smaller size, though in reality the diameters of brown dwarfs are more variable than those of large gas giants.





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Donatelo200Date: Friday, 25.04.2014, 04:02 | Message # 258
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Brown dwarfs actually will be smaller than most gas giants. After about 2 Jupiter masses gas giants shrink in size under their own weight and gravity. Brown dwarfs take this a little farther and the most massive are only supported by electron degeneracy pressure since they can't really fuse anything causing them to generally be significantly smaller than Jupiter. (besides Deuterium which is the marker between gas giant and brown dwarf.)
EDIT: ninja'd





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Edited by Donatelo200 - Friday, 25.04.2014, 04:03
 
HarbingerDawnDate: Friday, 25.04.2014, 05:12 | Message # 259
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Quote Donatelo200 ()
they can't really fuse anything
besides Deuterium

And lithium in the most massive ones. Anyway, that fusion does produce energy, which would compensate for some of the gravity. There are also a couple of brown dwarfs which are thought to be significantly larger than Jupiter:

https://en.wikipedia.org/wiki/CT_Chamaeleontis
https://en.wikipedia.org/wiki/GQ_Lupi_b





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apenpaapDate: Friday, 25.04.2014, 09:47 | Message # 260
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Is the size of brown dwarfs age-related? That would make sense considering their heat mainly comes from contraction, and those two huge ones are both very young.




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HarbingerDawnDate: Friday, 25.04.2014, 16:11 | Message # 261
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Quote apenpaap ()
Is the size of brown dwarfs age-related?

Probably.

Also, I moved this discussion here since it was extremely off-topic in the work progress thread.





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WatsisnameDate: Thursday, 10.07.2014, 12:53 | Message # 262
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Yeah, they do shrink over time via gravitational contraction, but this is most relevant during their formative phases. I suspect that, in general, their size depends more on initial mass, and distribution of brown dwarf sizes probably does not correspond to age very well.

Incidentally, gravitational contraction is an excellent source of heat. This was proposed as an explanation for the sun's energy before we understood nuclear fusion. The rate of contraction necessary to produce observed energy output would have been too small to observe.





 
Zaddy23Date: Tuesday, 29.07.2014, 02:12 | Message # 263
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Is neutronium a real thing? I have seen it mentioned countless times in SF and non-fiction but has it ever been created in a lab?




Along with fezes and bowties, brown dwarves are cool.

Edited by Zaddy23 - Tuesday, 29.07.2014, 02:13
 
Billy_MayesDate: Tuesday, 29.07.2014, 08:17 | Message # 264
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Neutronium is a substance composed entirely of neutrons.
It is theorized to exist in the cores of neutron stars.

But, unfortunately,
Quote
it's currently impossible to make it in a lab, there hasn't been a chance to test its physical properties





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Zaddy23Date: Tuesday, 19.08.2014, 02:38 | Message # 265
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Aww, that sucks. Would be really interesting to see what it would do though.




Along with fezes and bowties, brown dwarves are cool.
 
WatsisnameDate: Saturday, 23.08.2014, 21:06 | Message # 266
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A pure neutron substance like "neutronium" probably does not appear anywhere in a neutron star. They have a differentiated structure, from an ion-electron crust to an increasingly neutron-rich mixture of nuclei and other particles deeper down. This might not reach 100% neutron density, as we expect at densities sufficient to fully combine protons and electrons into neutrons, other transitions may become more favorable. Neutron star cores could be more exotic, like quark-gluon plasma.

If neutronium is a real thing, then to describe it we must know the behavior of free neutrons. In normal matter, neutrons are bound in nuclei with protons, and surrounded by electron orbitals which are what produce your standard chemical properties and reactions. Outside of nuclei, neutrons are unstable, and undergo a form of beta decay (converting to a proton, electron, and neutrino) with a half-life of a few minutes. It is possible to produce free neutrons with particle accelerators, and they also appear in nuclear reactors.

But these are single free neutrons, not a material made up of them. The only way we know of to make that is with those extreme pressures. The problem then is that, even if we can make it, it would be impossible to subject to standard tests for physical and reactive properties. The pressures involved are so high that any apparatus would be destroyed, and it's not like you could put a window on the enclosure and watch what it's doing.

We might instead try to determine is properties under standard conditions. Unfortunately here we run into a similar problem as of trying to take a deep-sea creature out of its native environment, magnified a few trillion trillion times. It will very violently decompress (explode). And I believe that answers your question of what neutronium "would do". smile





 
WatsisnameDate: Friday, 05.09.2014, 01:34 | Message # 267
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Per spacer's question in the chat box: Here's the basic idea for how to calculate the total number of atoms in the observable universe.

By dimensional analysis, we can write



We used mass/volume since we recognize this as density, and multiply by the total volume of the universe. This will help us later. We should also be careful with how we count mass -- most of the mass of the universe is dark matter and dark energy. We'll multiply by the fraction fbaryonic so that we only count the mass which is in the form of atoms.



The true density of the universe is very close to the critical density, ρc, or the density which would make the universe flat. It's easy to calculate the critical density through cosmological equations, which is why we started with mass/volume.



Now we'll plug in formulae for critical density and volume, and write expressions for the average atomic mass u (or number of nucleons) per atom in the universe. Most of the atoms are hydrogen and helium, with u=1 and u=4 respectively. Very little is in the form of heavier elements, so we can safely neglect these. We arrive at



where
H = Hubble's Constant ~68km/s/Mpc
Robs = Radius of observable universe (comoving) ~14Gpc
G = Gravitational Constant = 6.67x10-11m3*kg-1*s-2
fb = fraction of total mass density in form of baryonic matter ~0.049
fH;He = fractional contribution of Hydrogen and helium atoms ~ 0.74 and 0.24, 0.9 and 0.1, respectively.
u is a conversion factor ~1.66x10-27kg per amu.

Plug and chug, WolframAlpha tells us this is on the order of 1080 atoms.

We can google this question to see how good our answer is. Universe Today says 1078 to 1082. Not bad!





 
spacerDate: Friday, 05.09.2014, 08:52 | Message # 268
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Quote Watsisname ()
Universe Today says 1078 to 1082. Not bad!

they wrote: "then the total number of hydrogen atoms should be roughly 10^82"
so its all the atoms or just the hydrogen atoms?





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SpaceEngineerDate: Friday, 05.09.2014, 09:05 | Message # 269
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Hydrogen atoms is 90% of all atoms.




 
WatsisnameDate: Friday, 05.09.2014, 10:51 | Message # 270
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Quote SpaceEngineer ()
Hydrogen atoms is 90% of all atoms.


For the cosmological elemental abundances today? That seems high -- what's the source? Periodictable.com cites Mathematica with a Hydrogen abundance of 75%. We could also be concerned about including the next few most abundant elements, as they raise the estimate of average atomic mass of atoms in universe. But such corrections are higher order, only approaching average atomic mass of ~2 (if using above source -- less if 90% H is correct). I'm not interested in factor of 2 accuracy, I care about order of magnitude.

Quote spacer ()
they wrote: "then the total number of hydrogen atoms should be roughly 10^82"
so its all the atoms or just the hydrogen atoms?


They are speaking of total number of protons. Note that their approach is quite different, almost Drake-equation like, by starting with estimated number of galaxies, then average number of stars per galaxy, then average mass of stars, and multiply by average number of protons per unit mass. That method is not as rigorous or precise, but is useful in providing a quality check on the results when compared to different approaches. That such a different method produces an answer which differs by only about 2 orders of magnitude out of 80 is very encouraging.





 
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