[Fluid_thinker (OP)]

Are all stars Hydrogen based in the Visible spectrum?

(Excluding failed stars and dead ones that could have been or have been conventional Stars.

[DoctorBeaver]

No. As stars age they use up all their hydrogen. When that is exhausted the star contracts and heats up enough to burn helium (there is more involved, but that'll do for now). When all the helium is gone it starts on carbon and so on. As each successively heavier element is used the star contracts further, heats up more, and starts burning the next element. Iron is the end of this process.

HERE is a site that explains it nicely.

Your thread title does not actually correspond to the way you phrased the initial post.

But to answer it anyway:

Dark matter is very strange beastie and no-one really knows what it is. It does appear to interact through gravity so I suppose it is possible that stellar-size clumps of it could accrete. However, it is assumed that it does not interact through any of the other known forces so the processes that cause nuclear fission could not occur.

[Vern]

Nice link DoctorBeaver; it explains the main sequence very well. I glean from the link that the main sequence stars end with the helium burning process and never get to the iron creation stage. So to make iron, maybe we need monster stars out of the main sequence.

[DoctorBeaver]

Nice link DoctorBeaver; it explains the main sequence very well. I glean from the link that the main sequence stars end with the helium burning process and never get to the iron creation stage. So to make iron, maybe we need monster stars out of the main sequence.

As far as I am aware, all main sequence stars go to the iron stage. Certainly our sun will become a red giant eventually and there is nothing special about it.

[Vern]

During this phase some Carbon and Helium will fuse

12C + 4He --> 16O

resulting in the formation of a Carbon-Oxygen core. When the Helium is exhausted in the core of a star like the sun, no further reactions are possible. Helium burning may occur in a shell surrounding thecore for a brief period, but the lifetime of the star is essentially over.

I guess it was this paragraph that seemed to indicate that the helium burning process was the final process leading to Carbon-Oxygen. I think I saw somewhere else that iron was produced in more massive stars. But, I'm wrong a lot. []

Edit: The reason I have a mind set for this is that it has implications of the age of the universe. Since our solar system is iron rich, old sol can't be a first generation star. It is made of star dust. If the sun is going on five billion years old, that iron had to come from stars that died before the solar system formed.

[astrobabe]

So - two questions here which i will tackle one at a time:

1 - the sun will not get to the point of burning up to iron, it takes a star at least a few time heavier than ours - the red giant phase is just part of the running-out-of-hydrogen effect and we will burn helium and little more to get a C-O white dwarf. However - for all stars, even the really big ones that explode (supernova) and fuse all elements bigger than iron during that explosion, the Hydrogen is never completely depleted. Enough is fused that the density decreases below what can be pushed together to fuse. This still leaves most of the star as Hydrogen - especially in the outer layers. In most supernovae you can still see the lines from hydrogen showing a significant mass left in hydrogen (the exception being stars that have 'blown-off' most of their outer layers before exploding).

2 - dark matter -> dark stars. There seems to be an interesting property to dark matter (if it exists) in that it doesn't clump together much. Sure it concentrates to form the large scale structure we see as galaxy clusters etc. but never more than that. There was some work done by some Cambridge astronomers looking at dark matter distribution in ver small galaxies suggesting the density flattened toward the centre - as if the dark matter particles (or what ever it is made of) do not like to get too close together.

[DoctorBeaver]

astrobabe - thank you for correcting me about main sequence stars.

I should have put "when all the fusionable hydrogen is gone", my error.

With regard dark matter, I had not come across that Cambridge research. That makes dark matter even stranger - interacts with gravity but stays apart. hmmmmm...

[Fluid_thinker (OP)]

Sorry for the confusion. I was specifically interested in the Dark Matter foming Dark Stars.

If it interacted with Gravity does it form dark stars.

[Vern]

Sorry for the confusion. I was specifically interested in the Dark Matter foming Dark Stars.

If it interacted with Gravity does it form dark stars.

I haven't seen any evidence that dark matter could form dark stars. It doesn't seem to clump and seems to avoid the inner areas of galaxies. We have evidence that something is there, but we have no idea what it is.

[Soul Surfer]

Dark matter interacts only by gravity and it cannot collect to make small dense bodies like stars.  The reason for this is that it cannot cool down by radiating electromagnetic energy it can only cool down by radiating gravitational energy.
 
A star forms when cold material contracts under gravity  as it does this it gradually heats up.  one way of looking at this is that the particles accelerate as the fall in the self gravitational field and eventually gets hot enough for the atoms to interact with each other to radiate heat and light  that's why we see it as a star  (the nuclear reactions come later in the centre of the stars)this allows the particles to cool down and collapse further

Dark matter cannot do this so the particles continue to accelerate towards the center and never slow down so the just zoom at high speed through the centre of any mass concentration and vanish out into space.

Large gravitating bodies like stars and even planets could concentrate the mean density of dark matter slightly and attempts are being made to detect these effects but as yet no clear evidence has been found.

[DoctorBeaver]

Dark matter interacts only by gravity and it cannot collect to make small dense bodies like stars.  The reason for this is that it cannot cool down by radiating electromagnetic energy it can only cool down by radiating gravitational energy.

How does it get hot in the first place?

[Chemistry4me]

I heard this on one of the podcasts.
How hot is x-ray hot?

[Soul Surfer]

If a body falls under gravity it accelerates. This applies to everything including subatomic particles. If a subatomic particle is moving quickly it possesses energy and this energy in a solid liquid or gaseous body is called heat.

This apples even to particles that interact only by gravity  it's just that instead of continually bumping into each other and sharing out the energy like electromagnetically interacting particles like atoms, dark matter particles just accelerate up to high speeds and pass like comets trough any gravitational fields that they encounter and that includes going right through the middle of stars without slowing down or bumping into anything.

The cross section for gravitational interactions between even quite high mass particles is so tiny they virtually never collide. People are trying to look for these collisions but they are extremely rare or they would have easily been detected by now.

[Fluid_thinker (OP)]

So if the Dark matter is accelerating and passing through concentrated matter, then is Dark Matter widely dispersed or in a uniform distribution. I thought this was not the case.

[astrobabe]

With regard dark matter, I had not come across that Cambridge research. That makes dark matter even stranger - interacts with gravity but stays apart. hmmmmm...

newbielink:http://arxiv.org/abs/astro-ph/0608528 [nonactive]

[Chemistry4me]

I heard this on one of the podcasts.
How hot is x-ray hot?

No takers?

[Chemistry4me]

Why can't I find something about this on Google? []

[yor_on]

I heard this on one of the podcasts.
How hot is x-ray hot?

No takers?

X-rayhot? Would that mean a star?

Context: X-ray surveys carried out with the Einstein and ROSAT satellites have resulted in rather unexpected detections of X-ray emission from late B-type and early A-type stars. http://arxiv.org/abs/astro-ph/0703524

As for how hot those stars can be you might look here. X-ray stars seems to be of both A and B variety. There is also a big difference between a stars surface temperature and inner temperature. http://www.astronomy.ohio-state.edu/~ryden/ast162_2/notes8.html And here http://web.utah.edu/astro/about.html.

If you look at the Sun (a G2-star) it is 10,000 (10 thousand) degrees F at its surface, going in to its core you will find its temperature to be around 25,000,000 (25 million) degrees F. And a normal A-star would then have a outer temperature around 35540 F. http://spaceplace.nasa.gov/en/kids/phonedrmarc/2005_may.shtml

[Chemistry4me]

Yes. A star.

[Professor Mega-Mind]

           Dark Matter Kugelblitz
     Logically , the answer is yes !
Dark Matter has mass .  It may be slippery , it may be diffuse , but it won't escape a black hole anymore than a photon ( see Kugelblitz ) .  The trick here is to cram enough Dark Matter into a tiny B.H. to make a basically D.M. black hole .
Would there be a difference ?  No .
.............P.M.
Note : The above applies only if Dark-Matter actually is matter , not fields or virtual-mass .