Dark matter question

[scotthm]

I just read an article (http://www.space.com/scienceastronomy/080617-st-dark-matter.html) about astrophysicists looking to the sun for clues about dark matter, and it got me to thinking.

If dark matter comprises about 90% of the mass in our universe, and the only discernable property it has is mass, then what would keep it from comprising the majority of a star's mass?

It's thought that galaxies are filled with and surrounded by dark matter, making spiral galaxies (for example) spin faster than the visible matter could account for. Likewise, how do we know that a large percent of the mass of a star isn't dark matter, and that the star's 'normal' matter is far less than assumed? Wouldn't this have a significant impact on a star's life cycle if it's mass is mostly from dark matter? And if this isn't the case, why isn't it the case?

Anyone have any insights?
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[Jadzia]

Well I'm no expert here.

I think its dark because it doesn't absorb/radiate photons so is essentially invisible to optical instruments, so we don't know its there or not except from gravitational effects. It is thought to be neutrinos. For these particles at least, they interact so gently that they can pass straight through the sun. I think the sun makes them in fusion reactions, plus heavy atomic nucleii, and proton rich nucleii make them in beta decay.

 

[Pavonis]

I think the term "dark matter" arose simply to denote matter that wasn't shining, so to speak, so that "dark matter" in a star, which by definition is luminous and therefore not dark, is not really possible. Besides, we can determine the mass of a star, and there's no reason to invoke the presence of "dark matter" in a star. In galaxies it is invoked to explain the discrepancy between observed mass (stars) and the calculated mass. But dark matter probably isn't anything too exotic, probably just brown dwarfs and other non-luminous material (dust, gases, etc).

At least that's how I understand it.

 

[scotthm]

But dark matter probably isn't anything too exotic, probably just brown dwarfs and other non-luminous material (dust, gases, etc).

If this is the case then I don't see any problem either, but many astrophysicists apparently believe that the bulk of Dark Matter is composed of 'exotic' particles. If this is the case, shouldn't it pose a problem for solar physics.

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[Pavonis]

^ I don't see why. I'm not aware of any major unsolved problems in solar physics. Last I heard, the solar neutrino problem had been solved, and that's the only issue that might touch on both exotic matter and solar physics.

But then, I'm a geophysicist, not a solar physicist, so I'm certainly not up on the latest research on the sun.

 

[Christopher]

It's thought that galaxies are filled with and surrounded by dark matter, making spiral galaxies (for example) spin faster than the visible matter could account for. Likewise, how do we know that a large percent of the mass of a star isn't dark matter, and that the star's 'normal' matter is far less than assumed? Wouldn't this have a significant impact on a star's life cycle if it's mass is mostly from dark matter? And if this isn't the case, why isn't it the case?

The reason it's called "dark" matter is that it's mass whose gravitational effect can be observed but that can't be accounted for by the things we see out there in space. Stars are things we can see, so all of their mass falls under the category of regular matter.

Unless you're proposing that a large percentage of the observed mass that we count under the "regular" column (because it's in stars and is therefore part of the "bright" matter) is made of the same invisible stuff that we describe as "dark matter," so that there's more dark matter and less "bright" matter than we recognize there to be. Based on observational evidence alone, I suppose that can't be ruled out. But as others have pointed out, the indications are that dark matter is largely made of exotic, non-baryonic particles (sorry, Pavonis, but microlensing studies have shown that massive compact halo objects like brown dwarfs cannot account for the majority of dark matter, since if there were that many of them, we'd see much, much more microlensing than we do). And our models of how stars work and what they're made of have proven reliable, so there's no reason to doubt that they contain just as much hydrogen and helium and other baryonic matter as we think they do.

Besides, as Carl Sagan liked to say, we're made of starstuff. The elements that make up planets and people were forged in stars, blown out into space in supernovae, condensed into nebulae and new stars and planets, etc. So if stars were largely made of a given type of matter, we would be too.

 

[scotthm]

Unless you're proposing that a large percentage of the observed mass that we count under the "regular" column (because it's in stars and is therefore part of the "bright" matter) is made of the same invisible stuff that we describe as "dark matter," so that there's more dark matter and less "bright" matter than we recognize there to be. Based on observational evidence alone, I suppose that can't be ruled out. But as others have pointed out, the indications are that dark matter is largely made of exotic, non-baryonic particles.

This is exactly what I'm talking about.

If Dark Matter clumps together with regular matter because of gravity, I don't see how galaxies can consist largely of Dark Matter if stars don't. What would keep the two types of matter seperated from one another? They should be gravitationally drawn together, and stars should consist of 90% dark matter just like the universe seems to.

Wouldn't this make a star of a given mass have much less 'regular' matter to use as fuel than what we might assume, compared to a star of the same mass consisting only of 'regular' matter?

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[Christopher]

The leading theoretical contender for the particles that make up the bulk of dark matter are "WIMPs" -- weakly interacting massive particles -- or similar things like neutrinos. That means that they don't interact much with ordinary matter -- or with each other -- except by gravity. So they wouldn't really clump together into concentrated masses the way baryonic matter does. Also, although abundant, they'd individually be very low in mass and therefore moving very quickly, unlikely to be captured by the gravity of a protostar and trapped inside it. Occasional capture by a star is possible, but:

...halo WIMPs may, as they pass through the Sun, interact with solar protons and helium nuclei. Such an interaction would cause a WIMP to lose energy and become "captured" by the Sun .... As more and more WIMPs thermalize inside the Sun, they begin to annihilate with each other, forming a variety of particles including high-energy neutrinos.

So even if a star somehow started out with a large quantity of dark-matter particles within it, they wouldn't survive.

 

[scotthm]

The leading theoretical contender for the particles that make up the bulk of dark matter are "WIMPs" -- weakly interacting massive particles -- or similar things like neutrinos. That means that they don't interact much with ordinary matter -- or with each other -- except by gravity. So they wouldn't really clump together into concentrated masses the way baryonic matter does.

Well, I don't really understand why this would prevent WIMPS from coalescing into compact shapes due to the influence of gravity, but since I don't know much about it, I'll take your word.

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[Jadzia]

^Compare these wimps with light. Light doesn't coalesce.

 

[Christopher]

No, light is a bad analogy, since photons are massless. These are particles that have small mass, but that don't form electromagnetic or nuclear bonds, so they can't hold together the way normal matter can. They're just loosely bound by gravity, which is very weak. And they don't clump too tightly because they're usually just moving too fast. They're slow enough to be below the escape velocity of a galaxy or galactic cluster, so they tend to stay in halos around galaxies, but not slow enough to be below a star's or planet's escape velocity, and therefore they have too much energy to condense into something as small as a star or planet.

By analogy, think of the difference between water vapor and ice. The molecules in water vapor are too high in energy, moving around too quickly, to condense. So the most you can get is a loose cloud of vapor. They have to lose a lot of energy in order to collapse into smaller forms. The difference is that water molecules can lose that kinetic energy by converting it into electromagnetic binding energy when they link together into liquids or crystals, and simply by transferring energy to the air or metal or whatever around them. WIMPs can't do that. Since they don't interact much with anything, they don't have many mechanisms for shedding the energy they have. So they remain in a loose "vapor" state rather than coalescing into "solid" clumps.

 

[scotthm]

water molecules can lose that kinetic energy by converting it into electromagnetic binding energy when they link together into liquids or crystals, and simply by transferring energy to the air or metal or whatever around them. WIMPs can't do that. Since they don't interact much with anything, they don't have many mechanisms for shedding the energy they have. So they remain in a loose "vapor" state rather than coalescing into "solid" clumps.

That does make sense. Thanks.

Now, if scientists can only prove they exist...

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[Christopher]

On the other hand... it turns out there may be rare circumstances in which a star could contain a large quantity of dark matter. That would keep it from behaving like a normal star, though:

http://space.newscientist.com/article/dn14197-frozen-stars-could-shed-light-on-dark-matter.html

If dark matter particles are made up of heavier versions of already known particles, an idea known as supersymmetry, as many scientists believe, they could lose energy through interactions with normal matter and sink to the centres of the stars.

Trapped, the dark matter particles would collide and annihilate into a spray of elementary particles and energy.

A star that captured enough dark matter particles would still emit radiation, but its fires would no longer be fuelled by nuclear reactions. As a result, it would be caught in a state of arrested development.

Previous modelling work suggested population III stars could remain in this frozen state for hundreds of thousands of years before using up enough local dark matter to resume normal stellar evolution.

However, new research by Bertone and his team shows that if the first stars were born in exceptionally dense dark matter regions – such as those near the centres of galaxies, they could remain frozen indefinitely.
...
And the team says these stars could be detected. "A frozen star would appear much bigger and colder than a normal star with the same mass and chemical composition," says colleague Marco Taoso.

Of course, this would only happen if dark matter is made of supersymmetric counterparts of known particles. So it might not be true at all. But if such stars could be found, it would help us understand what dark matter is.

 

[scotthm]

^
Very interesting theory. Thanks for the update.

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