White Dwarfs Could be Dark Matter

Porfiry writes "An international team of astronomers has detected what could be a significant portion of the galactic dark matter that has eluded astronomers for nearly 70 years. Scanning digitized images of the southern sky, the team found 38 previously unseen cool white dwarfs within about 450 light years of Earth. If the density of these newly discovered white dwarfs is indicative of the rest of the galaxy's halo, these dead stars would comprise at least three percent of the dark matter of the halo, and perhaps as much as 35 percent."

Re:Dark matter

[Christopher Thomas]

Obviously, no one has seen dark matter, but only attributed to it unexplainable effects. Being as dense as it supposedly is with such great gravitational attraction, how does it keep from folding in on itself like a black hole?

"Dense" is a relative term. If there's 10 times as much dark matter in the galaxy as luminous matter, it's still pretty tenuous. If most of the dark matter is in intergalactic space, then it's even more tenuous. (In practice, you have dark matter both inside and between galaxies; how much is present in each location is open to debate).

So, dark matter may avoid forming black holes the same way luminous matter does - by there not being enough in one place to collapse.

Secondly, many candidates for dark matter are particles that travel at or near the speed of light. This is much less readily confined than slower matter, which would explain both why it hasn't formed black holes and why there's so much of it (relative to luminous matter) in intergalactic space.

In the case of white dwarf cinders, they don't collapse because they're in more-or-less stable orbits within the galaxy, just like most of the other stars.

Food for thought.

[Christopher Thomas]

What strikes me as interesting is that there are so many of these white dwarf stars relatively nearby. If there are any within a dozen or so light-years of earth, they would make interesting targets for sail-based probes.

White dwarf stars are mostly made up of degenerate matter (they have a shallow "skin" of normal matter on top). The theoretical properties of degenerate matter are well-known, but having a probe over a white dwarf would give us a vast amount of new information about it (mainly via seismic data; yes, there are "starquakes"). There would also be a lot of (relatively) cool, dense matter on the surface and in the atmosphere. This would give us a lot of information about how materials behave under extreme conditions. We'd also learn about how matter interacts with the white dwarf's strong magnetic field (white dwarf stars, like neutron stars, keep much of the parent star's magnetic field, compacting it into a much smaller space).

This environment is very different from anything found within our solar system, so it would be quite interesting to study.

(Yes, we can build probes that can reach there within a reasonable amount of time. A ship that carries its own fuel would take generations or centuries, but a sailcraft launched with stationary lasers could get there much more quickly. If you make the craft and the laser system more complicated and expensive, you can even slow it down at its destination.)

Sailcraft.

[Christopher Thomas]

And it's not nearly as easy as you might think; your sailship would take a century or more, and would require very powerful lasers if they were to have noticeable effects at even interplanetary distances.

Actually, the main engineering challenge with the laser isn't its strength - it's its diameter. In order to have most of the beam hitting the sail even when the probe is a few light-years away (as is needed if you want to use a "forward"-style system to slow down), the sail and the effective laser aperture have to both be about 1000 km in diameter. This means you have a space-based array of lasers which use a lot of optics trickiness to stay in phase with each other.

My father and I were kicking around numbers for this thing a few weeks ago (we're both into physics). It turns out it's buildable (though quite expensive).

A century is a reasonable length of time for the trip to take. How short a trip you can manage depends on how bright you can make the laser and how light you can make the sail. Laser brightness scales linearly with cost (just make it wider). My sumbers suggest that in theory you can make the sail thin enough to get the ship moving at close to C for most of the trip, but more realistic estimates give about order-10% C as the peak velocity for an order-10-LY trip.

White dwarfs and dark matter

[smoondog]

Interesting. If white dwarves are really to account for the enormous difference in predicted vs seen dark matter, then there must be an awfull lot of white dwarves out there. Don't white dwarves give off some radiation strongly taht should be detectable if they are really that common?

-Moondog

just the BBC article

[jeffsenter]

The BBC has a normal article [bbc.co.uk] on this.

Obligatory "Bloom County" Reference

[Transition Cat]

Andover, racial epithets are in violation of FCC regulations....

....

Distance to Sirius

[Kotetsu]

According to the current best measurement (Hipparcos [estec.esa.nl]), Sirius is 8.60 light years away. (I'm severely nit picking, I know) The rest of your post is exactly correct. There's a good reason that "astronomical" is used as an adjective to mean "so large as to defy comprehension". The distances to objects, even in our solar system, are so great that travel to any of them by any means takes years. Pluto is less than 1/1000 of a light year away. The Voyager 2 mission took 12 years to reach Neptune (which was farther from the Sun than Pluto at the time). At that pace it would take over 100,000 years to reach Sirius. Even allowing for continuous acceleration between the stars, probes will take centuries to reach their destinations.

Re:Dark matter

[skwang]

Being as dense as it supposedly is with such great gravitational attraction, how does it keep from folding in on itself like a black hole?

There are two answers:

1.

What determines the final stage of a collapsed star is its main sequence mass. Your sun's main sequence mass is 2 x 10^30 kg or one solar mass (Mo). Stars that are (less than)1-3 Mo become white dwarves, 3-10 Mo become neutron stars, >10Mo become black holes. Note, these numbers are approximate, look in a astronomy text for details.

2.

White dwarves are "held up" by what is known as degenerate electron pressure. To understand this phenomenon, I have to explain the Pauli-Exclusion Principle. The Pauli-Exclusion Principle states that no two electrons can occupy the same energy state. (In general the Pauli-Exclusion Principle says that no two identical fermions can occupy the same state. Fermions are particles with a spin of one-half.) A good analogy (which was given to me) is as follows: think of a classroom with 50 seats. Since only one student can occupy one seat, there are exactly fifty students in the room. If the room suddenly shrinks to 30 seats, 20 students must leave, and only 30 students remain.

As a main sequence star with mass (less than)1-3 Mo collapses, it reaches a point where every electron fills every energy state. At this point, there is an outward pressure because all the electron energy states are filled. This outward pressure balances against the inward pressure of gravitational collapse. At this point, we have a white dwarf.

If a star is sufficiently large, the gravitational pressure is enough to force electrons to bind with protons making neutrons. Neutrons are also fermions. So they also obey the Pauli-Exclusion Principle. Neutron stars are "held up" by degenerate neutron pressure.

I don't understand how black holes form...

Re:Dark matter

[Kalrand]

Simple, the dark matter is using those really cheap K-Mart folding tables that absolutely refuses to fold after the 4th or 5th time they've been used.

Kalrand

-the voice of reason

Re:Dark matter

[caffeinated_bunsen]

The term "dark matter" does not, in general, refer to matter any different from what you're made of. The distinction is that it is not emitting light, like stars, and so is not easily observed at any significant distance. Astrophysical models tell us that the matter we see as stars and dust clouds falls short of the total mass of the galaxy. The missing mass is collectively referred to as dark matter. As this article explains, previously unobserved dim white dwarfs may comprise a significant percentage of that mass.

There has been much speculation about the possible existance of non-baryonic dark matter. Non-baryonic just means not composed of protons, neutrons, or their unstable cousins. Nobody has offered any good explanation of the precise nature of this matter, if it even exists.

Re:Food for thought.

[Actinophrys]

The nearest white dwarf is the companion to Sirius, eleven light-years away. But almost anything at that distance would be an interesting target for a probe; we know very little, for instance, about stars other than the sun. And it's not nearly as easy as you might think; your sailship would take a century or more, and would require very powerful lasers if they were to have noticeable effects at even interplanetary distances.

Re:Dark matter

[NonSequor]

Well for one thing, the fact that neutrinos have some sort of mass could explain some of the missing mass (they must have a mind-bogglingly small mass though). Anyway, I was thinking, what if there were particles that only interacted by means of gravity. Such a particle could be even more prevalent than the neutrino and we would never notice it. Even if it had a moderate mass (as compared to the neutrino) we would have trouble detecting it. This is just speculation though.

Er... Well, y'know. You can't make an omelette without um... destroying a forest. Or something.

Dark matter

[Dancin_Santa]

Obviously, no one has seen dark matter, but only attributed to it unexplainable effects. Being as dense as it supposedly is with such great gravitational attraction, how does it keep from folding in on itself like a black hole?

Dancin Santa

Re:Dark matter

[redux94]

Well, what happens when something forms a black hole is you get a large amount of matter in a small area, and there's enough mass there that the force of gravity can overcome the Pauli exclusion principle and it can collapse beyond certain limits to a point in space. Assuming dark matter is distributed somewhat evenly throughout the universe, even if it is still massive, I doubt it'd collapse to a point. And besides, we don't really know what's out there or what mass we aren't seeing. There are theories about primordial black holes and other things of a similar nature.

The whole point of dark matter, though, is based on Freidmann models of whether the universe will collapse in on inself or will just keep expanding. Present calculations on the mass of the universe with known matter bring it to about 1% of what's needed for it to collapse in on itself. With dark matter, that goes to about 10% of the mass needed to cause the universe to begin to contract.

There are three Freidmann models. One is that the universe has enough mass that it will collapse in on itself. The second is it just barely does not have enough mass to collapse in on itself. The third is that it just keeps expanding, and the mass is well below the critical mass we calculate. Simply put, dark matter is matter that doesn't emit radiation that we can detect, because we only add in the mass of known matter to our calculations. We presume there is vast amounts of matter that aren't putting out detectable radiation. These could come from a number of sources, namely interstellar matter that never formed stars, burned out remnants of stars, and as long as these are distributed relatively evenly, these would not collapse to form a singularity for a very long time. I bring this all up, though, as dark matter was discussed because of the question of if the universe would collapse to a singularity, which seems to be related to your question.

Re:Dark matter

[TopherC]

As I understand it, the degeneracy pressure works like this:

The "seats in the classroom" you refer to are particular values for the momenta of the particles (electrons in the case of white dwarfs) in some volume. So as you try to compress the material, in you're adding more particles to any given volume of the stuff. These particles must pick discrete values of momenta that aren't already "taken", so they must be fast. All of the lower momentum seats are already taken, if you consider a white dwarf to be "cold". Even though a white dwarf is very hot in ordinary terms, it's "stone cold" for these purposes because the pressure due to temperature is much less than what's needed to compete with gravity.

So the more dense the white dwarf becomes, the higher the average momentum of the electrons must be. This average momentum translates into pressure. So even though the white dwarf is not hot enough to support itself against gravity by ordinary classical gas pressure, there is a minimum pressure of the gas that comes, basically, from the Pauli exclusion principle.

As you increase the pressure more and more (we're considering more and more massive stellar cores), the electrons you're forcing into a given volume will need to adopt larger and larger values of momentum. Eventually the momentum of the electrons will become larger than the rest mass of the electrons, and they become "relativistic" (moving very close to the speed of light). In this case, as you add more electrons and increase the average momentum, you aren't actually increasing the speed of the electrons by much -- they're already going (very nearly) the speed of light. At this point, the pressure doesn't build up as quickly with density, and "gravity wins".

So if the core of the star is massive enough, the force of gravity will overcome this electron degeneracy pressure, and you could be left with a neutron star or a black hole. In the case of the neutron star, the protons and neutrons (also spin 1/2) will get close together and create their own degeneracy pressure. If the density is so great that the protons and neutrons become relativistic, it will collapse into a black hole (well, that's the best theory anyway). The protons and neutrons need a much higher momentum to move near the speed of light, because their mass is about 2000 times greater than the electrons.

For even more complicated reasons, many of the protons and electrons in a newly-forming neutron star will interact to form neutrons and neutrinos. The neutrinos leave, cooling the core and helping out with the supernova. The neutrons stay behind of course, forming a large percentage (70%?? I forget) of the composition of the neutron star, hence the name.

I hope this helps! I guess the short answer is that special relativity spells doom for massive would-be white dwarfs (or would-be neutron stars).