Dark stars

Dark stars, ordinary stars that have expended their fuel and have cooled off to the point where they don’t emit significant visible light, show up from time to time in science fiction. There was one in Doc Smith’s Skylark of Space and another in Campbell’s The Black Star Passes. In Fritz Lieber’s “A Pail of Air”, a dark star has captured the Earth and taken it away from the sun where it’s so cold that the atmosphere has frozen. There’s even a dark star in Bujold’s Vorkosigan series. There’s probably been others, but those are the only ones I can think of at the moment.

My question is: Is it possible for a star to be that cold in the universe today? Not brown dwarfs or black holes, but white dwarfs or neutron stars. Has there been enough time since stars first formed for either of these types of objects to have cooled off that much?

If all nuclear reactions has ceased, would it still be a star? Wouldn’t it just be a big planet/oid/rock/body?

Good question, never thought of it before. Wikipedia suggests an inert core of iron and oxygen will be left behind after the outer shells of other matter are whipped away. Iron can’t take part in further nuclear reactions so it just sits there in a big lump.

Wouldn’t that be rust?

No, it’d be an iron core surrounded by shells of fusing atoms. If you’ve got a star that’s massive enough to have an iron core near the end of its lifetime (one that’s at least 8 times the mass of our Sun), you end up with a star with a striated structure; it has an inert iron core, surrounded by shells that are fusing (or did fuse) silicon, oxygen, neon, carbon, helium and hydrogen (in that order from the core out). By the time you’ve got a large iron core, things are going to explode rather quickly, in the form of a supernova, blasting away all the outer shells of material, and forming elements heavier than iron.

I think we don’t know if there are black dwarfs yet, (that’s the term I remember for the idea of a white dwarf which has cooled down and stopped shining,) because they’d be very hard to see if they were out there. Even white dwarfs are hard to spot from direct observation.

And I think they’d be considered still stars because they’re the remains of main sequence stars and they’re too big to be planets or asteroids, even if they’re not producing energy or shining. The matter in a black dwarf would be so dense and closely packed as to not resemble any kind of matter we’re directly familiar with. The usual cliche is that a teaspoon of white dwarf matter, if it could be brought to the earth, would weigh at least a ton. (That’s usually expressed as being matter from the core of a dwarf star, but I think that the surface wouldn’t be much less dense comparatively.)

Neutron stars are a different question, since they’re an order of magnitude or two denser than dwarf stars, at least, and I’m not sure if they ever really ‘shine.’ Some spin and emit narrow beams of radiation, (pulsars,) but I have the notion that most neutron stars are pretty dim. Can’t give a cite on that though.

So could a space faring civilisation conceivably use these star remnants as mines of some sort?

I suppose so, although mining stuff at several thousand G would be a bit of a challenge. Easier to just strip down asteroids, probably.
Unless you specifically wanted superdense matter, but then you’d need gravity control to maintain pressure and stop it turning into normal matter. And if you had gravity control, you could use that to squish normal matter and wouldn’t need to bother mining anything…
Incidentally, if you wan’t a rip-snorting adventure featuring a civilization of little amoeboid critters living at 67 billion gravities, have a read of starquake

For a star to be “dark” I guess we want it to cool down to the point where it’s emitting very little visible light.

From an introduction to stars. Not exactly the ApJ, but other sources seem to agree.

So I think we might have to wait around for a few more billion years if we want to see a black dwarf.

There are actually two different types of black dwarf. A black dwarf can be the remnant of a white dwarf, or of a red dwarf. We’re quite certain that there are no formerly-red dwarfs in the Universe, since red dwarfs (being very frugal) are expected to last for hundreds of billions or even trillions of years. Eventually, though, they will run out of fuel, and cool into black dwarfs composed mostly of hydrogen and helium (reds aren’t hot enough to fuse past helium).

A formerly-white black dwarf is at least plausible, though I’ve been out of the astrophysical loop for a few years, so I’ll take Podkayne’s word that there hasn’t been quite enough time yet for them, either. A formerly-white black dwarf (like a currently-white dwarf) would be composed largely of iron, but would have layers of all of the lighter elements, since no star can ever completely fuse all of its fuel.

Neutron stars glow by the same mechanism as do white dwarfs: They’re born hot, and it takes them a while to cool down. Eventually, neutron stars will also become cold and dark. However, since a neutron star is both smaller and more massive than a white dwarf, and since they’re born hotter, it would take a neutron star even longer to cool down than it would a white dwarf (the oldest neutron stars are also older than the oldest white dwarfs, but I don’t think this is enough to make up the difference). So there are even less likely to be any cooled neutron stars in the Universe.

Hmmm… now this is an interesting factoid I hadn’t come across before. Our sun, pretty much a yellow dwarf, is expected to expand into a red giant when it runs out of hydrogen fuel, and then collapse into a white dwarf. I always kind of expected that even a tiny little red dwarf would do this.

Do we have any idea where the dividing line is, either in terms of luminosity, main-sequence spectrum, or stellar mass? I remember hearing that there are stellar mass dividing lines for whether a star will become a white dwarf, a neutron star, or a black hole when it collapses… is there a similar line for determining if a star will ever expand into a red dwarf or just quietly fade away to darkness??

And apologies if I’m not phrasing the question in the correct terms. :slight_smile:

I guess I’ll have to answer my own question. Or at least do some googling to find the answer. On this page, I found

The surface temp of the sun is about 5800 K, so these objects should still be radiating fairly brightly in the visible. They have low luminosity (the L above) because they are much smaller than the sun. But if you get close enough to them, they won’t be dark.

Too massive to be planets, you mean. In terms of size, they are about the size of the Earth or smaller.

Well, they start off with temperatures greater than white dwarfs, so initially they should be very bright, although no doubt more in the UV than in visible. I couldn’t find a specific number on the temp of the oldest neutron stars, though.

Just want to note that I started that last post before Podkayne’s post and didn’t preview.