How big would Jupiter look if it were a sun?

[Chronos]

What I was asking was a little more subtle than that: given unlimited resources (like say the early big bang conditions) would it be possible to compress a mass like Jupiter’s just enough to form a star but not a black hole and have it be stable by gravity alone, or would it explode/evaporate/disintegrate in a relatively short timespan?

If you somehow artificially induced fusion to start in the core of Jupiter (say, by compressing it in the grandmother of all vice grips), then as soon as you let go of the vice grips, the heat produced by the fusion would cause the body to expand out to a point where it again would not support fusion.

[Anne_Neville]

@Superfluous_Parentheses

What I was asking was a little more subtle than that: given unlimited resources (like say the early big bang conditions) would it be possible to compress a mass like Jupiter’s just enough to form a star but not a black hole and have it be stable by gravity alone, or would it explode/evaporate/disintegrate in a relatively short timespan?

As far as we know, yes, it would be stable once you got it to form.

[Polycarp]

So evaporation by Hawking radiation has been reasonably well disproven, then? (What little I know of it is all from popular articles by Jerry Pournelle, so I’d welcome non-technical explanation of why.)

[Anne_Neville]

@Polycarp

So evaporation by Hawking radiation has been reasonably well disproven, then? (What little I know of it is all from popular articles by Jerry Pournelle, so I’d welcome non-technical explanation of why.)

Well, stable for a while, at least. As far as we know, all black holes will eventually evaporate via Hawking radiation.

[Anne_Neville]

@Anne_Neville

Well, stable for a while, at least. As far as we know, all black holes will eventually evaporate via Hawking radiation.

From a quick calculation, stable for about 10[sup]57[/sup] years.

[Chronos]

Well, there are some contrived situations where you might get a black hole that doesn’t evaporate at all, or which evaporates only enough to asymptotically approach a stable state, but they’re not at all practical. When I said that a Jupiter black hole would be stable, I was neglecting Hawking radiation, since it would take considerably longer than the age of the Universe for it to even begin shrinking that way.

[Anne_Neville]

@Chronos

When I said that a Jupiter black hole would be stable, I was neglecting Hawking radiation, since it would take considerably longer than the age of the Universe for it to even begin shrinking that way.

It would only take 10[sup]57[/sup] years for it to evaporate via Hawking radiation. That’s 10[sup]47[/sup] times the present age of the Universe.

[Chronos]

I said longer than the age of the Universe for it to even begin shrinking. A black hole in the current Universe would actually be growing, even if it’s in the middle of one of the great voids between galaxy superclusters, because it’d gain more from the cosmic microwave background radiation than it’s losing to Hawking radiation. So to even get started, you first have to wait for the Universe to cool off by a factor of a few million or so.

[Icerigger]

Speaking of Red Dwarfs, is it true they will “burn” for a trillion years or about 1000 times longer than the sun?

[Anne_Neville]

@Icerigger

Speaking of Red Dwarfs, is it true they will “burn” for a trillion years or about 1000 times longer than the sun?

Yes, for at least some red dwarfs (I’m not sure for the most massive ones).

More massive stars burn their hydrogen faster than less massive stars, and don’t live as long. Red dwarfs also have more convection inside them than larger stars, so they can burn more of their available hydrogen.

Like all Main Sequence stars, red dwarfs get brighter and hotter as they age. They will eventually become blue dwarfs.

[Polycarp]

Thanks, Chronos and Anne, for the clarification on Hawking radiation. I’d formed a completely erroneous impression of the time scale involved. :o