PDA

View Full Version : when stars get really old and become cold, are they just planets?



WaxRubiks
2006-Mar-26, 01:49 PM
Surely it is the radiation that is given off by a body which makes it any kind of a star, once that has gone is there anything to distinguish it from a planet?

antoniseb
2006-Mar-26, 02:14 PM
So far we have used terms like neutron star, or white dwarf, or stellar mass black hole, or (more broadly) stellar cinder to describe old stars. Since they weigh about as much as a star (thousands of times more than planets), it wouldn't be consistant with our current use of the language to call them planets.

DuaneW
2006-Mar-29, 12:02 AM
I read an SF story a few years back (it was by either Connie Willis or Nancy Kress) about a group of miners who worked on the surface of a black dwarf star and mined it for minerals. In the story, the surface was covered with a thick, heavy dust and the temp of the surface was around 100C (boiling point of hydrogen dioxide ;) )

My question is.... Could a cold, dead star have a solid surface that you could stand on? (Not that you'd want to, of course, unless you don't mind your leg bones fusing together at the knees).

Ilya
2006-Mar-29, 02:33 AM
My question is.... Could a cold, dead star have a solid surface that you could stand on? (Not that you'd want to, of course, unless you don't mind your leg bones fusing together at the knees).
All existing white dwarves are still mostly hydrogen, and won't have solid surfaces until some 10^20 years from now, when they are cold enough for hydrogen to freeze. After a tiny fraction of that time -- mere trillion years or so, -- so much hydrogen in the universe will have converted into heavier elements, the universe will be full of stars with composition unheard of today: more carbon, silicon and oxygen than hydrogen. When these latecomer stars burn out and turn into white dwarves, they will solidify much faster, into essentially gigantic Earth-composition "planets".

Finally, there are in fact dead stars with a solid surface right now -- but you definitely do not want to stand on them! All neutron stars are covered with a layer of dwarf-star matter several tens of meters thick and on top of that a layer of ordinary matter a few centimeters thick. Oldest neutron stars already cooled off enough for that layer to solidify (it starts out as plasma, of course). So very old neutron stars are covered by very ordinary solid iron; I suspect the entire crust is a single iron crystal, that being the lowest possible energy state.

Disinfo Agent
2006-Mar-29, 03:02 PM
Surely it is the radiation that is given off by a body which makes it any kind of a star, once that has gone is there anything to distinguish it from a planet?There is the density. Brown dwarfs are much denser than any planet.

Cougar
2006-Mar-30, 04:26 AM
All existing white dwarves are still mostly hydrogen...
You're kidding! I thought white dwarfs were remains of stars like our sun after they converted all their hydrogen to helium and then helium to heavier elements, etc. Yes, after the core hydrogen is gone, there is still hydrogen burning in outer shells, but doesn't most of that expand into space from the growing stellar winds and eventual red giant phase?

In fact, a little googling clarifies that a white dwarf (http://en.wikipedia.org/wiki/White_dwarf) consists mostly of carbon and oxygen. Still, NOT a "planet". NOT a place to take a walk.

Tobin Dax
2006-Mar-30, 04:50 AM
You're kidding! I thought white dwarfs were remains of stars like our sun after they converted all their hydrogen to helium and then helium to heavier elements, etc. Yes, after the core hydrogen is gone, there is still hydrogen burning in outer shells, but doesn't most of that expand into space from the growing stellar winds and eventual red giant phase?

In fact, a little googling clarifies that a white dwarf (http://en.wikipedia.org/wiki/White_dwarf) consists mostly of carbon and oxygen. Still, NOT a "planet". NOT a place to take a walk.

They should mention ONeMg white dwarfs in that article, since we've seen a number of dwarf novae where the white dwarf is made of oxygen and neon. There are also helium white dwarfs, but he's right, Ilya. Any hydrogen on a white dwarf (if it's there at all) will be in a very thin layer on the surface, or a slightly less thin layer of accreted hydrogen from a binary companion.

Ilya
2006-Mar-30, 03:00 PM
Any hydrogen on a white dwarf (if it's there at all) will be in a very thin layer on the surface, or a slightly less thin layer of accreted hydrogen from a binary companion.
Yes, you are right. But that "thin layer" is still many times the height of a man, and at the depth where it gives way to carbon-oxygen, it is probably liquid. Still could not stand there. Swim, maybe? :)

Tobin Dax
2006-Mar-31, 04:48 AM
Yes, you are right. But that "thin layer" is still many times the height of a man, and at the depth where it gives way to carbon-oxygen, it is probably liquid. Still could not stand there. Swim, maybe? :)

True, but that's not what I'm arguing. I was responding to the "mostly hydrogen" comment.