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View Full Version : How long does it take for a star to become a red giant?

AstroSmurf
2004-Feb-12, 02:51 PM
Suppose you have a star at the end of its main-sequence lifetime, and all or most of the fusile(?) hydrogen has turned to helium. Now, from what I've read in my astrophysics textbook, what will happen is that the star will grow smaller and brighter, since the radiation pressure no longer balances gravity. Eventually, the core will have compressed enough to initiate helium fusion, and the star will expand to a red giant.

What I'm wondering is how long this process takes. The starting point is a little vague - my impression is that a star is more or less constant in temperature and luminosity up to a certain point in its lifetime, so call that the start. How long would it be between this point and the start of helium fusion, and from helium fusion until the star stops expanding?

Also, how does the luminosity vary during this process? I have some sort of idea, but it would be useful if someone has a pointer to a diagram over time of some kind.

The situation came up during an RPG session, believe it or not... I didn't know, and couldn't find the answer, so I had to fudge it. :oops:

Normandy6644
2004-Feb-12, 05:31 PM
Suppose you have a star at the end of its main-sequence lifetime, and all or most of the fusile(?) hydrogen has turned to helium. Now, from what I've read in my astrophysics textbook, what will happen is that the star will grow smaller and brighter, since the radiation pressure no longer balances gravity.Eventually, the core will have compressed enough to initiate helium fusion, and the star will expand to a red giant.

When the gravitational force compresses the star, it begins to fuse the heavier elements (as you said). When this happens, more outward radiation pressure overwhelms the gravitational force, and the star expands.

What I'm wondering is how long this process takes. The starting point is a little vague - my impression is that a star is more or less constant in temperature and luminosity up to a certain point in its lifetime, so call that the start. How long would it be between this point and the start of helium fusion, and from helium fusion until the star stops expanding?

I would guess several billion years, since the sun is due to become a red giant in a few more billion years. I'm not exactly sure it takes for it to stop expanding. I guess it depends on the mass of the star and how much (and how quickly) it will fuse helium.

Also, how does the luminosity vary during this process? I have some sort of idea, but it would be useful if someone has a pointer to a diagram over time of some kind.

Luminosity is a function of temperature, which is a function of mass. Red giants tend to be cooler, yet more luminous than comparative main sequence stars, due to their higher mass. This site (http://home.cwru.edu/~sjr16/advanced/stars_avgdeath.html) seems to answer some of your questions. Hope this helps!

AstroSmurf
2004-Feb-13, 08:38 AM
When the gravitational force compresses the star, it begins to fuse the heavier elements (as you said). When this happens, more outward radiation pressure overwhelms the gravitational force, and the star expands.
(...)
I would guess several billion years, since the sun is due to become a red giant in a few more billion years. I'm not exactly sure it takes for it to stop expanding. I guess it depends on the mass of the star and how much (and how quickly) it will fuse helium.
Yes, but what I was talking about is the time starting from the point where the core stops fusing hydrogen until the surrounding layer starts up. The site said:

Gravity will contract the star, and eventually a shell of hydrogen around the helium core will become hot enough to fuse H -> He.
It was this "eventually" that I wished to get more specific. It would also be interesting to know if the photosphere radius decreases during this part of the process, or if this entire thing affects only the core. But this is a quibble - my main interest was the time scales.

I've gotten an idea about how the luminosity varies - a slow increase throughout the process, ending with ~100 times the earlier luminosity (ouch, that can't be good for any surrounding biospheres).

Red giants tend to be cooler, yet more luminous than comparative main sequence stars, due to their higher mass.
Er, no - since we're talking about the same star, the mass will remain constant, right :) You're quite right as long as we're looking at a population of main-sequence stars, but in this case, we're looking at a specific star as it transforms into a giant-stage star. So the equations necessarily become different ones - same mass, higher luminosity.

It seems counter-intuitive until you start thinking about it - a star that is dying should become fainter, not brighter. However, at 100 times luminosity, it will consume resources much faster than before. (And I'll finally get to describe a dying world lit by a monstrously huge, red, sun...)

dgruss23
2004-Feb-13, 12:52 PM
James Kaler gives some numbers on this his textbook "Astronomy!". For about a billion years after the end of hydrogen fusion, the Sun's luminosity is expected to remain fairly constant - even though its outer shells will be expanding. Then once the nuclear reactions in the expanding shells begin to intensify, the Sun's luminosity will increase and it will take about a billion years for it to climb the red giant branch. While its expanding into a red giant, the core is shrinking.

AstroSmurf
2004-Feb-13, 01:22 PM
Thanks dgruss23, that was exactly the kind of answer I was looking for. I knew someone here would have it =D>
I figured it would go faster, on the order of a few million years until helium fusion starts, and a few hundred million years climbing the red giant branch. Shows what I know :P