by stormculture » Wed Jan 06, 2010 9:18 pm
What holds Betelgeuse's thin outer layers "in orbit" is the same as how Earth's thin outer atmosphere stays in place despite solar wind - gravity. Yes, Betelgeuse is a star with a solar wind that is pushing against the outer layers - but that's why Betelgeuse is so large in diameter. Assuming it is at equilibrium, the "surface" of Betelgeuse is the point at which those thin wisps of hot plasma experience equal forces from gravity pulling them back to Betelgeuse's core and the solar (particle) wind, pushing them out.
That said, Betelgeuse could very well NOT be at equilibrium - it's at the end of it's life and would be experiencing the "bounces" that occur as a star runs out of the easier fuels (Hydrogen, then Helium, and so on) and has to start fusing larger atoms. Each time there is a shift to a larger atom, there is a "bounce" that causes a sudden increase in the pressure against the outer shell of material, but only after a period of contraction. This is why you see multiple, distinct "shells" in planetary nebulae from dying or dead stars.
The actual process, put more clearly, is this - lets say we're talking about our Sun, just for a concrete example:
- Sun starts running out of Hydrogen available to fuse
- total real-time fusion decreases, so the force pushing out on the outer layers (and total radiation) decreases
- Sun starts to collapse - ie its apparent radius diminishes much faster than the normal loss due to radiation/solar wind, and it's density increases
- density in the core increases, which increases the temperature
- at a particular point, the temperature and pressure become great enough to start fusing Helium into Beryllium. This suddenly increases the amount of fusion (and thereby also increases the "radiation" the core is producing.
- the outer shells that had been collapsing on the core are suddenly met with new expanding radiation and particles from the new type of fusion taking place, and so are "blown" back. This is violent - and gets more violent each time it happens - each time the star moves up to heavier fusion. The outer shell now starts expanding again and "bounces" out to an equilibrium point that is actually further away from the core than it had been. Some material is also completely ejected into space at very high speed - this is what we see in a planetary nebula.
This cycle repeats until all that's left in the star is Iron atoms, or the density becomes so great that atomic nuclei can no longer stand the gravitation pressure on them, and they, too, collapse - which is the process that creates neutron stars, black holes, and supernovas.
So the core of dying stars, by necessity, increases in density and temperature continually (until the star "dies" - becomes merely a white dwarf, neutron star, or black hole), but the outer shell and the "apparent diameter" of the star increases with each "bounce", until death.
What holds Betelgeuse's thin outer layers "in orbit" is the same as how Earth's thin outer atmosphere stays in place despite solar wind - gravity. Yes, Betelgeuse is a star with a solar wind that is pushing against the outer layers - but that's why Betelgeuse is so large in diameter. Assuming it is at equilibrium, the "surface" of Betelgeuse is the point at which those thin wisps of hot plasma experience equal forces from gravity pulling them back to Betelgeuse's core and the solar (particle) wind, pushing them out.
That said, Betelgeuse could very well NOT be at equilibrium - it's at the end of it's life and would be experiencing the "bounces" that occur as a star runs out of the easier fuels (Hydrogen, then Helium, and so on) and has to start fusing larger atoms. Each time there is a shift to a larger atom, there is a "bounce" that causes a sudden increase in the pressure against the outer shell of material, but only after a period of contraction. This is why you see multiple, distinct "shells" in planetary nebulae from dying or dead stars.
The actual process, put more clearly, is this - lets say we're talking about our Sun, just for a concrete example:
- Sun starts running out of Hydrogen available to fuse
- total real-time fusion decreases, so the force pushing out on the outer layers (and total radiation) decreases
- Sun starts to collapse - ie its apparent radius diminishes much faster than the normal loss due to radiation/solar wind, and it's density increases
- density in the core increases, which increases the temperature
- at a particular point, the temperature and pressure become great enough to start fusing Helium into Beryllium. This suddenly increases the amount of fusion (and thereby also increases the "radiation" the core is producing.
- the outer shells that had been collapsing on the core are suddenly met with new expanding radiation and particles from the new type of fusion taking place, and so are "blown" back. This is violent - and gets more violent each time it happens - each time the star moves up to heavier fusion. The outer shell now starts expanding again and "bounces" out to an equilibrium point that is actually further away from the core than it had been. Some material is also completely ejected into space at very high speed - this is what we see in a planetary nebula.
This cycle repeats until all that's left in the star is Iron atoms, or the density becomes so great that atomic nuclei can no longer stand the gravitation pressure on them, and they, too, collapse - which is the process that creates neutron stars, black holes, and supernovas.
So the core of dying stars, by necessity, increases in density and temperature continually (until the star "dies" - becomes merely a white dwarf, neutron star, or black hole), but the outer shell and the "apparent diameter" of the star increases with each "bounce", until death.