What makes an ordinary star become a red giant?

Answer 1

Most stars will go into a red giant phase after their main sequence life is over. Once all the available hydrogen in the core is consumed, hydrogen fusion stops and the star continues the gravitational collapse that was interrupted when it started fusing hydrogen. This quickly builds up enough pressure to restart fusion, first in a shell of hydrogen around the collapsed helium core, and then in the helium core itself. These fusion phases generate more energy than the original hydrogen core fusion. This increased energy causes the outer, non-fusing portion of the star to expand, and the star blows up to many times its original size. Even though it is now producing far more energy than it did on the main sequence, it has so much surface area that its surface temperature is cooler than it was, causing the star to appear reddish.

The more massive stars that go on to become supernovae also pass through red giant phases, but they become supergiants (luminosity class I). The really massive ones burn so hot they become blue supergiants. Only red dwarfs below a certain mass threshold are exempt, and will go directly to white dwarfs when their hydrogen core fusion stops.

Answer 2

Stars have two basic forces working against each other. A nuclear fusion reaction in the core which pushes outward and gravity which pulls the star inward. For most of its life, while a star "burns" hydrogen as its fuel, the two forces are balanced and the star's size remains fixed. When the hydrogen fuel runs out, the nuclear reaction stops and so does the outward force. Gravity begins to collapse the star but as it collapses, its core temperature rises and a new fusion reaction begins at the core, this time "burning" helium. (the waste product of the original reaction). This new "burning" creates a new outward pressure that outdoes gravity, swelling the star up again - this time bigger than before because the new reaction pushes a little harder than the first. The star stabilizes at a new, larger size until all the helium is gone. This is what we call a red giant. It's red because its energy is spread out over a larger surface so it is cooler than it was before. For stars like our sun, after that its game over but very large stars can go through several cycles like this burning other products successively. Sometimes if the star is large enough, it can explode violently in a supernova or in extreme cases, gravity wins and the star collapses inward to become a black hole.

Answer 3

Stars with an initial mass of less than 8 solar masses become red giants when they run out of hydrogen in their core. (They've been busy turning this hydrogen into helium through nuclear fusion) The helium is heavier than the hydrogen, so it sinks to the star's core (there may still be hydrogen in the outer shell). The star collapses, the pressure and temperature rises, and the star expands into a red giant. Then the star begins fusing helium into carbon.

-edit-

A red giant will NEVER become a supernova. Red Giants end their lives as white dwarf's after having thrown off a planetary nebula.
Supernovas are formed by stars with an initial mass of greater than 8 solar masses. Some supernovas end their lives as neutron stars. Other supernovas (the most massive ones) end their lives as black holes.

Red Giants NEVER become supernovas, nor visa-versa. They are two different branches of stellar evolution. Which branch the star takes is driven by initial mass.

I hope this helps.

Answer 4

(PhD physicist responds) Well, the reason they're blue and giant is because they're so massive. Usually 3 times the mass of our own sun or more. And very hot! That's the reason they're blue. These are type O and type A stars (our sun is a G, and it goes OBAFGKMNS from brightest to dimmest) Chandrasekar has shown that blue giants burn up their fuel quickly (a few million years rather than a few billion years) and then impode upon themselves becoming a supernova. The remnant of a supernova is either a neutron star or a black hole. To see a blue giant, go out at night and look at the constellationOrion, now in the south sky. Orion's right knee is a blue giant.

Answer 5

Our sun is destined to become a red giant. The above answers miss the main reason for this expansion. The sun will reach a level of fusion that will be its last. Millions of tons of mass is lost every second during fusion, it is easy to see what this means for the total mass of the sun after millions of years of decreasing mass, the gravitational force is also diminished because of the loss of this mass. Radiation will no longer be able to be overcome by gravitational pressure and the sun will begin to expand and continue to do so until fusion finally ceases, by that time the sun will have expanded to include the planets, then gravity is able to begin the final collapse of the mass of gas and the sun will become a white dwarf, it will remain hot for a few million years years and then become nothing more than a cinder.

Answer 6

When the star begins running out of hydrogen and begins to fuse helium. Eventually it will fuse other elements and stop at iron. It will then become a supernova and fuse all the rest of the elements.

Answer 7

The gases are running out causing the star to expand and prepare for death.

Answer 8

http://www.historyoftheuniverse.com/starold.html