Science Buddies: "Ask an Expert"

Thanks for all the experts,especially Jhon Dreher,Dharman Kothari,WW.I have selected to next level in tana global science fair which was world wide science fair.watch this website http://www.tanaglobalsciencefair.com/ My project ID was TP1033 and project title was The Suns Death And End To Mercury,venus And Earth.In this level I have to face an interview.I had doubt about my project is "How many years did the sun remains in red giant state?" Please answer my question.And once again thanks for all experts and this forums.

Congratulations!
It's great to hear that you have moved ahead in the competition. As for your question, one article states that the red giant phase will probably last 100 million years: http://www.universetoday.com/18847/life-of-the-sun/. Other predictions say it will last 1 billion years.
Good luck on your interview, I hope you continue to do well in this global science fair!

munnyastrophia123,

Congratulations and good luck on the next round of competition.

WW

My congratulations too. We enjoy helping where we can.

As to red giants. Stars of different masses have very different lifespans. As a rule of thumb, the lifetime of a star varies as its mass to the -3.5 power. What happens when a star uses up the hydrogen in its core (having converted all of it to helium) can be very complex. These complex behaviors depend sensitively on the initial mass of the star and the initial composition of the star. The first two steps are fairly simple. In a few billion years from now, the sun will grow in size a little while staying about the same in surface temperature — as a result, its luminosity will go up by, very roughly, a factor of three. This will not swallow up the earth (maybe Mercury, I’m not sure off hand), but it will boil off the oceans and, I guess, blow off the atmosphere. So we should probably plan on leaving by then!! Next, fairly rapidly, the sun will swell up and become a red giant. This will swallow up the inner planets. The red giant phase will last about a billion years (check this out on, say, Wikipedia - my memory is not so reliable as it once was). What happens next is too complicated to bother with if you are not an astronomer with interests in stellar evolution. Basically the dying star thrashes about from being very hot and luminous to being very cool and super luminous; the details are still being worked on by specialists in modeling stellar structure. There are three end states: 1) a white dwarf, 2) a neutron star, and 3) a black hole. The sun will end up as a white dwarf, a star that is no longer producing energy from fusion and is held up against gravity by electron degeneracy pressure. I can’t think of a way to explain this pressure in simple, everyday terms. To get to this final state the sun will shed the tenuous outer layers of its red giant incarnation either gently as a stellar wind or explosively (forming a planetary nebula). Since a white dwarf is no longer generating energy by fusion, it will very slowly cool off, eventually ending as a nearly invisible black dwarf.