[4CJ]

And how massive do you think the sun is?

1.99e30 kg.

How much would you expect to remain?

A lot less. Why do stars collapse? A .03% change in mass shouldn't cause it should it?

[Doctor Stochastic]

It isn't just the change in mass. As the hydrogen burns to helium, the star no longer produces enought energy to counteract its self-gravitation. Check with an astrophysics book or one of the better encyclopaedias (Britannica is good.)

[Physicist]

1.99e30 kg.

Very good. Now do you see why your reply #10 was wrong?

A lot less. Why do stars collapse? A .03% change in mass shouldn't cause it should it?

Why does a hot air balloon collapse, when you turn the burner off?

[ThinkPlease]

Why do stars collapse?

It's all about pressure balance. Pressure outwards is from radiative pressure, photons actually pushing on the matter, pressing inward is our old friend gravity. In nearly all stars, the pressure balance is enough to prevent collapse.

Most stars never collapse, the only ones that do are stars that go supernovae. As was said earlier, that only happens if the the star doesn't output enough energy to counteract the inward pressure of gravity. This occurs when the star runs into a fusion process that doesn't create energy, but actually takes energy to react. That process is the process of fusing two iron atoms together, and it only occurs at temperatures of several hundred million degrees celsius. This only happens in very high mass stars, like most O type stars.