The Sun shines by converting hydrogen in its core into helium, and in the process loses
that is converted to energy via Einstein's E=mc2. It has been doing this for about 4.5
billion years, and is expected to do so for 5 billion more years. After that time, its
hydrogen fuel will be depleted. With its internal energy source shut down, gravity will
cause the core to collapse. That collapse will generate enough heat to expand its outer
layers, turning our Sun into a red giant that will expand beyond the Earth's orbit. (So
don't make any plans for the year 5,000,002,000.)
The collapsed core will become a white dwarf, composed of degenerate matter supported by
the inability of two electrons to occupy the same space. A star more massive than our Sun
eventually becomes a neutron star via a similar process. The most massive stars collapse
to form black holes.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor
'The strength and weakness of physicists is that we believe in what we can measure. And if we can't measure it, then we say it probably doesn't exist. And that closes us off to an enormous amount of phenomena that we may not be able to measure because they only happened once. For example, the Big Bang. ... That's one reason why they scoffed at higher dimensions for so many years. Now we realize that there's no alternative... '