First off, the Sun is only half way through its life and won't explode for 5 BILLION years! So there is nothing to worry about yet. When the Sun does come to the end of its life, it is believed that the Sun's outer layers will puff up waaaaayy out into the solar system, beyond Earth and Mars and then collapse in on itself to become a white dwarf. Like I said, this won't happen for a very very long time yet. The reason we can figure this out is by studying how other stars evolve. This is usually described using a "Hertzsprung Russell" diagram (http://en.wikipedia.org/wiki/File:HRDiagram.png). This is a bit like taking a snapshot of every human being on the planet at one single time. If we measure everyones age and height and weight all at once, we can get a good idea of how many people there are, how old they are, how long they live, what changes happen etc. Since we can't study a star from beginning to end, we can do the same thing with all the stars in the universe: study them all for a little while and get an idea of what characteristics different stars have.