Why Do Stars Change Throughout Their Lives?
Gravity is the key to understanding the changes a star undergoes during its lifetime.
Gravity causes the protostar to collapse out of the surrounding interstellar gas and dust cloud. Just as with the egg on the left, the internal pressure is not strong enough to halt the collapse. Click on the panel to see what happens next.
When the nuclear reactions begin in the core of the star, converting hydrogen to helium with the release of energy, the internal pressure is increased. Like the bowling ball on the left, high internal pressure halts the gravitational collapse. The star enters the main sequence and is stable for a large part of its life. Click on the bowling ball to see what happens.
During the remainder of time, the star periodically shrinks under the influence of gravity, expands as the internal pressure due to some process takes over, or temporarily pauses in this sequence as the internal pressure and gravity balance.
The end, however, is inevitable. Gravity always wins.
What's left at the end depends on how massive the star was to begin with. So let's look in more detail at the lives of low- and high-mass stars, separately.