Page 30 - Curriculum Visions Dynamic Book

Novas
As a pair of stars (binary stars) end their lives, the larger one may begin to swell up and leave the main sequence, while the smaller one remains part of the main sequence. When this happens, the gases in the envelope of the larger star spread out and are attracted by the gravitational field of the smaller star. As a result, streams of gas flow from the larger to the smaller star until the once smaller star becomes the larger of the two. This makes the growing star leave the main sequence as well and become a giant star.
That causes an enormous release of light, and a faint star may be visible in the sky for the first time. That is why it is called a nova, from the Latin meaning “new.” The situation then reverses, with material from the giant star flowing back to the smaller star, and so the brightness rapidly decreases. Notice that the stars return to their former brightness. In sUpernovas (described on pages 31–33) this results in a huge explosion.
The death of massive stars
The sequence described above is what happens to a small star. During its final life stages a massive star (a Population I star in one of the arms of a spiral galaxy) can become so hot that it triggers nuclear reactions in the outer shell, and then the resulting death throws are very different.
The last stages of a massive star begin as a red giant. But in a massive star more reactions, first in the core and then in the outer shell, can produce even heavier elements, such as silicon, sulfur, nitrogen, and calcium. Iron will eventually be formed in the core.
At this stage the process stops because an iron atom cannot be forced to release nuclear energy by adding more protons or neUtrons. This is the planetary nebula phase (see pages 26–28).
 This is the nebula NGC 6751 in the constellation Aquila. You can see the hot star in its center. It was responsible for ejecting the clouds of gas now seen encircling it.
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