Page 24 - Curriculum Visions Dynamic Book

Main-sequence stars
The longest part of the life of a star is when it is mature. It has formed enough gravity for nuclear reactions
to be well under way, but it still has enough hydrogen fuel left to keep it burning steadily. This long period of stability is referred to as the main seqUence. It is when the star shines steadily, and when it transforms hydrogen to helium in its core.
About nine out of ten stars are in the main sequence, and every one of them is a dWarf star. A star is only classified as a larger size as it swells toward the end of its life.
This means that most of the mass of a galaxy is in main-sequence stars. In fact, most stars are small mass stars. But since they are not as bright as large stars, they are not as conspicuous. As a result, more starlight is produced by the larger stars than you would think by simply comparing star mass.
Small stars do become brighter as they age. The Sun is becoming brighter in this way. Even later, near the end of their lives, stars may become cooler and redder.
Old age for a star
A star is a mass of swirling, burning gas. Gas is easily compressed. The great mass of a star ensures that it has enormous gravity. Gravity therefore tends to collapse the star in on itself all the time. But for much of its life the star burns so fiercely that the gases are constantly expanding. A balance is set up between expansion due to burning and the squashing effect of gravity. This lasts all of the time the star is in the main sequence.
The star begins to die when there isn’t enough fuel in the form of hydrogen to maintain the nuclear reactions needed to keep the gases burning and expanding.
Stars, however, do not all end the same way. Just as at birth, what happens at death depends on size.
For more on our Sun see Volume 2: Sun and Solar System.
dWarf star A star that shines with a brightness that is average or below.
gravity The force of attraction between bodies.
main seqUence The 90% of stars in the Universe that represent the mature phase of stars with small or medium mass.
mass The amount of matter in an object.
stellar Wind The flow of tiny charged particles (called plasma) outward from a star.
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