Most of the stars that populate the galaxy are in the prime of life. It’s a phase that can last anywhere from a few million years to a trillion years or longer.

In astronomical jargon, that phase of life is known as the main sequence. And a prime example of a star that’s on the main sequence is Altair, the leading light of Aquila, the eagle. The star is high in the southeast at nightfall, at one corner of the bright Summer Triangle.

The main sequence is named for a relationship between a star’s brightness and its surface temperature or color. More than a century ago, two astronomers noticed that if you plotted those two characteristics on a graph, most stars lie along a narrow line — the main sequence. And it’s all because of what’s going on inside those stars.

Altair is bigger and brighter than the Sun, for example, and its surface is thousands of degrees hotter. Yet both are on the main sequence because they’re both “fusing” the hydrogen fuel in their cores to make helium. That keeps both of them shining steadily.

A star’s time on the main sequence depends on its mass. The Sun, for example, will spend about 10 billion years in that phase. But Altair is almost twice the Sun’s mass, so it “burns” through the hydrogen much more quickly. As a result, it’ll spend just a quarter as much time on the main sequence. After that, it’ll puff up and enter the next phase of life: a red giant.

We’ll have more about Altair tomorrow.

Script by Damond Benningfield