The main sequence was a single band running from the upper left to the lower right. What decides where along that band a star lands is the mass it gathered when it was born.
The heavier the star, the harder its own weight presses on the center. The harder the press, the higher the temperature, and the fiercer the fusion. So a heavy star is hot and bright, at the upper left of the band.
A light star is the reverse. The temperature at its center is barely enough, so the fusion is slow and the star is faint and red, at the lower right of the band.
A position on the main sequence is set by mass alone. That is why the band comes out as a single line rather than a scatter.
Measure mass against luminosity and the stars of the main sequence line up neatly. The luminosity is proportional to about the power of the mass.
A power is steep. Double the mass and the luminosity goes up times; raise it tenfold and the luminosity goes up about times.
Drawn on a scale where both axes are powers of 10, the relation becomes a straight line. The slope is , and the stars of the main sequence all sit near that line.
A small difference in mass makes an enormous difference in brightness. That fact settles the question of lifetime that comes next.
The fuel of a star is the hydrogen at its center, and the amount is roughly proportional to the mass. A heavy star holds a great deal of fuel.
The rate of spending is the luminosity. What is made at the center leaves through the surface, so the brighter the star, the faster it is spending.
The lifetime is the fuel divided by the rate of spending. An amount proportional to divided by a rate proportional to makes the lifetime proportional to about .
A heavy star holds far more fuel and still runs out far sooner. The rate of spending climbs much more steeply than the amount held.
The sun spends about billion years on the main sequence. Its present age is billion years, so it is not yet halfway.
A star of times the mass lasts only about million years, less than a three-hundredth as long.
A star of times the mass lasts more than a trillion years. Only billion years have passed since the universe began, so not one such star has finished yet.
Stars sitting on the same band stay there for wildly different spans. The brighter the star, the shorter the window in which it can be seen at all.
A star cluster is a group of stars born at about the same time. Their masses vary, but their age is shared.
Plot the stars of a cluster on the HR diagram and the main sequence breaks off partway. The heavy stars leave first, so the band is eaten away from the upper left.
The point where it breaks off is called the turnoff. The lifetime of a star sitting at the turnoff is the age of the cluster.
A young cluster keeps its band up to the upper left, and an old one is eaten far down to the lower right. Read the position of the turnoff and the birth date of the group follows.