How stars age

When a gas cloud forms stars, it forms many at the same time, with a wide variety of masses. The more massive ones are more luminous and have hotter surface temperatures, so they appear at the upper left of this diagram, called the Hertzsprung-Russell diagram or simply HR diagram. The diagonal line of stars seen here is called the main sequence, with the least massive stars at the lower right end. All stars begin their lives on this sequence, fusing hydrogen into helium.

The more massive stars are so much more luminous that they use up their hydrogen more quickly despite starting with a larger store. (Note: luminosity is plotted on a magnitude scale here, so the uppermost points here are nearly a million solar luminosities.) Thus, stars leave the main sequence in an orderly fashion: most massive first, and then gradually less and less massive stars. At any given time, them, the "main sequence turn-off" (the kink where stars are just leaving the main sequence) is a key indicator of the age of that cluster of stars. Move the slider to the right and see for yourself!

Food for thought:

Note how the age in years changes as you move the slider. Why do we devote the first half the slider to the first 200 million years and then squeeze the subsequent 11.8 billion years into the last half of the slider? [The notation "3.9e+06" means 3.9 times 10⁶ or 3.9 million years; "3.9e+07" would mean 39 million years; "3.9e+08", 390 million years; and so on.]
For each star, we show its first stage of evolution after leaving the main sequence. What happens to them after that? Which ones explode in a supernova and which ones become white dwarfs? Where would white dwarfs appear on this plot?
How luminous are the least luminous stars on this plot? The clues you need to answer this question are in the text above.