Until this century, almost all astronomy involved the study of the planets, moons, comets, the Sun, and stars. It's easy to see why. When you look at the sky at night, all you see is a few planets, and occasionally some wondrous intruder, such as comet Hale-Bopp that passed our way in Spring 1997 -- and stars.

In Boulder, where the city lights make the sky bright at night, you can see a few dozen of the bright stars that define the constellations. But at a dark place on a moonless night, you can see a few thousand stars. Now, with telescopes like Hubble, we can see literally billions of stars. We now know that many of these stars are rather similar to the Sun. But some are blue giant stars hundreds of thousands times more luminous than the Sun; some are equally luminous red giant stars, hundreds of times bigger than the Sun. The sky is also filled with stars too faint to see with the naked eye. Some are red dwarf stars, not much bigger than Jupiter and hundreds of times less luminous than the Sun; and some are white dwarf stars, as massive as the Sun but no bigger than the Earth.

Moreover, we know how these stars work. Beginning early in this century with the pioneering work of Lord Kelvin, Sir James Jeans, and Sir Arthur S. Eddington, and continuing after World War II with the work of nuclear physicists such as George Gamow and Hans Bethe, astrophysicists have developed a theory that explains how normal stars burn, why they are as bright as observed, how long they will live, and how they die. For all intents and purposes, normal stars are a done deal. The basic questions are answered. Nowadays, most astronomers have moved on to other subjects. But, as you will see, there are plenty of things left to learn about stars: there are many abnormal stars that we do not understand, and there are still some big unanswered questions, especially about the brightest and the faintest stars.

Like much of Western Civilization, it began in Greece, some 2100 years ago. Once upon a time, on the beautiful island of Rhodes, there lived a man named Hipparcos ...