‘O’ Stars

The hottest and brightest stars in the galaxy are also the rarest — they account for about one of every 10 million stars. Yet they’re so dazzling that several of them are visible to the unaided eye.

Astronomers classify these stars as class “O.” They’re at the top of a system that uses the letters O, B, A, F, G, K, and M.

An O star that’s in the prime of life can be anywhere from about 30 to more than a hundred times as massive as the Sun. Because of that great heft, the nuclear reactions that power the star take place at a furious rate. Energy from those reactions makes the surface of the star tens of thousands of degrees hotter than the Sun, which makes the star look blue-white.

The star also looks tens of thousands of times brighter than the Sun. Because of its temperature, though, most of its energy is in the ultraviolet. When you add that to the visible light, the brightest O stars can shine up to a million times brighter than the Sun.

The Milky Way’s population of O stars is probably about 20,000 — out of a total of hundreds of billions of stars. In part, that’s because the galaxy gives birth to far more small stars than big ones. But it’s also because an O star lasts only a few million years. When its nuclear fuel is gone, the star’s core collapses to form a black hole or a neutron star. The star’s outer layers may then explode as a supernova — briefly adding to the luster of these already lustrous stars.

More tomorrow.

Script by Damond Benningfield