If you put your face close to a hundred-watt light bulb, it looks pretty darned bright. But if you move back a few feet, it loses some of that luster. The bulb isn't any dimmer, it just looks that way because of the greater distance.
The same thing is true of the stars. Some of the ones that look faint are actually just as bright as some of the ones that look bright, they're just farther away.
A good example is two stars in Leo, the lion.
The star that looks brighter is Regulus, which is to the upper left of the Moon as darkness falls this evening.
A curving pattern of stars that looks like a backwards question mark stretches to the upper left of Regulus. The star at the end of the question mark's loop is Epsilon Leonis.
To the eye alone, it looks about one-fifth as bright as Regulus. But if you could place the two stars side by side, and add up all of their energy, Epsilon would be just as bright as Regulus. Epsilon is bigger, but Regulus is hotter, so a given surface area emits more energy.
Both stars are a few times as massive as the Sun, so the nuclear reactions in their cores take place at a much faster rate. Epsilon is the more massive of the two, so its reactions are faster. In fact, the star is closer to the end of its "normal" lifetime, even though it's tens of millions of years younger. It'll soon shed its outer layers, leaving only its hot, dense core -- a tiny stellar light bulb with the wattage turned way down.
Script by Damond Benningfield, Copyright 2011
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