Appearances are Deceiving

Most stars look alike. There are exceptions, of course. Betelgeuse ("beetle-juice"), glowing red in Orion, stands out even among the brilliant stars of winter. But, for the most part, stars look like white dots, some dazzling, some so dim that we can barely see them. Besides their relative brightness, there seems to be little to differentiate them.

Stars are just big fires -- thermonuclear fires. There are two things that could make one fire appear brighter than another. The fires might be the same size, but one is closer than the other. Or, even if they were at the same distance, one might be a bigger fire than the other. The relative brightness of the stars is caused by their distance from us, their intrinsic brightness (how bright they are in reality), or some combination of the two.

Two stars blazing in the sky tonight dramatically show this difference. Sirius, the brightest star in the nighttime sky, and Rigel, a brilliant white star blazing to its right, seem almost equally bright. But Rigel is over a hundred times further away from us than is Sirius. To shine so brightly at such a tremendous distance, Rigel must be a really giant star. It is 50 times as big as the sun, 60,000 times as bright, and one of the brightest stars in the galaxy. (By comparison, Sirius is only about twice as big as the Sun, and twenty times as bright.)

To separate appearance: how bright a star looks -- from reality: how bright it really is -- we need to know the star's distance from us. But that's not really very hard. We use the method of triangulation -- taking a bearing on the star from opposite ends of the Earth's orbit. These bearings differ, for even the closest stars, by only fractions of a second, but that's enough to reveal their distances.

(03/07/03))