Sirius
 The "Dog Star" Sirius and its companion, known as the Pup, move through space in this artist's concept. The Pup, at right, is a white dwarf -- the dense, crushed core of a once-normal star. Recent observations revealed that the star is almost exactly as massive as the Sun, but only as big as Earth. Sirius is the brightest star in the night sky, and shines in the south on winter nights. [Credit: NASA/ESA/G. Bacon (STScI)]
Sirius appears so bright in part because it really IS a fairly bright star, but mostly because it's one of our closest neighbors -- less than nine light-years away.
That closeness made Sirius a favorite target for astronomers who were building new telescopes and new techniques in the 19th century. In 1844, for example, German astronomer Friedrich Bessel discovered that something was tugging Sirius back and forth a little bit -- an unseen star. Alvan Clark first saw this star in 1862. It's small and faint, so it's hard to find beside much-brighter Sirius. Since it's a little companion to the dog star, astronomers called it the Pup.
A half-century later, astronomers found that the Pup was unlike any other star they'd ever seen. Its surface is extremely hot, yet the star produces so little light that it must be tiny -- no bigger than Earth. But it's about as massive as the Sun, which is far larger than Earth.
The Pup was the first of a previously unknown type of star called a white dwarf -- the crushed corpse of a once-normal star. More about the Pup tomorrow.
Script by Damond Benningfield, Copyright 2002, 2005
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