Stellar Magnitudes

The Greek astronomer Hipparchus devised the first magnitudes in the second century B.C. He classified stars according to how bright they looked to the eye: the brightest stars he called "1st class" stars, the next brightest "2nd class," and so on down to "6th class." In this way all the stars visible to the ancient Greeks were neatly classified into six categories.

Modern astronomers still use Hipparchus' categories, though in considerably refined form. With modern instruments astronomers measure a quantity called V, the star's brightness in the visual portion of the spectrum. Since visual light is what our eyes most readily detect, V is analogous to Hipparchus' classes. For example, Hipparchus listed Aldebaran, the brightest star in the constellation Taurus (the Bull), as a 1st class star, while today we know that for Aldebaran V = 0.85. Astronomers often refer to a star's visual brightness as its apparent magnitude, which makes sense since this describes how bright the star appears to the eye (or the telescope). You will hear an astronomer say that Aldebaran has an apparent magnitude of 0.85.

Hipparchus' scheme defined from the outset one of the quirks of magnitudes: they run backwards. The fainter the star, the larger the number describing its magnitude. Therefore, the Sun, the brightest object in the sky, has an apparent magnitude of -26.75, while Sirius, the brightest star in the sky other than the Sun and visible on cold winter nights in the constellation Canis Major (the Big Dog), has an apparent magnitude of -1.45. The faintest star you can see without optical aid is about +5 (or +6 if your eyes are very sharp), and the faintest objects visible to the most powerful telescope on Earth have an apparent magnitude of about +30.