Appearance and Reality

What, then, is an object's real brightness? How much total energy is it sending to us at all wavelengths combined, visible and invisible? The answer is called the bolometric magnitude, m_bol, because total radiation was once measured with a device called a bolometer. The bolometric magnitude has been called the God's-eye view of an object's true luster. Astrophysicists value it as the true measure of an object's total energy emission as seen from Earth. The bolometric correction tells how much brighter the bolometric magnitude is than the V magnitude. Its value is always negative, because any star or object emits at least some radiation outside the visual portion of the electromagnetic spectrum.

Up to now we've been dealing only with apparent magnitudes — how bright things look from Earth. We don't know how intrinsically bright an object is until we also take its distance into account. Thus astronomers created the absolute magnitude scale. An object's absolute magnitude is simply how bright it would appear if placed at a standard distance of 10 parsecs (32.6 light-years).

Seen from this distance, the Sun would shine at an unimpressive visual magnitude 4.85. Rigel would blaze at a dazzling –8, nearly as bright as the quarter Moon. The red dwarf Proxima Centauri, the closest star to the solar system, would appear to be magnitude 15.6, the tiniest little glimmer visible in a 16-inch telescope! Knowing absolute magnitudes makes plain how vastly diverse are the objects that we casually lump together under the single word "star."

Absolute magnitudes are always written with a capital M, apparent magnitudes with a lower-case m. Any type of apparent magnitude — photographic, bolometric, or whatever — can be converted to an absolute magnitude.

(For comets and asteroids, a very different "absolute magnitude" is used. The standard here is how bright the object would appear to an observer standing on the Sun if the object were one astronomical unit away.)

So, is the magnitude system too complicated? Not at all. It has grown and evolved to fill every brightness-measuring need exactly as required. Hipparcus would be thrilled.