Magnitude definition in Science

This wide-field view of the sky around the bright star Alpha Centauri was created from photographic images forming part of the Digitized Sky Survey 2. The star appears so big just because of the scattering of light by the telescope's optics as well as in the photographic emulsion. Alpha Centauri is the closest star system to the Solar System. Image released Oct. 17, 2012.

A glance at the night sky above Earth shows that some stars are much brighter than others. However, the brightness of a star depends on its composition and how far it is from the planet.

Astronomers define star brightness in terms of apparent magnitude (how bright the star appears from Earth) and absolute magnitude (how bright the star appears at a standard distance of 32.6 light years, or 10 parsecs). Astronomers also measure luminosity — the amount of energy (light) that a star emits from its surface.

Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz.

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Star Quiz: Test Your Stellar Smarts

Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz.

0 of questions complete

From Greek to modern times

More than 2, 000 years ago, the Greek astronomer Hipparchus was the first to make a catalog of stars according to their brightness, according to Dave Rothstein, who participated in Cornell University's "Ask An Astronomer" website in 2003.

"Basically, he looked at the stars in the sky and classified them by how bright they appear — the brightest stars were 'magnitude 1', the next brightest were 'magnitude 2', etc., down to 'magnitude 6', which were the faintest stars he could see, " Rothstein wrote.