What is absolute and apparent magnitude?

How do we measure the brightness of objects in the night sky?


The Tarantula Nebula, seen here, has an apparent magnitude of 8.

Asked by Tina Brown

The magnitude of an astronomical object, such as a planet or star, is a measure of its brightness. As you might have read in our guide to getting started in astronomy in All About Space issue 7, the brightness of objects from Earth with respect to each other are measured on a magnitude system. The lower the magnitude number, the brighter the object. Also a difference of five magnitudes implies a difference in brightness of one hundred times. The brightness of an object as seen from our planet is referred to as the apparent magnitude. Astronomers define this magnitude as the brightness of an object in the absence of the Earth’s atmosphere (which can scatter some of the light, causing it to dim).

Absolute magnitude is slightly more complicated. It is the apparent magnitude of an object if it were 32.6 light years away (that’s equivalent to 10 parsecs) from us and in the absence of any sources that could interfere with its brightness – these sources being the dust and gas in the Universe that absorbs or scatters emitted light. Fixing the distance allows astronomers to directly compare the brightness of stars.

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