MadSci Network: Astronomy
Query:

Re: How much brighter is the moon than an average star?

Date: Sun Jan 15 11:23:58 2006
Posted By: Neil Saunders, Research fellow
Area of science: Astronomy
ID: 1132632142.As
Message:

Hi, Antonio,
Astronomers use a system called the magnitude scale to measure the relative brightness of objects in the night sky. There are 2 kinds of magnitude: absolute magnitude and apparent magnitude. Absolute magnitude measures how bright an object would be if it was at a standard distance (10 pc, about 30 light years) while apparent magnitude measures how bright things appear to us here on Earth.

The magnitude scale is a logarithmic scale. It is defined such that a difference in 5 magnitudes corresponds to a factor of intensity (brightness) of 100. This means that for every change in 1 magnitude (1m), the change in brightness is a multiple of the 5th root of 100 (about 2.512). Also, the smaller the value, the brighter the object. So for instance, a first magnitude object is 2.512x brighter than a second magnitude one, 2.512 x 2.512 = 6.31 times brighter than a third magnitude one, 2.512 x 2.512 x 2.512 = 15.851 times brighter than a fourth magnitude, and so on.

The brightest star in the sky is called Sirius and its magnitude is -1.4m. The planet Venus can be as bright as -4m and the Sun is -27m. The faintest stars that the unaided eye can see in dark skies are about 6m. The brightness of the full moon is about -12.7m. So in magnitude units, a full moon is about 11m brighter than the brightest star and and about 19m brighter than the faintest stars that we can see. If you raise 2.512 to the power of 11 and 19 respectively, you see that this corresponds to about 25 000 times brighter than the brightest star or about 40 000 000 times brighter than the faintest.

You can read more about the magnitude scale at Curious about Astronomy? Ask a Cornell University astronomer.

Neil


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