In 120 B.C. Hipparchus, a Greek astronomer, ranked the brightness of stars in the sky on a scale of one to six. He classified the brightest stars he could see as first magnitude and ranked the rest down to the faintest at sixth magnitude.
Astronomers still use this scale to measure the brightness of celestial objects, although it has since been modernized.
The Magnitude Scale
The magnitude scale is logarithmic, so a difference of one point in magnitude is equal to a difference in brightness of about 2.5 times.
A magnitude-one star is about 2.5 time brighter than a magnitude-two star, and a hundred times brighter than a magnitude-five star.
The lower the magnitude, the brighter the object.
The brighter planets and stars have negative magnitudes. The sun, the brightest object in the sky, has a magnitude of -26, followed by a full moon at magnitude -12.6.
Objects with a magnitude of six or less can be seen without optical aid under ideal observing conditions away from all artificial light.
Where Do Objects Fit in the Scale?
The table below is a list of well-known celestial objects and roughly where they fall on the magnitude scale—some objects, such as Venus, vary in brightness. The magnitude values have been rounded.
| || |
|Venus (the brightest planet)||-4|
|Sirius (the brightest star in the sky)||-1|
|Vega (the brightest star in the Summer Triangle)||0|
|Polaris (the North Star) and the Stars of the Big Dipper||+2|
|The Andromeda Galaxy||+4|
|Uranus and the Faintest Stars Visible with the Naked Eye||+6|
|Objects You Need Binoculars to See||+7 and greater|
A Few Handy Terms
Here are a few handy terms to keep in mind when reading about the appearance of celestial objects.
Luminosity is the intrinsic brightness of a star—compared to the sun—as it would appear if you were there in orbit around it, rather than viewing it from Earth. The sun's luminosity is 1. Sirius has a luminosity of 23 and Betelgeuse has a luminosity of 55,000.
Brightness is the light given off by a celestial object as seen from Earth. Brightness depends on luminosity and the distance from the object.
Magnitude is a logarithmic brightness scale. Magnitude-one objects are 2.512 times brighter than magnitude-two objects, which are 2.512 times brighter than magnitude-three objects, and so on. The difference between magnitude one and magnitude five is one hundred times. The higher the magnitude, the fainter the object. The lower the magnitude, the brighter the object. The brightest stars have negative magnitudes.
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