The Magnitude of Stars

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You may have heard astronomers or others use the term “magnitude” when talking about stars’ brightness. Just what does this “magnitude” mean and where did it come from?

The magnitude system was developed by ancient Greeks. The astronomer Hipparchus is often credited with this system. He put the stars that they could see into six groups. The brightest ones were in the first group and called magnitude 1. The next brightest into the second group, and so on until all of the visible stars were grouped.  The dimmest were in the sixth group and called magnitude 6.

The brighter the star the lower the number.  Of course, with the telescopes of today, we can see dimmer stars so many of the stars have magnitudes in the negative numbers. Without a telescope, you can barely see stars that are magnitude 6 but the Hubble telescope can sometimes see stars that are magnitude 30.

There are two kinds of magnitude.  Apparent magnitude, which is what most star charts use, and absolute magnitude.  Apparent magnitude is how bright the stars look to us from earth.  Because some stars are very distant from us and others are close their apparent brightness differs.  A dim star that is close to Earth will appear to be brighter than a bright star that is far away.  If you could line up all of the stars at the same distance, the magnitude would be different than the way we normally see them.  That’s what Absolute magnitude is.  Astronomers can calculate the brightness of stars as they would appear at equal distances from Earth (about 32 and a half light years away).  That means that if all of the stars were lined up at the same distance from Earth, the magnitude of our sun would only be 4.83 on the absolute magnitude scale as opposed to minus 26.8 on the apparent magnitude scale.

The brightest star in the sky is Sirius with an apparent magnitude of 1.4 where the full moon has an apparent magnitude of minus 12.6.  Other well known bright stars include Arcturus, Vega, Capella, Rigel, Procyon,  and Betelgeuse.

Learn more about stars’ magnitude and a variety of other interesting astronomy facts at AstronomyNotes.com.

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