Diamonds in the Sky Short course on star magnitude

As we have just recently watched fireworks on the Fourth of July, the topic of star magnitude comes to mind.

We saw the fireworks just in front of us being very bright, while fireworks exploding a few blocks away were dimmer, so too the brightness of a near star is brighter than a star that is far away. But that is not always true; there are near stars that are dim. This is due to the real brightness of the star.

For example: there is the star called Barnards star that is nearby (5.96 lightyears) but past the limit of human eye visibility and has a magnitude of 9.5. This is because Barnards star is a red dwarf star, both smaller and dimmer than our own sun, while the nearest sun-like star is Alpha Centauri, which is +1.3. a somewhat bright star which can be seen if you live where you can see the far southern sky, like in Hawaii. Alpha Centauri is brighter than Vega, which we see in our summer night sky at 0 magnitude.

How is brightness determined? Back in the 2nd century BC the Greek astronomer Hipparchus set up the classification using a six point scale based on the brightness of a star that the human eye could see. The higher the number, the dimmer the star.

Why the larger number the dimmer? It was a class system. A first class star is the brightest and a second class star is a bit dimmer, down to a sixth class star, anything lower than five was a sixth class star. The stars of the Milky Way were all sixth class, though Hipparchus did not know that the Milky Way was made up of stars.

Remember, there was no light pollution back in the second century BC. Today, out in the dark skies of eastern Washington and Oregon, seeing a sixth class star is not too much of a problem. If you can find a dark sky here in western Washington, you may have a good chance of seeing down to sixth magnitude. If you are very lucky, you may be able to see way down to a seventh magnitude star.

What is the difference between a magnitude 1 star and a magnitude 2 star? It is a difference of 2.5 times.

By Greg Smith

Thus a Magnitude 1 star is 2.5 times brighter than a magnitude 2 star. Each magnitude is 2.5 times. The difference between magnitude 1 and a 3 is 6.25. Thus between 6 and 1 is 97.6 or almost 100 times dimmer.

The same range also applies as you go into the negative range. The moon is –13 and the sun is –26. Venus at max –5, Jupiter at max is –3, and Polaris (the North Star) is 1.98 or considered a 2.

Longview resident Greg Smith is past president of Friends of Galileo. Meet him and other club members at monthly meetings in Longview. For more info about FOG, visit friendsofgalileo.com.

When looking up star magnitudes you will find that stars have magnitudes of 2.23 or some other decimal. That is because modern astronomers have the equipment that can get finer readings on the brightness of a star.

I hope this makes things clearer for you to understand the magnitude scale of the objects in the night sky. •••

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