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Starry, Starry Night


Paul Kohlmiller


The summer triangle consists of three stars. Vega lies to one side of the Milky Way and Altair lies on the other. The third star in the triangle appears to be equally as bright and resides in Cygnus the Swan. It is called Deneb. Deneb means “tail” and it is the tail of the swan if you wish it. If you see a swan in Cygnus you are the rarity. Most see a kite or a cross. Cygnus is often called the Northern Cross.

Despite the fact that all three stars in the summer triangle are at nearly the same apparent magnitude, Deneb is much brighter intrinsically. Vega is only 22 light-years away and Altair is slightly closer. But Deneb is at least 1500 light-years away.

Deneb (Alpha Cygni) is an A2 star. Vega is slightly warmer but Deneb has the luminosity of 160,000 suns. It may be the most luminous class A star in the galaxy.

You can already guess that Deneb is a very large star and you are right. It is a supergiant star - 200 times the diameter of the sun. Place Deneb where the sun is and the star would reach out to Earth’s orbit. This does not make it the largest star in the galaxy.

The constellation Cygnus is fun for other reasons. It contains the double star Alberio which the folks up in Berkeley like to call the Cal Star because it is blue and gold (well, yellow). It contains a blackhole candidate called Cygnus X-1. It also contains at least two stars that are known to be planetary systems: 16 Cygni and HD 188753. It also contains the Messier objects 29 and 39 (open clusters) and the Dumbbell Nebula (M27) is in nearby Vulpecula.

Sometimes people stop and say to me, "Hey, Paul". Because that's my name. The "Paul" part, the "Hey" is optional. They say, "Hey, Paul. How do you know so much about stars." Well, I don't. I do know how to use Google - a little. And if you want to find out about most any star that has a real name (instead of a number) you can try this: Go to Google, enter "Kaler starname" where starname is Vega, Beutelgeuse or whatever. Then hit "I'm feeling lucky". This will not work with every star but I bet it works more often than not. You will also find that there is not a lot of agreement on some things like a star's size or distance. That's because accuracy is difficult especially if a star is busy puffing up into a red supergiant or is losing a lot of matter through its stellar wind. But this information is free.


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