Thursday, July 11, 2013

Star of the Week: Deneb

If I had a pet swan, and I'm not sure why I'd ever have one because swans are rather mean creatures, I'd have to name him or her "Deneb". This isn't because "Deneb" means "swan". The name comes from the Arabic word for "tail". Rather it is because Deneb is the brightest star in the constellation of Cygnus, the swan, and is so closely related with swans in my mind that I cannot help imagining one when I hear the name.

This  is why I would never have a pet swan
Credit: Flickr user: ellenm1
Not only one of the brightest stars in the sky, Deneb is one of the most luminous stars as well. The difference between luminous and brightness is explained by distance. Astronomers measure how bright something seems with two numbers: apparent magnitude, or how bright it appears on Earth, and absolute magnitude, or how bright it would appear at a distance of 10 parsecs (32.6 light years). Because stars are at varying distances, using absolute magnitude allows them to compare the stars.

What makes Deneb so luminous? It is very hot and very big. Deneb is a type A supergiant (I will explain what this means in a separate post) with a temperature of 8400 K. Although it is much larger than the sun, it is only 20 times as massive, meaning that actually it is rather tenuous, as is common for large stars. Like Polaris, Deneb is a variable. In fact, Deneb lends its name to a class of variables, the Alpha Cygni variables. In these stars, one part of the star might be expanding whilst another is contracting!

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