14 September 2010

Blue Star, Red Star, Yellow Star

When conducting a star party, I always point out star colors. Most of the time, people see the stars as uniformly white, but in fact upon closer inspection it's easy to see that stars have color, sometimes very dramatic color. This time of year there are several colorful bright stars that illustrate nicely the range of what you can see in the sky.

The southern sky is dominated by the distinctive shape of Scorpius, the Scorpion of the Zodiac constellations. The "heart" of this constellation is the bright red supergiant star Antares. It is in the middle of the body of the scorpion and it is one of the biggest stars we can see, so big that if places in the Solar System it would enclose Mercury, Venus, Earth and Mars. To the observer, it looks a reddish-orange color.

High above this time of year is the Summer Triangle, featuring three of the brightest stars in the sky. One of these three is Deneb in the constellation Cygnus, the "Northern Cross." Deneb appears blue to the observer and in fact is indeed a blue-white supergiant star, similar to Antares in terms of massive size, brightness and distance from the Sun.

These two beautiful stars should be enough to whet your appetite for detail when you look at the night sky. They are bright enough and easy to spot and display color quite nicely. But don't stop there. A simple pair of binoculars gets you enough resolution to see an amazing array of color in so many of the stars in the sky.

The colors of the stars is an indication of their temperature. Like the different levels of heat in flame, the colors of stars follows a similar pattern with red being cooler and blue being hotter. There are yellow stars, hotter than the red giants, and next in line are white stars, cooler than the blue stars. Details of Stellar Classification and Color Index are documented on very fine websites for those who want to learn more about star colors.


Paulie said...

Star color seems to be a big hit at public star parties. I'm not much of a double star observer, but the beautiful blue/orange combination of the Albireo system has been one of my best received targets at star parties lately, especially ones in the city of Chicago, where light pollution hinders faint fuzzies. I was slow to appreciate Albireo, but sometimes showing something easy to the public reminds us to take another look, and marvel with delight.

The Urban Astronomer said...

Thanks for the comment, Paulie. I will take your advice and look more often at Albireo since it is well placed in the sky this time of year. I have a star party tomorrow night and was planning to talk about star colors so I will definitely point the scope at the pair. And I hope I can develop an appreciation of Albireo as you have -- I admit, I have not warmed up to it but repeat viewings will likely move me toward being a fan :-)

-- Paul

Paulie said...

It's not something I appreciated at first either, but I think you'll get a good reaction from it, and anything that pleases the crowd, pleases me.

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