The Elusive Green Stars

12/10/06 – 12/16/06
by C. Zaitz

Stars come in different colors. We’ve all heard about “red giants” and “white dwarfs”, blue hot stars and orange cool stars. But what about green stars? Where are all the green stars? Our streets and houses are lit up this time of year with lights of red, blue, purple, green, orange and yellow. The stars come in those colors too, all but green. So what’s wrong with green stars?

Green is a great color. It's my mom’s favorite color, so I usually buy her clothes with green in them to make her happy. Green is the color of life, of plants and trees. Grass bleeds green when you cut it, and to me the color even "smells" like freshly cut grass. Green is the color of ”go” and avocados, girl scouts and money. Green is great. But not in stars. Our sun radiates energy mostly in the green part of the spectrum, so technically it would be a green star, if stars could look green. But they can’t. Why not?

Remember Roy G. Biv? Scientists have done away with color indigo for the most part, so it could be shortened to Roy Gbv, but that’s not as memorable, and hard to pronounce. Roy is the red end of the spectrum, and Biv is the blue end. Green is right in the middle of the light spectrum, and that is the problem. We can easily see when a star’s energy peaks in the red end of the spectrum. Most of the light we can see is reddish, so these cooler stars often appear red, even to the naked eye. The star Betelgeuse in the right shoulder of Orion is a classic red giant star. Very hot stars can peak in the blue end of the spectrum, giving them a bluish-white cast. The star diagonal from Betelgeuse in Orion’s left foot is called Rigel, and it is a very large, hot, bluish star. When you compare the tint of these two stars in the sky, it’s pretty easy to see the difference. However, since green is smack dab in the middle of the spectrum, when a star sends out most of its radiation in that wavelength, it is also sending out almost as much in the adjacent wavelengths. So all the colors- the reds, greens and blues- all mix together and make white. If we looked at our sun from above the atmosphere it would look white. In the sky it often appears yellow, or even red near the horizon. This is not because the sun has changed color. It is because the air is scattering blue light to make the sky blue, and the color that’s left when you take light blue out of white is yellow. Later on, as the light from the sun has to pass through a lot of atmosphere at sunset, most of the blue light has been scattered out of the sunlight, and only the long reddish wavelengths are left, giving us really rich sunsets.

If you want to see pretty Betelgeuse and bright Rigel, wait a little while after sunset, and you will see the three bright stars of Orion’s Belt. They are, from east to west, Alnitak, Alnilam, and Mintaka. Betelgeuse is above Alnitak, and Rigel is below Mintaka. All three stars in the belt are white-blue stars, very hot and very big. You can look for a green star among the thousands up there, but unless you’re wearing green-tinted glasses, you won’t find one!

Until next week, my friends, enjoy the view.