Green “Stars” for Real?

June 10th, 2008

In light of the recent posts about the red M stars, as red as they’re perceived to be anyway, I wanted to bring up the issue of the non existence of green stars. A star with the appropriate temperature, not too dissimilar from that of our own Sun, has a spectrum that peaks in the green. Yet there are no green stars. Why not?

I’ve asked this from time to time as an extra credit question on introductory astronomy exams. I usually only get a few correct answers out of 120 students. It does take different thinking from the every day.
The reason there are no green stars is that those stars of the appropriate temperature also put out a lot of red light and blue light both. Emitting all colors, they appear white.

But if you look carefully enough, there are some green star-like objects. I was discussing this with a friend at the American Astronomical Society meeting last week. These greenies aren’t bright enough to be seen with the naked eye, or indeed with any telescope I’m aware of that you can use with an eyepiece. [We'll ignore the philosophical question of whether or not they're actually green if you can't see them with your own eye.]

They’re a subset of quasars/radio galaxies of redshift 3 or so, which places the strong Lyman alpha line (the strongest emission line of hydrogen) at an observed wavelength of about 500 nanometers. The light at shorter wavelengths is eaten by intergalactic absorption. The light at longer wavelengths can be much fainter than the Lyman alpha emission.

This is the best example I can find in a quick search. The color composite cannot be trusted as it is created using broad band images and the band with the Lyman alpha is a blend of blue and green. The combination of the strong Lyman alpha line and the eye’s peak efficiency both in the green should make this object appear green to the eye if seen through a large enough telescope. At least I think so. I’d love to hook up a regular color camera to a telescope (it would need to be close to a meter in diameter at least) and test this. Perhaps as an experiment this summer at Launch Pad we can try it.

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