Spider Star
The human colony on the planet Argo has long explored and exploited the technology left behind by an extinct alien race. But then an archaeology team accidentally activates a terrible weapon...
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-- Locus Magazine"Star Dragon is steeped in cosmology, the physics of interstellar travel, exobiology, artificial intelligence, bioscience. Brotherton, author of many scientific articles in refereed journals, has written a dramatic, provocative, utterly convincing hard science sf novel that includes an ironic twist that fans will love."
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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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I’d be leery of trusting a camera to accurately tell you whether a star is green. But if it’s a reasonably well-studied star, someone surely has a well-calibrated spectrum for it. Just feed that spectrum into some code that models the eye’s response and you can find out what colour the star should look. Of course to find out whether you’re right you’d want to actually look at it, but a camera’s colour sensors respond rather differently to light than our eyes do – for example film cameras tend to be over-sensitive to UV, while CCDs tend to be over-sensitive to IR…
Yes, the combination of physics and physiology is necessary, absolutely. It seems to be tricker to model, however. I’d love to write such a program in the future.
I actually wrote code that converts a spectrum to a visible colour (in one of the standard colour spaces). The key tool is the CIE XYZ colour space. X Y and Z here are closely connected to the actual stimuli in the eye (specifically, there are response functions, which you can simply numerically integrate your light spectrum against to get the X Y and Z values), and there are well-defined transformations from XYZ to (for example) sRGB. I was able to implement the code in python after a day or two of reading.
Re green stars: An interesting corollary question is whether it is possible to *create* an obviously green star. Let’s say ,for instance, that a team of very-well-funded astroengineers belonging to some super-advanced alien civilization *wanted* to convert a star (at a certain location) into an extremely green star (at least to our eyes), maybe for use as a stellar “marker buoy”. It seems to me that the only way that this could be done would be to feed the star an extreme over-abundance of a specific element/compound. The question then would be, what substance would work (if any)?