h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

As part of my investigation into green stars, I went out to look at Zubeneschamali. It was hidden by the brightness of the full moon.

Speaking of which, I decided to sleep through the "Blood Moon" last night. But it did turn out great in our area. We had perfectly clear skies. The local news showed some great viewer pics.

My theory is that there are no green stars but descriptions of Zubeneschamali say that it is green. I want to prove them wrong.

I remember this discussion. I was thinking about that just the other day.

What is the basis of your theory, Gnomon?

Starlight is what is known as "black body radiation", which can be red, orange, yellow, white or light blue but never green.

But is starlight truely black body radiation? stars also emit radiation in many other frequency bands, UV, x-rays, gamma rays, and in the other direction, radio waves...

If a star was moving fast enough, would the doppler effect be enough to shift UV light down into the visible spectrum... violet, and if it were shifted a bit further, green?

Just found an on-line doppler calculator - ain't the internet great!

Near visible UV light at 400 Nm, doppler shifted due to the emitter moving away from the observer at 0.2 C , would actually drop down into the Green part of the spectrum.

Oh, and of course doppler shift works both ways... A nice yellow sun would appear green if it were moving directly towards us at just 0.1 C... Although if someone at NASA detected the second case, I seriously doubt they'd worry about the colour

It's because they give out black body radiation that they emit in all those regions. Their output is spread over lots of different frequencies, so they'd never be just green.

Amazing! I zoomed in on Zubeneschamali with my Stellarium programme just the other night, to check if it was green or not. It didn't look green to me. I can't see it through my binoculars as my eastern horizon isn't clear, the street lights are in that direction.

I don't think the star is green, it's classified as a blue-white dwarf so if anyone sees it as green it's possibly a trick of the light.

GB

Ok Gnomon, that figures...

I'm playing devils advocat here (from a position of ignorance as well - so feel free to shoot me down)... Yes, they're emitting all frequencies,but not all stars are emitting all those frequencies at the same amplitude.

What about those stars we see that have a bluish tint, a quick search gives the example of http://en.wikipedia.org/wiki/Eta_Carinae , which appears blue to us... Surely if that star was further away and moving faster from us it'd shift into the green part of the spectrum.

To shoot myself down over this - I suspect that any star which could be doppler shifted by that amount would be so far away it wouldn't be visible to any optical instrumentation we've yet developed, but it might still exist in theory.


The other possibility is that even if a star wasn't actually emitting majority green frequencies, is there any possiblity of a gas cloud between the star and ourselves which would filter out some of the other frequencies and leave us with what appeared to be a green star?

Thanks Whisky. It's worth stating all these objections so that I can see the kind of things people will think of.

1. Stars do emit different colours, but always over a range of frequencies. When that range is mainly at the red end of the spectrum, we can't see the infra-red parts of it, so we see it as red. When it's mainly across the visible spectrum, we see it as white and when it's mainly at the blue end of the spectrum, we can't see the ultra violet portions so we see it as blue.

If you Doppler shift a blue star towards the red, the ultra-violet parts will become visible and it will look slightly whiter. But this would be a tiny effect - doppler shifts are very small.

Filtering the light through a nebula is a good way of turning the starlight green - if we could find a nebula which glows magenta (a mixture of red and blue), then I think starlight travelling through it would have the red and blue parts filtered out, leaving green. That could work, but I don't know of any examples.

If you're after any kind of green items in the Galaxy, then I've seen it reliably written that some believe the entire universe was sneezed out of the nose of the Great Green Arkleseizure, and they live in perpetual fear of the time they call 'The Coming of the Great White Handkerchief.'

I should be looking for the elusive 'Snotstar', then, should I?

Here is my favorite astronomy blogger's explanation of why you won't see any green stars.

http://blogs.discovermagazine.com/badastronomy/2008/07/29/why-are-there-no-green-stars/#.U0_HJvnER8E

The TL;DR version is simply that since stars(blackbody radiators) emit light at all visible frequencies, when their temperature is such that green is the prevalent frequency (like our Sun) we see the color as white since it's also emitting roughly similar amounts of blue and red light. To be seen as green, a star would have to emit ONLY green light.

However, in a follow-up post he discusses other objects that appear green and how some stars may appear green by contrast with neighboring stars. He also specifically mentions Zubeneschmamali. But he leaves it a bit indeterminate. Some see it as green while others see it as white. He suggests that its rapid rate of rotation may influence its appearance.

http://blogs.discovermagazine.com/badastronomy/2008/08/28/followup-green-objects-in-space/#.U0_KLvnER8E

That's a tantalising post because it completely fails to explain the question I'm asking while suggesting that someone might actually know the answer.

Well, I felt it answered some of the questions others here were discussing.

So, what is your actual question? I'm quite intrigued.

Ok, so this has got me thinking... (probably a little too laterally)...

Assumption one: black body radiation will never create a green glow...

So, what we need is some kind of monochromatic source of light...

How about Cherenkov radiation... On earth it's usually found when charged particles pass through a dielectric medium (water surrounding a nuclear reactor) and it gives off a blue glow...

So, we need a massive source of charged particles and a dielectric medium (water works well, so does nitrogen, various other gases and molecules, as well as, strangely enough, some plastics.

So, take a neutron star, wrap it in clingfilm and get it moving at a fair percentage of the speed of light away from the observer and bob's your uncle - a green star.

Am I getting a little bit too weird here?

Well, that second link above discusses how the charged particles in the cosmic gas clouds of nebulae can glow green like a massive fluorescent light bulb.