
What could make a star green? - e-sushi
http://worldbuilding.stackexchange.com/q/64344/16029
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Rooster61
We can never perceive a green star with our eyes alone without some sort of
substrate between the two blocking the red and blue light emissions. This is
because green falls smack dab in the middle of the visible light spectrum.
When a star a appears blue, most of what is being emitted by the star is
photons with a frequency that appears blue, and photons with a frequency of
ultraviolet or higher. Same goes for red stars, except instead of ultraviolet,
it emits frequencies in infrared and lower, resulting again in light that our
eyes cannot pick up. We only see the red photons that our eyes are able to
pick up.

If we could see further into the ultraviolet and infrared spectrum, "blue" and
"red" stars would most likely disappear in favor of whatever colors
blue+ultraviolet and red+infrared look like. This is what we see when we look
at a white star. It's a star emitting most of its light in the green spectrum,
but also emitting reds and blues, thus appearing white.

(Fun side note: Our sun is actually what a "green" star would look like if we
could actually see them. It's white when viewed from space. It only changes
colors and appears primarily yellow due several factors affecting the photons
in our atmosphere. Absorbption of certain wavelengths, Rayleigh scattering,
ect.)

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dhimes
_We can never perceive a green star with our eyes alone without some sort of
substrate between the two blocking the red and blue light emissions._

A doppler shift of a red or blue star would do it.

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Rooster61
How so? A doppler shift would only shift the apparent wavelengths that are
already present. For it to create a green star effect, it would need to
actively remove the upper and lower ends of the star's spectra.

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ainiriand
Exactly, deeper red or deeper blue, but not green.

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hencq
Completely unrelated, but the green stars reminded me of the chapter from
Surely You're Joking Mr Feynman where he's judging math books [1]:

> Finally I come to a book that says, "Mathematics is used in science in many
> ways. We will give you an example from astronomy, which is the science of
> stars." I turn the page, and it says, "Red stars have a temperature of four
> thousand degrees, yellow stars have a temperature of five thousand degrees .
> . ." \-- so far, so good. It continues: "Green stars have a temperature of
> seven thousand degrees, blue stars have a temperature of ten thousand
> degrees, and violet stars have a temperature of . . . (some big number)."
> There are no green or violet stars, but the figures for the others are
> roughly correct. It's vaguely right -- but already, trouble! That's the way
> everything was: Everything was written by somebody who didn't know what the
> hell he was talking about, so it was a little bit wrong, always! And how we
> are going to teach well by using books written by people who don't quite
> understand what they're talking about, I cannot understand. I don't know
> why, but the books are lousy; UNIVERSALLY LOUSY!

[1]
[http://www.textbookleague.org/103feyn.htm](http://www.textbookleague.org/103feyn.htm)

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arethuza
As an aside: there is a rather splendid explanation for stars _going_ green in
Ken MacLeod's SF novel _Learning the World_ :

[https://en.wikipedia.org/wiki/Learning_the_World](https://en.wikipedia.org/wiki/Learning_the_World)

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scrumper
Argh I'm halfway through Absolution Gap now, and there's a quote from the
bloody epilogue halfway down the page!

Managed to avoid reading most of it fortunately.

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based2
[https://en.wikipedia.org/wiki/Planck%27s_law](https://en.wikipedia.org/wiki/Planck%27s_law)

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the8472
Would dumping enough copper into the star work or would that still glow white?

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marcosdumay
If you spread enough of it around the star, sure, it will do. But _into_ the
star won't help you.

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astrobe_
If there's a redshift, there might be a greenshift?

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antisthenes
In a non-Euclidian geometry perhaps.

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Pica_soO
greenfly

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jccalhoun
envy?

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jstanley
> You cannot [...]

Says who? This StackExchange is for people writing fiction. They can do
whatever the hell they want.

It's fine to advise against certain things that might detract from
credibility, but telling people they "cannot" do things comes across awfully
self-important.

EDIT: Maybe I'm getting downvoted because of the self-importance of telling
people they cannot tell people that they cannot do things? :)

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pjc50
Worldbuilding.SE is weird. People are very unclear whether they want
"plausible fiction" or "hard science" answers for things, and to some extent
it's also treated like a puzzle - which I think is what the phrasing means
here. "You may/may not" phrasing appears in exam questions.

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acbabis
This hasn't been my experience. Worldbuilding has hard-science and soft-
science tags, and the better posts use them. Puzzle questions tend to get
flagged and closed.

