# What could make a star green?

## Stars are never green.

When a star's spectra "peaks" in the green range, it also releases a significant number of waves of the adjacent colors - so "green" stars appear yellow or white.

I want a green star anyways.

How can such a star come about if they do not exist that way in nature (as far as we know)? What natural circumstances would change the appearance or composition of a star in this way (so that it emits green light)?

You can

• Have elements or molecules outside the star (exotic if you wish) as long as they are stable wherever you put them and can as long as they can form naturally in real life.
• Change the composition of the star itself with (exotic if you wish) matter as long as it is stable and produces the desired effects
• Provide a solution that will eventually change color when the star expands
• Provide a somewhat speculative explanation but it should be based in real science
• Have the star "capture" whatever makes it green after formation or have it form with this quality in the first place

You cannot

• Simply change the atmosphere of a nearby planet so it looks green; it should appear green(ish) from space
• Change the eyes of creatures viewing it; assume human eyes
• Have intelligent intervention; all circumstances should be possible in nature (rare is fine)
• Change the laws of physics or the characteristics of light
• Create the illusion of green color from either an actual binary or an optical binary; this star should be standalone
• Create the illusion of green color from movement of the star / color shift

Edit: Most current answers are acceptible, and I have one in mind to accept, but I wouldn't mind a different approach - changing the composition of the star instead of the adjacent material. The one I accept likely won't change but many of the ideas presented now are similar.

• Comments are not for extended discussion; this conversation has been moved to chat. Dec 15 '16 at 1:53
• I don't think you could make a star green. They throw out all kinds of radiation, cause massive planetary warming, enable the precursors of fossil fuels... Dec 15 '16 at 9:06
• @DavidRicherby I'm sure a couple of the answerers would disagree - and there are ways to deal with that radiation Dec 15 '16 at 12:50
• Just bombard the star with tons of copper. that usually makes my campfires green XD Nov 15 '19 at 14:54

# Give it a circumstellar cloud of oxygen.

Some planetary nebulae, such as NGC 6826, appear green because of ionized oxygen.

Image in the public domain. Yes, this is a true-color image.

I see no reason why you couldn't surround the star with an extremely dense cloud of hydrogen, containing a relatively high fraction of oxygen, which would absorb light and reradiate it at green wavelengths. This is the same effect that we see in auroras. The emission of light at the 557.7 nm wavelength is the cause of the green tinge.

The stability of such a cloud is, of course, a problem. Radiation pressure, responsible for the dissipation of molecular clouds surrounding newborn stars, can disperse quite a lot of gas. In fact, planetary nebulae can last for only about $$\sim10,000$$ years, an extremely short time relative to the lifespan of stars.

Obviously, the optimal way to combat this would be either a continuous resupply of gas, possible via accretion from a companion (unlikely, in the case of oxygen, although I don't have a source for that) or an extremely large gas reservoir. Extreme mass loss, possibly due to extremely strong stellar winds, is a possibility. Walmswell & Eldridge (2012) suggested that "superwinds" from red giants could be responsible for self-sustaining circumstellar envelopes of gas, which would actually decrease the star's luminosity (they were searching for a solution to the red supergiant problem).

Mass loss in the case of our star would require there to be a large amount of ionized oxygen in the stellar atmosphere and outer layers of the star - possible, given that red supergiants should be fusing heavier elements, and given also that higher-metallicity stars may have substantial quantities of these elements, known as "metals". I'm not confident that such a supply is totally realistic, but I see no reason to dismiss it entirely. While hydrogen dominates photospheric spectra in most stars, chemically peculiar atmospheres have been observed in many other stars nonetheless.

## Technical details about forbidden lines

For anyone more curious, here's a bit more detail on how a cloud of oxygen can turn green.

An emission nebula gets its distinctive colors from photons emitted by different elements in its gas. Hydrogen is, of course, the most plentiful, and so $$\text{H}\alpha$$ emission often dominates the spectra of such nebulae. $$\text{H}\alpha$$ occurs when an electron in a hydrogen atom becomes excited and jumps from the third energy level to the second. The transition leads to the emission of a photon, which in the case of $$\text{H}\alpha$$ is red. (The process is actually more complicated than this, and evolves ionization and recombination, but the key issue here is that there is no perturbation by another electron).

Oxygen, however, emits light through a different process - the poorly named forbidden transitions. Emission here is the result of the collision of a free electron with an electron in an atom of doubly ionized oxygen, denoted $$[\text{O III}]$$ (not a typo - there are indeed three $$\text{I}$$s). This collisional process happens more often at higher temperatures, as the mean velocity of electrons increases as temperatures increase. Therefore, hotter nebulae are more likely to be green than cooler nebulae with the same concentration of $$[\text{O III}]$$. Oxygen can then become strong in the nebula's spectrum, often almost as strong as $$\text{H}\alpha$$.

There are plenty of other emission lines (about 263, to be exact, for oxygen alone) that cause photons with green wavelengths ($$500\text{ nm}<\lambda<565\text{ nm}$$) to be emitted. However, this particular transition is preferred because of $$[\text{O III}]$$'s abundance in space and because of the high probability of this particular transition.

• Is it actually green? Space photos are often colourised to represent different wavebands, and I couldn't find mention of actual colour on linked wiki, nor pages linked to by wiki. Dec 12 '16 at 15:32
• @Miech It's actually green, yes. It's not a false-color image. Dec 12 '16 at 15:33
• NGC 6826 is difficult to observe in a telescope; when you look directly at it, the central star overwhelms the light from the nebula, so it takes careful observation with averted vision to see the nebula (and that means you see it in grey scale, not green). Observation from nearby is problematic as well--O III is caused by intense ultraviolet radiation stripping away electrons from oxygen. Being close to that amount of ultraviolet would be unhealthy, to say the least. OTOH, we're probably getting into an area that's pretty easy to ignore/handwave. Dec 12 '16 at 19:10
• It's worth noting that Copper also does this in a less vibrant 510nm green, but also emits gold at 578.2nm. Together they produce some electric green colors. However, oxygen will be the brightest green to human eyes, as we're most sensitive to green at 555nm. The effeciency of copper-light conversion is usually much, much higher than oxygen, however. Dec 12 '16 at 20:23
• Excellent answer. I was also thinking of wrapping the star into a cloud, but did not knew oxygen was a candidate. Dec 12 '16 at 20:36

Phil Plait of the Bad Astronomy fame:

But is there a star that’s intrinsically green? Zubeneschamali is the second brightest star in the constellation of Libra. It’s somewhat hotter than the Sun, and some people claim it looks green to them, while other say it looks white. It’s unclear why; some people’s sensitivities are different, but it might also have to due with the star itself: Zubeneschamali is a young star and a rapid rotator, which might affect its colors (the emitted light from a star is not really a blackbody, and its youth and rotation might have some influence on its emitted colors).

So maybe, maybe, there is one intrinsically green star, but even then it’s controversial.

There are several lessons here. One is that stars really can’t be intrinsically green; they need to be seen against a contrasting color to look green, and even then it’s just a trick. Also, there are green objects in space, but they are very different than stars (gas clouds and planets). And finally, the color we see from an object depends on how that object emits light, which can be just as important as the light emitted itself.

(From "Green objects in space" by Phil Plait)

• The first link in the quote leads to a 404 error page. Dec 12 '16 at 19:49
• @Philipp: 2016-12-12 19:51:50 here it is just veeeery slow to load, but it finally works. Dec 12 '16 at 19:53
• @AlexP are you behind a university firewall?
– Tim
Dec 13 '16 at 23:13
• @Tim: No, not a university firewall. Dec 13 '16 at 23:39
• There is also issue of photopic vs scotopic vision which means that very dim light sources cannot have color at all. Starry sky falls under mesopic (mixed) vision but it means that color discrimination is already impaired. Dec 15 '16 at 15:22

The best I can come up with is to have the star's corona dominated by something with a strong green emission spectra.

Here are 11,000 emission lines in the green range (495-570 nm). You'd have to go through them and find one that does not have any (or at least many) other emission lines in the visible range. I'm not going to do that. But supposing you did, then you might have a plausible reason for a green star, although I don't know how you can explain a corona full of Yttrium or Thallium.

• You could use ionised oxygen... but I see another answer's already covered that. Dec 12 '16 at 18:45
• Copper burns green. Is there any way to introduce a helluvalot of copper into a star? Say, ejection from a supernova (which is where all heavier elements originate) into another smaller star?
– SRM
Dec 13 '16 at 4:45
• @SRM "Copper burns green" brings us back to needing oxygen, since that's what "burning" is. And if we already have the oxygen, we don't need copper for green. Dec 14 '16 at 22:28
• Huh. I never thought about that. You're right. It's one of those things I discovered as a kid and never mentally updated for adult chemistry knowledge!
– SRM
Dec 14 '16 at 23:28

The other answers focus on purely natural chemical processes, but why not a cause with a sentient origin?

# Chlorophyll

... When she looks into the sky she sees only a handful of stars: those bright enough to shine through the moonlight and the sparkling river of the ring. Of the green star that the butterflies have revealed there is no longer any sign. But she knows it is still there, just too faint to be seen. Once revealed, it is not something that can ever be forgotten.

She knows that there is nothing actually wrong with the star. Its fusion processes have not been unbalanced; its atmospheric chemistry has not been perturbed. It shines as hot as it did a century ago, and the neutrinos spilling from its core attest to normal conditions of pressure, temperature and nucleotide abundance. But something very wrong has happened to the system that once orbited the star. Its worlds have been unmade, stripped back to raw atoms, then reassembled into a cloud of glassy bubbles: air-and-water-filled habitats, countless numbers of them. Vast mirrors—forged in the same orgy of demolition and reconstruction—trap every outgoing photon of starlight and pump it into the swarm of habitats. Nothing is wasted; nothing is squandered. In the bubbles, the sunlight feeds complex, teetering webs of closed-cycle biochemistry. Plants and animals thrive in the swarm, machines tending to their every need. People are welcome: indeed, it was people for whom the swarm was made in the first place.

From the epilogue of Absolution Gap by Alastair Reynolds.

• This is interesting but it's worth noting that stars are somewhat .. hot to evolve sentient life, even directly on the surface Dec 12 '16 at 23:31
• @Zxyrra They are suggesting you surround the star with an envelope of life. Sure, you might have a vacuum gap to attenuate it; but a shell at approx 1 AU will have roughly the right amount of light hitting it to grow plants. It will be a big sphere, mostly full of vacuum. We can give it a name. Like Dyson Sphere. After the vacuum company. >_> <_<
– Yakk
Dec 13 '16 at 16:22
• @Yakk how do you suggest evolving life in a place with sparse matter and an abundance of ionizing radiation? This is creative but it seems very difficult to do Dec 13 '16 at 21:29
• Who says they evolved there? It could be part of a colonisation effort by an alien species of your choice.
– Riot
Dec 13 '16 at 22:02

@riot already mentioned Absolution Gap. In general, something else around the stars causes the color, not the surface of the star.

In my own work in progress, a green laser is used to get the attention of the planet on the receiving end of an attempt at interstellar communication. Because, as you note, stars are not green, using a green beam (making a star appear green) will make it conspicuous.

• Interesting scenario in the second half - consider also violet (I may do a follow-up question on violet stars) or a spectral emission that wouldn't occur naturally (lines in the wrong places, etc). I digress. Dec 13 '16 at 5:44
• Violet: some stars are blue-hot. Lines: indeed, small number of lines to keep power in small number of channels, but provide correlation for noise rejection. Dec 13 '16 at 5:52
• A similar situation occurs in The Mote in God's Eye, in which it is deduced that the Mote must harbor an alien civilization because it had turned green for a while, as a result of turning on the launching equipment for a laser-sail starship. Dec 13 '16 at 18:52

Larry Niven's science fiction novel, "Integral Trees," written in 1984 featured a gas giant orbiting a neutron star outside of its Roche Limit. The neutron star leaching atmosphere from the gas giant led to a surrounding gas torus environment capable of supporting life in the thickest part of the halo. I have no idea about the physics, but one could imagine a circumstance where a gas torus rich in oxygen, copper, chlorophyll, or any combination thereof could result in a green star. Perhaps a habitable (thus the chlorophyll) gas torus fed by an oxygen/copper rich gas giant, orbiting close to a cool white dwarf would make an interesting scenario.

Alternatively, a nebula rich in oxygen or copper slowly collapsing into white dwarf might produce such an object. Something like the Egg Nebula or Stingray Nebula without the purple and other colors comes to mind: https://en.wikipedia.org/wiki/Egg_Nebula https://en.wikipedia.org/wiki/Stingray_Nebula

Not sure if these are the actual colors or if the images have been color corrected. Also, not sure if there could be a habitable zone within a planetary nebula. Can't say if any of these really fit your criteria. I guess Kermit was right; it ain't easy bein' green... in space.

• This is a good first answer, welcome to Worldbuilding SE! A nebula sounds promising although I ask for a standalone star so I'm not able to work with a binary system as you mention in the first part. Dec 14 '16 at 6:02
• Ok. Thanks. I suggested it because I didn't see anyone talking about neutron stars or white dwarfs. However, it's conceivable that a nebula could collapse into a single dwarf star, isn't i?. I wanted to give a visual and I only had limited examples to post. That's why I chose those particular nebula. There were others that were even more green, but it was difficult getting information on them (for instance some were probably color corrected) so those were what I had to work with. Dec 14 '16 at 17:15

I am no physicist, but can you imagine a star that radiates blue light going so fast away from you that its redshift would make it appear green?

Or something red going towards you very fast?

• I think the questioner is implying that the star has a habitable planet orbiting it. Dec 13 '16 at 15:48
• @AndrewRecard not necessarily but the idea is that it looks green when observed at nearly any place - this would still look blue up close. Dec 13 '16 at 17:13
• Nothing looks the same color from all viewpoints. That's the point of relativity. Red stars blue shift when viewed from some views.
– SRM
Dec 13 '16 at 20:27
• Red-shifting a (black-body) blue star just makes it white again (and then red), no green in between. Same for blue-shifting a (black-body) red star. Dec 13 '16 at 22:35
• Plus, if you redshift a blue star, it will appear white, not green. That's because the redshift applies to the whole spectrum so when the blue in it becomes green, the ultraviolet becomes blue and the white becomes red. This resulting in a white blend to the eye.
– KPM
Dec 14 '16 at 4:09

A planet orbiting within a red dwarf's habitable zone is likely to be tidally locked to its star. If so, any human or alien inhabitants will likely be confined to the strip of land or water along the planet's terminator, where temperatures are comfortably balanced between the deep cold of the night side and the extreme heat of the subsolar point on the day side.

An observer stationed along the terminator might then see the primary star in a permanent "sunset", and if the star is partially blocked by the horizon the observer may very well see a permanent green flash given the right atmospheric conditions.

• Creative but the question specifically says "green seen from space" and mentions something about not having it appear green just from a particular planet Dec 12 '16 at 21:34
• «likely to be tidally locked» no, it’s likely to have a half odd multiple spin:orbit resonance (like our own Mercury). Dec 13 '16 at 5:47
• @JDługosz - That's a good point. It's thought Mercury's 3:2 spin-orbit resonance arises from its high eccentricity, which itself may be due to perturbations from other planets. In tightly planetary systems orbiting red dwarfs I imagine there could be even stronger gravitational perturbations. Dec 13 '16 at 14:44
• I mean to post a self-answerd Q on this, but need to track down some references. Yes, eccentricity encourages the case as preferred over 1:1, and outer planets cause eccetricity. Dec 13 '16 at 16:45

Red or blue shift of a wolf-rayet star. WR stars are characterized by emission spectra (i.e. strong emission lines relative to the black-body component, rather than absorption lines in a black-body spectrum) so it should in principle be possible for one to appear green with the appropriate doppler shift.

• Welcome to Worldbuilding. I appreciate the answer but I cannot accept it. The question implies that the star's color should look the same in different places, including up close; and other answers have already suggested this. Dec 14 '16 at 21:24