Communities

Writing
Writing
Codidact Meta
Codidact Meta
The Great Outdoors
The Great Outdoors
Photography & Video
Photography & Video
Scientific Speculation
Scientific Speculation
Cooking
Cooking
Electrical Engineering
Electrical Engineering
Judaism
Judaism
Languages & Linguistics
Languages & Linguistics
Software Development
Software Development
Mathematics
Mathematics
Christianity
Christianity
Code Golf
Code Golf
Music
Music
Physics
Physics
Linux Systems
Linux Systems
Power Users
Power Users
Tabletop RPGs
Tabletop RPGs
Community Proposals
Community Proposals
tag:snake search within a tag
answers:0 unanswered questions
user:xxxx search by author id
score:0.5 posts with 0.5+ score
"snake oil" exact phrase
votes:4 posts with 4+ votes
created:<1w created < 1 week ago
post_type:xxxx type of post
Search help
Notifications
Mark all as read See all your notifications »
Q&A

What could make a star green?

+0
−0

Stars are never green.

enter image description here

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.

History
Why does this post require moderator attention?
You might want to add some details to your flag.
Why should this post be closed?

This post was sourced from https://worldbuilding.stackexchange.com/q/64344. It is licensed under CC BY-SA 3.0.

0 comment threads

1 answer

+0
−0

Give it a circumstellar cloud of oxygen.

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

Image of NGC 6826
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.


History
Why does this post require moderator attention?
You might want to add some details to your flag.

0 comment threads

Sign up to answer this question »