# What could make a star violet?

Part of a series I'm starting since the recent What could make a star green? did quite well and others could use the information here in their worlds.

## There aren't any "violet" stars.

When a star emits a significant amount of violet light, it also emits blue light, which humans are better at detecting. If any stars out there are actually violet, we see them as dark blue.

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

The criteria for this are the same as they were with the previous question:

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 violet 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 violet; it should appear violet(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 violet color from either an actual binary or an optical binary; this star should be standalone

While the ideas provided in the answers here may overlap those in the sister question to some degree, there are significant differences: the likes of oxygen and chlorophyl, for example, will not produce violet.

• A blue dwarf star which has been told a man's joke doesn't count, I suppose. – Alexander von Wernherr Dec 15 '16 at 13:10
• Generally "purple" is taken to mean the non-spectral colors intermediate between blue and red. The object needs to emit both reddish and blueish wavelengths. – AlexP Dec 15 '16 at 13:20
• @AlexP Updated question accordingly – Zxyrra Dec 15 '16 at 13:30
• Create the illusion of green color from either an actual binary or an optical binary; this star should be standalone - you need to adjust this for violet, not green – Raisus Dec 15 '16 at 13:44

## Give it a circumstellar cloud of Argon

Some planetary nebulae, such as the Crab Nebulae appear purple because of ionized Argon.

Human eyes have evolved to view yellow and green radiation, presumably because our sun emits radiation primarily in those wavelengths.

A green star is radiating right in the center of the visible light spectrum, which means it is emitting some light in all the possible colors. The star would therefore appear white — a combination of all colors. Earth's sun emits a lot of green light, but humans see it as white.

Purple stars are something the human eye won't easily see because our eyes are more sensitive to blue light. Since a star emitting purple light also sends out blue light — the two colors are next to one another on the visible light spectrum — the human eye primarily picks up the blue light. Reference here

• Astrophysics isn't really mine but wouldn't the cloud get sucked into the star? Or how fast would the gas have to rotate around the star, so it is not sucked in by gravity? – Alexander von Wernherr Dec 15 '16 at 13:56
• Argon is a Noble gas, generally speaking, it's not very reactive, plus with something like the Crab Nebulae, it was formed from stellar remnants – Raisus Dec 15 '16 at 14:05
• I guess it would depend on how close the gas would be to the star for it to be effectively making the star appear purple/violet – Raisus Dec 15 '16 at 14:07

How about a star that is violet, when seen from some viewpoints, via the doppler shift?

If a blue-dominant star (say peak emission nominally 470 nm) were coming toward an observer fast enough, the light -- including that dominant blue peak -- would decrease in wavelength. Enough speed, and the peak would sit in the violet band of the spectrum. A back of the envelope calc. suggests that the star needs about 10% of the speed of light to so shift it to violet (420 nm nominal.) A cooler (longer wavelength) star would need a greater approach speed. Seen from behind, the star would appear red shifted.

To appear violet, the star doesn't have to be coming straight at the observer. But, what if it is headed right at you?

• Purple stars are something the human eye won't easily see because our eyes are more sensitive to blue light. Since a star emitting purple light also sends out blue light — the two colors are next to one another on the visible light spectrum — the human eye primarily picks up the blue light. – Raisus Dec 15 '16 at 14:08
• I assert that for a sufficient doppler shift, the eye's greater sensitivity to blue (based on the photochemistry of our cone cells in the retina) could be overcome, so call it 15% of lightspeed, instead of ten. I also note that there are substantial variations in ability to discriminate colors. Some people see indigo as a distinct color, though I cannot. – Catalyst Dec 15 '16 at 14:11
• See the answers on the sister question - this will not be the case for all perspectives, so I cannot accept this answer. – Zxyrra Dec 15 '16 at 14:11

A couple of points, if I may.

That gorgeous lavender you see in pictures of massive stars and the nebulae around them is hydrogen. The Balmer alpha line (656.3 nm) is red. Hydrogen alpha photos of the sun, for instance, are true-color pictures. But the Balmer delta line (410.2 nm) is purple, as is Balmer-epsilon (although that one may be outside the range of many peoples' vision). The physics to understand what I just said is in Wikipedia, so I won't bother to explain. On a world in a starburst galaxy, you could be close enough to something like the Tarantula Nebula or 30 Doradus to see that glowing hydrogen with the naked eye.

A number of kinds of star are actually colored ultraviolet. Only a small part of the light they emit is reddish enough for us to see. Central stars of planetary nebulae, for example, are usually hotter than 50000 K, which we see as blue-white. If you could shift such a star's spectrum until its peak emission was near 400 nm, like hydrogen's Balmer epsilon, you would see those stars as distinctly blue, probably with some purple thrown in. (Assuming human eyes. Our own world has creatures which see well into the ultraviolet.)

Thanks.

A solar system where an abundance of Arcanite asteroids near the center of of the system would allow the introduction of potassium sulfide into the outer shell of the sun and be expelled as a burst of violet plasma.