# Blacklight Star

Tangentially related to The evolution of neon kittens

Is is possible for a star to radiate in the ultra violet spectrum but not much in the visible light spectrum?

If not, is there any mechanism that would block a lot of the visible light spectrum while still allowing ultra violet to pass through, or change the visible light to UV like the coating on a black light bulb, but on a planetary scale?

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Theoretically, yes. At least, according to our friends over at Astronomy.

According to them, an exceptionally cold star (for a star, of course) won't emit light in the visible spectrum, and neither will a star with a massive gravitational redshift.

But, as Lacklub points out, both of these factors will cause a redshift in the light emitted by the star. What we need for this to happen would be a blueshift, the opposite of a redshift.

A blueshift occurs when an object moves towards an observer. Hurling a star at a planet seems hazardous for the native cats, so let's try something else.

A blueshift also occurs when a photon is falling into a gravity well. If the planet had a significant gravity well, light from the star would appear in the ultraviolet spectrum. (Note: hypergravity not considered conducive to felines.)

Of course, you (well, anyone belonging to a society actually on the Kardashev scale) could always install a mechanical shell around the star that adds energy to the outbound photons, thus inducing an artificial blueshift.

Which leads to the questions: Who built this thing?

And why?

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@Frosfyre excellent answer – Lacklub Mar 11 at 16:37
@Lacklub You get some credit for pointing out flaws. Thanks. – Frostfyre Mar 11 at 16:39
Which leads to the questions: Who built this thing? And why? I assume it was the people that want glowing cats. – AndyD273 Mar 11 at 16:53
Hmm, I wonder if you could get something similar to the blueshift from falling into a gravity well if you had gravitational lensing from a star/black hole binary system. – AndyD273 Mar 11 at 17:01
This is wrong. Stars emit roughly black body radiation. Very cool stars emit most of their radiation as infra-red, not ultraviolet. Very hot stars emit more UV than visible light, but they still emit more visible light than cooler stars. You can't blueshift the visible light to UV, without blueshifting the IR to visible. – James Kilfiger Mar 11 at 22:27

There are problems with two of your ideas, but one of them could potentially work.

Is is possible for a star to radiate in the ultra violet spectrum but not much in the visible light spectrum?

This really isn't possible due to the nature of light. As you may know, the wavelength and frequency of light is inversely proportional - the higher the frequency, the shorter the wavelength. The energy carried by a photon is proportional to its frequency, as you can see in this table on Wikipedia.

Ultra violet light has a higher frequency than visible light, so each photon in the UV range carries more energy than a photon in the visible light range. In order to emit UV light the source has to have more energy available, which means that there is also more energy with which to emit more visible and infrared light.

Something else that makes this not work that stars by their nature contain a variety of elements. Having a variety of elements is why the spectrum a star outputs is very broad. There's also no way to avoid having this variety - the nuclear fusion powering a star creates more elements.

change the visible light to UV like the coating on a black light bulb

This isn't actually how black lights work. Black lights work by blocking visible light with a black paint that lets UV light through. They do a little bit of the reverse though - they include some phosphors that absorb higher-energy UV light (the kind that is more dangerous) and re-emit it as lower-energy UV light. So for the most part, they are equivalent to the idea of blocking visible light and letting UV light through.

any mechanism that would block a lot of the visible light spectrum while still allowing ultra violet to pass through

This could work. As I just mentioned, black lights work by doing this. It's conceivable that you could have a more volcanic planet that tends to spew out some material into the atmosphere that blocks visible light but not UV light (with infrared optionally being let through as well). I don't know what compounds would be best at doing this.

Of course, with most visible light being blocked, plants aren't going to be able to grow. Life on Earth has photosynthetic life at the bottom of almost every food chain. However, on a volcanic planet it's plausible to have a greater abundance of chemicals that can be used in chemosynthesis - using certain chemicals as the basis for energy needed for life instead of using sunlight.

Also, any life that did develop would likely be incapable of seeing what we consider visible light - even with florescence, the amount of light that would be produced would be less than the available UV light. They'd be much more likely to either see infrared and/or UV.

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Any explanation for the drive-by-downvote? – Rob Watts Mar 11 at 18:47
Great answer, but transparency to UV but not visible is a rare property. – James Kilfiger Mar 11 at 22:32

May I suggest a very very dark planet? For all those who haven't heard or read the name TrES-2b, please please don't click on the link I have posted above :(

Basically it is a planet that that an atmosphere so horrific and ghastly, that it reflects less than 1% of the light that falls on it. I am not certain about UV spectrum, but since UV rays are on a higher energy level than visual spectrum, you just might have UV rays getting to the surface of the planet while all visible light gets absorbed in the atmosphere.

Now if you had such a dark planet, which was not as much of hell as TrES-2b is ...

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Why would you discourage people from clicking a link you included? – Frostfyre Mar 11 at 18:12
Because the planet is beyond horrific. Scary being too weak to convey its horror. – Youstay Igo Mar 11 at 18:16
@YoustayIgo In what way is the information about this planet even a little bit scary? It's just dark. – Rob Watts Mar 11 at 18:17
Maybe I just ... fantasy too much ... – Youstay Igo Mar 11 at 18:19

In general if a star follows https://en.wikipedia.org/wiki/Black-body_radiation, is very very hot, very very far away and moving very quickly towards the observer, it could emit in the UV strongly but not very much in the visible.

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