This is scientific American 60 second science. I'm Christopher in Tagyatta.

The sun spews out ultraviolet radiation and that's why you put on sunscreen.

But the sun isn't the only UV-producing

celestial body. Stars and supermassive black holes produce a huge amount of

UV radiation. Michaeli Fumagali, an astrophysicist at Durham University in the UK.

Some of this radiation can escape a galaxy,

and so this radiation builds up this cosmic UV background.

That cosmic UV background permeates the universe,

but it's diffuse, meaning hard to measure, especially from here on Earth.

And so the way we do this measurement is with a little trick.

That is when UV radiation hits gas,

he gives off a red glow.

So Fumagali and his team

used what's called the Muse Instrument

at the very large telescope in Chile

to stare for hours at the edge of a super-thin

galaxy until they saw that red glow. And since they knew how much gas was there, they

were able to calculate the intensity of the UV radiation hitting the gas, that cosmic

UV background. The finding is in the monthly notices of the Royal Astronomical Society.

That calculation offered clues to another celestial mystery, which is why we don't see as many small

galaxies in the universe as theory would predict.

Fumigali says the UV background radiation might strip away valuable star-forming gas from the small guys.

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