Young stars tend to be messy. That's part of the creation process, but after the 10-million-year mark or so, the gas and dust around them tends to clear up as they set about the business of creating planets and all the other things that normal, not so young stars do.
However, one star in the constellation Cetus, 49 CETI, doesn't seem to want to give up its ways and is still surrounded by huge amounts of gas 40 million years or so after it was created.
Astronomers have now figured out why such an old star has so much gas around it and it has to do with comets. Hundreds of trillions of comets, in fact, which you'll agree, is a fairly large number.
And most of these comets are a mile wide, 1.6 km, circling their star. With so many of them around the star, two comets collide roughly every six seconds, releasing gas, carbon dioxide, in the process. This has been going on for 10 million years.
This comet cloud is similar to the Sun's Kuiper belt. But whereas the Kuiper Belt, which contains a huge number of objects, only weighs about 10 percent as much as the Earth, and that's including Pluto, the comet belt around 49 CETI weighs about 400 times more than the Earth.
Astronomers believe that the comets there also contain a larger amount of CO2 than the ones around the sun. With so many comets bumping into each other, there's a lot of gas released around the star.
The study, published in the Astrophysical Journal, has looked at two stars that are too old to have this much gas around them. They have found similar conditions in both cases.
"Hundreds of trillions of comets orbit around 49 CETI and one other star whose age is about 30 million years," Benjamin Zuckerman, a UCLA professor of physics and astronomy and co-author of the paper, said.
"When they collide, the carbon monoxide escapes as a gas. The gas seen around these two stars is the result of the incredible number of collisions among these comets," he explained.
"We calculate that comets collide around these two stars about every six seconds. I was absolutely amazed when we calculated this rapid rate. I would not have dreamt it in a million years," he added.