Massive, mysterious stream of gas swirling around the Milky Way finally explained

Scientists have finally explained a massive, mysterious cloud that is swirling around our galaxy.

Billions of years ago, our Milky Way captured two smaller galaxies, known as the Small and Large Magellanic Clouds. As our galaxy grabbed them, a huge stream of gas called the Magellanic Stream was pulled from them – and now stretches halfway across the night sky, visible with the naked eye.

Astronomers have never been sure how this huge stream was able to form with such a mass. Taken together, it is more than a billion times the mass of the Sun – and there is no explanation of why.

But now researchers have found that a halo of warm gas is wrapped around the Magellanic Clouds, protecting it like a cocoon. That keeps it safe from our galaxy's halo, as well as giving account for most of the mass of the cloud.

When the smaller galaxies moved close to the Milky Way, the halo was stretched out and sown across space, leaving the Magellanic stream.

The researchers detail their findings in a new journa, published in Nature, which suggests that previous models of how the stream could form are wrong.

"That's why we came out with a new solution that is excellent at explaining the mass of the stream, which is the most urgent question to solve," adds Elena D'Onghia, a professor of astronomy at UW-Madison who supervised the research.

There is still more work to be done. That includes tests of the proposal: if the cloud formed as the new paper claims, then Hubble Space Telescope should be able to see the corona of gas that surrounds it.

"The stream is a 50-year puzzle," says Andrew Fox, one of the co-authors of the study and an astronomer at the Space Telescope Science Institute, which operates the Hubble Space Telescope. "We never had a good explanation of where it came from. What's really exciting is that we're closing in on an explanation now."