Giant hydrogen cloud headed for Milky Way

SAN JOSE, Calif. — A high-speed hydrogen cloud on a crash course with the Milky Way appears to be an exotic interloper, preliminary data suggest.

The cometlike streak, called the Smith Cloud, is as massive as a million suns and is shooting toward the galaxy at roughly 850,000 kilometers per hour. At about 40,000 light-years away, the cloud is on schedule to collide with the one of the galaxy’s spiral arms in roughly 30 million years. When it does, it could deliver extragalactic material to the Milky Way, said astronomer Jay Lockman of the National Radio Astronomy Observatory.

Lockman and his colleagues used the Green Bank Telescope in West Virginia to compare the Smith Cloud with others near the galaxy and seven massive ones that sit between the Andromeda and Triangulum galaxies. Smith’s Cloud is more similar to the ones that sit far-off, which suggests that the cloud originated billions of years ago in a distant region of space. Its trajectory indicates that the cloud has collided with the Milky Way before. With each interaction, it brings fresh, nongalactic material to the Milky Way, Lockman said February 15 at the annual meeting of the American Association for the Advancement of Science.

The results reinforce the idea that the space between galaxies is not empty. It’s messy, filled with “funny little clouds that seem to have a life of their own,” Lockman said.

Editor’s note: This story was updated February 17, 2015, to update the estimated speed of the Smith Cloud.