Since astronomers discovered the Smith Cloud, a giant gas cloud plummeting toward the Milky Way, they have been unable to determine its composition, which would hold clues as to its origin. Astronomers have now determined that the cloud contains elements similar to our sun, which means the cloud originated in the Milky Way’s outer edges and not in intergalactic space as some have speculated.
The astronomers found that the colossal cloud loud is as rich in sulfur as the Milky Way’s outer disk, a region about 40,000 light-years from the galaxy’s center and about 15,000 light-years farther out than our sun and solar system are. This means that it was polluted by material from stars. This would not happen if it were pristine hydrogen from outside the galaxy. Instead, the cloud appears to have had an intimate relationship with the Milky Way, but was somehow ejected from the outer Milky Way disk about 70 million years ago and is now boomeranging back onto its disk.