AUSTIN, TEXAS--It's large, it's fast, and it's heading toward the Milky Way. Less than 40 million years from now, a giant cloud of hydrogen gas, clocked at 250 kilometers per second, will smash into our home galaxy, likely setting off a huge burst of star formation. In fact, the cloud contains enough gas to form a million stars like our sun, astronomers reported here today at the 211th meeting of the American Astronomical Society. The finding also indicates that pristine material is still entering the relatively mature Milky Way.
Many clouds of hydrogen surround the Milky Way. But astronomers didn't start spotting them until a half-century ago--after the advent of radio telescopes, which are able to detect cold, neutral hydrogen gas. The early observations were not accurate enough to determine the clouds' distances, masses, or directions of motion, however.
Now, thanks to more powerful telescopes such as the 100-meter Robert C. Byrd Green Bank Telescope in Green Bank, West Virginia, these clouds are finally getting their close-up. The first to be spotlighted in extreme detail is Smith's Cloud, named after Dutch astronomy student Gail Smith, who discovered it in 1963. Curious about the cloud's elongated shape, a team of astronomers led by Felix "Jay" Lockman of the National Radio Astronomy Observatory in Charlottesville, Virginia, took tens of thousands of radio brightness measurements. The data reveal that the cloud is just 8000 light-years away from the Milky Way's central plane, making it the closest one known. Its cometary shape is apparently due to the tidal effects of the Milky Way.
According to team member Robert Benjamin of the University of Wisconsin, Whitewater, the cloud is also closing in very fast on our galaxy. At its current speed, Smith's Cloud will collide with interstellar gas in the Milky Way's disk in less than 40 million years, says Benjamin. The crash will happen far from us, but it could still put on quite a show as the cloud's gas condenses into tens of thousands of bright, massive stars that will explode as supernovas within a couple of million years.
W. Butler Burton, a retired radio astronomer and expert on these fast-moving hydrogen clouds, says the new observations are a further step in solving the riddle of these cosmic beasts: "We never knew whether the clouds were blown out of the Milky Way, only to fall back at a later stage, or whether they are pristine intergalactic clouds falling in for the first time." Because Smith's Cloud turns out to contain almost no elements heavier than hydrogen and helium, which are relatively common in the Milky Way, it looks like a first-time visitor. Says Burton: "We might be witnessing the final stages of the formation process of our galaxy."