Free Astronomy Magazine March-April 2016

SPACE CHRONICLES Monstrous cloud boomerangs back to our galaxy by NASA H ubble Space Telescope astronomers are find- ing that the old adage "what goes up must come down" even applies to an immense cloud of hydrogen gas outside our Milky Way galaxy. The invisible cloud is plummeting toward our gal- axy at nearly 700,000 miles per hour. Though hundreds of enormous, high-velocity gas clouds whiz around the outskirts of our galaxy, this so-called "Smith Cloud" is unique because its trajectory is well known. New Hubble observations suggest it was launched from the outer re- gions of the galactic disk, around 70 million years ago. The cloud was discovered in the early 1960s by doctor- al astronomy student Gail Smith, who detected the radio waves emitted by its hydrogen. The cloud is on a return collision course and is expected to plow into the Milky Way's disk in about 30 million years. When it does, astronomers believe it will ignite a spectacular burst of star formation, perhaps providing enough gas to make 2 million suns. "The cloud is an example of how the galaxy is changing with time," ex- T his diagram shows the 100-million-year-long trajectory of the Smith Cloud as it arcs out of the plane of our Milky Way galaxy and then returns like a boomerang. Hubble Space Telescope measure- ments show that the cloud, because of its chemical composition, came out of a region near the edge of the galaxy's disk of stars 70 million years ago. The cloud is now stretched into the shape of a comet by gravity and gas pressure. Following a ballistic path, the cloud will fall back into the disk and trigger new star formation 30 million years from now. [NASA, ESA, and A. Feild (STScI)] plained team leader Andrew Fox of the Space Telescope Science Insti- tute in Baltimore, Maryland. "It's telling us that the Milky Way is a bubbling, very active place where gas can be thrown out of one part of the disk and then return back down into another." "Our galaxy is recycling its gas through clouds, the Smith Cloud being one ex- ample, and will form stars in different places than before. Hubble's measurements of the Smith Cloud are helping us to visualize how active the disks of galaxies are," Fox said. Astronomers have measured this comet-shaped region of gas to be 11,000 light-years long and 2,500 light-years across. If the cloud could be seen in visible light, it would span the sky with an apparent diameter 30 times greater than the size of the