In 1963, an American student named Gail Smith was studying astronomy in the Netherlands when she discovered a large, high-velocity cloud of hydrogen gas located in the constellation Aquila using the 25-meter Dwingeloo Radio Observatory. She named her discovery Smith’s Cloud. (Her last name has since changed to Bieger.)
The cloud is getting headlines today for a reason Smith-Beiger could not have predicted 53 years ago: the cloud's trajectory. After forming about 70 million years ago in the outer regions of the Milky Way, the gas cloud was ejected out of the galaxy, but not far and fast enough to be able to escape its gravitational pull.
This means that Smith's Cloud is boomeranging back, and our galaxy’s tremendous gravitational force is shaping it to resemble a kind of giant comet 11,000 light-years long and 2,500 light-years across. To put this in perspective, if the hydrogen cloud was visible to the naked eye, it would show up in our night sky with a diameter 30 times bigger than the moon! (See the photo above to put that into perspective.)
The galactic cloud is now estimated to be moving at about 700,000 miles per hour. At this velocity, it should return to the Milky Way in about 30 million years, entering the galactic disc at an angle of about 45 degrees, at which point astronomers believe it will cause a massive star formation event, “perhaps providing enough gas to make 2 million suns,” according to NASA.
Because Smith’s Cloud is composed of gas invisible to the naked eye, astronomers have been tracking it using radio telescopes, including the Green Bank Telescope in West Virginia. This marvelous instrument has the distinction of being the world’s largest steerable radio telescope as well as the world’s largest moveable land object, according to the National Radio Astronomy Observatory.
Astronomers were able to tell the origin of Smith’s Cloud by using Hubble’s Cosmic Origins Spectrograph, which analyzed the chemical composition of the cloud by measuring tiny changes in how light interacts with it. If the cloud had come from outside our galaxy, it would be composed of hydrogen and helium atoms, the simplest and lightest building blocks of our universe. But Hubble detected sulfur in concentrations similar to the Milky Way’s outer disk, a region about 40,000 light-years from the galaxy’s center. The research appears in the January 2016, issue of The Astrophysical Journal Letters.
As you can see in the diagram above, Hubble's Cosmic Origins Spectrograph measured how the light from distant galaxies was affected as it passed through the cloud, revealing its chemical composition.