What is a Y dwarf?
For over a century, astronomers have used a system of stellar classification that described most stars as one of seven types depending on their temperature. The types were O, B, A, F, G, K, and M, but the letters do not stand for anything. The recent development of infrared detectors have allowed new types of stars to be discovered. Known as "brown dwarfs," these objects are not powered by nuclear fusion (converting hydrogen into helium), so they simply cool and fade over millions of years.
The new types of brown dwarfs were given the names L, T and Y. Y dwarfs are thought to be 450 Kelvins (350 degrees Fahrenheit) and cooler. Some of them are room temperature. Their atmospheres include simple molecules like methane (CH4), water (H2O), and ammonia (NH3). WISE J085510.83−071442.5 is both the coolest known Y dwarf and the closest. It is only 7.2 light-years away -- only three other star systems are closer to the Sun. Astronomers believe it is about 250K (-10F), cold enough that it should have clouds of water ice. If confirmed, it will be the first object outside the solar system known to have water clouds.
-John Gizis
