Using the NASA Wide-field Infrared Survey Explorer (WISE), astronomers have found the coolest stars yet, confirming the existence of the spectral class L dwarfs.
Stars are often classified by their spectral type which can be used to determine their surface temperatures (also known as effective temperatures). Since the confirmed discovery of brown dwarfs 1995, the hunt has been on for cooler and cooler stars and sub-stellar objects. Many brown dwarfs are of spectral class M, which is dominated by strong absorption of TiO and VO. But even cooler stars were found with the Two Micron All Sky Survey (2MASS), leading to a new spectral classification. The optical spectra of L stars show strong metal hydride lines (FeH, CrH, MgH, CaH) as well as neutral metal lines, while their near-infrared spectra show absorption bands of H2O and CO. L stars have effective temperatures of 1,400-2,000 K and are a mix of both very low mass stars and brown dwarfs (sub-stellar objects). Another new spectral class is the T dwarfs. Their near-infrared spectrum shows methane (CH4) absorption bands, similar to the atmospheres of the giant planets. Their optical spectra lack the metal hydrides of the L stars and instead has very broad metal Na and K absorption features. The surface of T dwarfs is between 700-1,300 K and these are all thought to be brown dwarfs.
What about even cooler stars (or sub-stellar objects)? Things get a bit murky as we move to cooler and cooler brown dwarfs, as it becomes tricky to separate them from giant planets. Deacon & Hambly (2006) proposed yet another stellar class comprising ultra-cool dwarfs called Y dwarfs.
These hypothesised Y dwarfs have effective temperatures less than 500 K. And this week the discovery of six Y dwarfs found with the NASA Wide-field Infrared Survey Explorer (WISE) was announced. These stellar objects have effective temperatures between 500 K and 300K, and the new record holder as the coolest brown dwarf yet discovered is WISE 1828+2650, with an effective temperature of just 300K, which is about 25°C – cooler than a human body!
Read more at the NASA press release NASA’S Wise Mission Discovers Coolest Class of Stars, and the discovery paper by Cushing et al. (2011).