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January 28, 2022

Scientists discover coldest class of stars

By Mo-Yu Zhou | September 7, 2011

NASA scientists have found a class of star-like bodies that are the coldest discovered to date. Their temperatures, which have been detected using the Wide-field Infrared Survey Explorer (WISE), can be even cooler than the human body.

These so-called Y dwarfs have been searched for unsuccessfully by astronomers for over a decade. The difficulty lies in the fact that a visible-light telescope does not suffice to detect them. Rather, an infrared-light telescope, such as WISE, must be used. WISE has the additional benefit of being able to produce an enormous amount of data; from January 2010 to February 2011, it scanned the entire sky about 1.5 times.

Because the Y dwarfs appear brighter – in fact, 5000 times brighter – at the longer infrared wavelengths, and because of the thoroughness of WISE's scanning capabilities, the telescope was able to spot six Y dwarfs at a distance of up to 40 light-years. These results were published in the Astrophysical Journal, with Michael Cushing of NASA's Jet Propulsion Laboratory in Pasadena, California as the lead author of the paper.

Y dwarfs are members of the brown dwarf family, which contains stars known as ‘failed' stars. These stars have masses too low to enable them to fuse atoms at their cores and generate energy at a level on par with other stars. In other words, unlike stars like our sun that can shine for billions of years, they cool and fade over time. They emit very little light, and this light is in infrared, rather than visible light form.

Brown dwarfs are interesting to scientists because they reveal information about how stars form and what the atmospheres of planets are like. In fact, gas-giant planets such as Jupiter have atmospheres that are similar to those of brown dwarfs. An additional benefit of brown dwarfs is that they are easier to observe away from the bright light of a star, such as the sun, as they are alone in space.

Data from WISE was supplemented by using some of the most powerful telescopes, such as the Hubble Space Telescope, to definitively confirm that brown dwarf candidates were indeed brown dwarfs and, in certain cases, Y dwarfs. This was done by splitting apart the objects' light and looking for signs of water, methane, or ammonia. Compared to the other brown dwarfs, the Y dwarfs had special spectral features that indicated their lower atmospheric temperatures.

In total, WISE data has revealed 100 new brown dwarfs. Most of the brown dwarfs, however, are much warmer than the Y dwarfs, the coolest of which has a lower temperature than room temperature (around 80 degrees Fahrenheit or 25 degrees Celsius).

More brown dwarfs are expected to be revealed as data from the latest scan continues to be analyzed. Currently, the closest of the Y dwarfs discovered so far is about nine light-years away, making it the seventh closest star system. Thanks to the capabilities of WISE, it is possible that scientists may find a brown dwarf closer to us than our closest known star, Proxima Centauri, at four light-years away.

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