Tuesday, December 16, 2014

The Coldest Stars in the Universe

Scientists using data from NASA's Wide-field Infrared Survey Explorer (WISE) have discovered the coldest class of star-like bodies, with temperatures as cool as the human body.
This artist's conception illustrates what brown dwarfs of different types might look like to a hypothetical interstellar traveler who has flown a spaceship to each one

Astronomers hunted these dark orbs, termed Y dwarfs, for more than a decade without success. When viewed with a visible-light telescope, they are nearly impossible to see. WISE's infrared vision allowed the telescope to finally spot the faint glow of six Y dwarfs relatively close to our sun, within a distance of about 40 light-years.

The Y's are the coldest members of the brown dwarf family. Brown dwarfs are sometimes referred to as "failed" stars. They are too low in mass to fuse atoms at their cores and thus don't burn with the fires that keep stars like our sun shining steadily for billions of years. Instead, these objects cool and fade with time, until what little light they do emit is at infrared wavelengths.

Y dwarfs belong to a larger family of objects called brown dwarfs. Brown dwarfs begin their lives like stars but they never accumulate enough mass to fuse atoms steadily at their cores and shine with starlight -- as our sun does so well. Instead, they fade and cool with time, giving off most of their light in infrared wavelengths. 
This artist's conception illustrates what a
This artist's conception illustrates what a "Y dwarf" might look like. Y dwarfs are the coldest star-like bodies known, with temperatures that can be even cooler than the human body.
While the O through K classes are all considered stars, M and L objects are a mixture of stars and brown dwarfs, and T and Y objects are all brown dwarfs. The term "brown dwarfs" was chosen because at that time, astronomers didn't know what colors these objects would actually have at the visible wavelengths our eyes see, and brown is not a true color of light (there are no "brown photons"). Astronomers now know that T dwarfs would appear reddish, or magenta, to the eye. But they are not certain what color Y dwarfs are, since these objects have not been detected at visible wavelengths. The purple color shown here was chosen mainly for artistic reasons. In addition, the Y dwarf is illustrated as reflecting a faint amount of visible starlight from interstellar space.

Astronomers study brown dwarfs to better understand how stars form, and to understand the atmospheres of planets beyond our solar system. The atmospheres of brown dwarfs are similar to those of gas-giant planets like Jupiter, but they are easier to observe because they are alone in space, away from the blinding light of a parent star.

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