Astronomy Picture of the Day

Discover the cosmos! Each day we feature a different image or photograph of our fascinating universe, along with a brief explanation written by a professional astronomer.

September 20, 1995
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GL 105C: The Coolest Star?
Credit: NASA, HST, WFPC 2, D. Golimowski (JHU)

Explanation: Is the dim star to the upper right of this false-color picture the coolest possible normal star? From this recent picture by the Hubble Space Telescope, astronomers have estimated its mass is just high enough for it to fuse hydrogen into helium in its core. In general, the higher an object's mass, the higher it's core density and temperature. Above a certain point, the intense core conditions cause hydrogen atoms to move so fast that many stick or "fuse" after collision, releasing enormous amounts of energy. Were this object less massive, the object would shine by gravitational contraction and so be termed a "brown dwarf" rather than a normal main-sequence "star." The star on the left is so much brighter than the "coolest star" that it creates the white streak and dramatic pattern visible in the image. More information is given by the Space Telescope Scientific Institute's press release.

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