S Monocerotis

StarDate: February 22, 2009

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Orion climbs across the south tonight, marked by his distinctive three-starred belt. To the east of mighty Orion is the much fainter constellation Monoceros, the unicorn. It's not much to look at, but it hosts some powerful stars.

An example is the system known as S Monocerotis. It actually consists of two stars that orbit each other once every 24 years.

Both stars are so hot that they emit most of their energy not as visible light, but as ultraviolet light, whose wavelength is shorter than the human eye can see.

As the ultraviolet energy streaks through the galaxy, it tears electrons from hydrogen atoms in big clouds of gas around the stars. When the electrons rejoin the hydrogen atoms, they produce red light. So this region of Monoceros abounds with gas clouds that glow bright red or pink.

Each star in the system is roughly two dozen times as massive as the Sun. Such hefty stars share a common fate: they will blast themselves to bits as supernovae. But the two stars won't explode at the same time. When the first one goes, it'll free the other one from their mutual orbit. Even so, the second star will also explode someday, ending the brilliant career of one of the most powerful star systems in the galaxy.

S Monocerotis is about 2400 light-years from Earth -- much farther than the brightest stars of Orion. Yet it's so powerful that you can see it with the unaided eye -- a pair of giants shining through the vast cosmic night.

Script by Ken Croswell, Copyright 2008

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