Look to the east of brilliant Orion tonight for the obscure constellation Monoceros, the unicorn. The Milky Way runs through Monoceros, which is home to one of the galaxy's most breathtaking sights: the Cone Nebula.
Lying 2500 light-years from Earth, the Cone Nebula is a dark pillar of gas and dust that stands in front of a red glow. Appropriately enough for the unicorn, the cone itself looks a bit like a horn. William Herschel spotted the nebula in the 1780s, a few years after he made history by discovering the planet Uranus.
The red glow behind the Cone Nebula comes from hydrogen gas. The ultraviolet light from a very hot star about 30 light-years from the nebula tears electrons away from the hydrogen atoms. When the electrons rejoin the atoms, they can emit red light.
The Cone Nebula itself is about seven light-years long. That's a little less than twice the distance from the Sun to the nearest star beyond the Sun. The dark gas that makes up the nebula is mostly molecular hydrogen -- two hydrogen atoms joined together. Dust mixed into the gas darkens the entire cloud.
This gas is dense enough that some of it has collapsed to give birth to new stars. Many of these stars resemble what the newborn Sun looked like four and a half billion years ago. And indeed, the Sun and Earth may well have been born in a dark cloud, now vanished, whose beauty rivaled that of the Cone Nebula.
We'll have more about Monoceros tomorrow.
Script by Ken Croswell, Copyright 2003, 2009
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