A dark cloud of dust forms the outline of a horse's head in this image shot through a telescope at McDonald Observatory. The outline, known as the Horsehead Nebula, is silhouetted against a more-distant cloud of glowing hydrogen gas, which is energized by hot young stars. The energy from those stars is slowly eroding the Horsehead. [Tom Montemayor/McDonald Observatory]
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If you watch the clouds on a lazy afternoon, you’ll see all kinds of shapes drift by — dragons, castles, presidents, and more. The same thing happens as you watch the clouds of gas and dust that fill the Milky Way galaxy — watch long enough, and familiar shapes will pop into view. One looks like California, another resembles North America, and still another looks like an eagle.
Like the clouds in an afternoon sky, though, the celestial clouds will evaporate and disappear from view.
Consider one of the most famous objects in the sky, the Horsehead Nebula in the constellation Orion.
It’s actually formed by two clouds. One is a bright pink cloud of hydrogen gas. It’s zapped by ultraviolet radiation from hot young stars, so it glows like a neon bulb. The second cloud is in front of this one, and it consists of cool, dark gas. It blocks the light of the bright cloud behind it, forming the outline of a horse’s head.
But the Horsehead is slowly vanishing. The same stars that cause the pink cloud to glow are eroding the dark cloud that forms the nebula. Within a few thousand years, it’ll disappear entirely.
For now, look for the Horsehead beginning in mid-evening. It stands just below Orion’s Belt, a short line of three bright stars that stands almost straight up from the southeastern horizon by about 8 o'clock. The Horsehead is small and faint, but if you have dark skies, it’s visible in fairly small telescopes.
Script by Damond Benningfield, Copyright 2011