Re: How was the horsehead nebula formed

Dear Sarah,

Nebulae are clouds of interstellar gas and dust. This gas and dust comes from a variety of sources. Much of the gas is hydrogen, some of which is essentially left over from the origin of the Universe. Other gas and the dust result from the death of stars. When stars die, either by forming planetary nebulae or supernovae, they blow off their outer layers. (Planetary nebulae are formed by stars like the Sun gently blowing off their outer layers; supernovae are massive explosions of very massive stars violently blowing off their outer layers.) Over time the matter blown off runs into the matter from other stars, comes to rest, and forms into gigantic clouds (10--100 light years across). The temperature of these clouds is often below 100 K (that's only 100 degrees above absolute zero or about -173 degrees C). At such cold temperatures, areas within the clouds start to contract, due to gravity, and form clumps. Once a clump becomes large enough it forms a new star.

The newly formed stars within the huge clouds begin to glow, which excites the nearby gases and they begin to glow as well. When another cold cloud of gas and dust lies between the bright nebula and us, it will absorb the light which we would normally see. The Horsehead Nebula is made up of just such a pair of gas clouds, one hot and bright, the other cold and dark. Why does it look like a horse's head? It's just a coincidence. If you've ever tried to make shadow puppets, every once in a while you'll get a shadow that looks just like a bird or fish. It's the same kind of thing. There are other nebulae that have reminded people of other things, such as the North American Nebula or the Eagle Nebula.

A good place to look at nebulae pictures is APOD Dark Nebula index.

Mike Francis
http://www.gis.net/~mtf/sm.htm