Zoo in the Sky
If
you wanted to see a pelican, a crab, or a swan, where would
you look? The answer is simple—a zoo!
But if you were an astronomer, you wouldn’t have to go to a zoo at all. With a powerful telescope and a little imagination, you could spot each of these creatures in the night sky.
This isn’t as strange as it sounds. The night sky has always excited people’s imaginations. When people of long ago looked up at the stars, they thought they could see the shapes of beasts, birds, and mythical heroes. Some of their fanciful sky pictures became our present-day constellations.
Much later, when astronomers began exploring the sky with telescopes, they saw—well, mostly they saw more stars. But they also saw strange, hazy patches of light.
Huge
Clouds
Astronomers
called each of these patches a nebula (NEB-you-lah), from
the Latin word for cloud. A nebula really is a cloud. But
it’s a cloud of gas and dust.
The more astronomers studied nebulae (NEB-you-lee), which means more than one nebula, the more familiar some of them looked. Just as you might see the figure of a clown or a cow in a cloud, astronomers saw ghostly shapes floating in space. For example, they called one nebula the Swan and another the Crab. They even named one of them the Horsehead.
Nebulae, like clouds, are in constant motion. But you would have to watch a nebula for years to notice a change. Nebulae are so huge and so far away that, to us, they seem frozen in space.
Star
Birth
Some
nebulae are the birth-places of new stars, and others are
the debris from dead or dying stars. But all nebulae have
one thing in common. They are made of the same materials
as stars.
The nebulae where stars are born are made of hydrogen gas, helium gas, and dust. Some of them are dark. We know dark nebulae exist because some of them block out the light of stars behind them. The most famous of all dark nebulae, the Horsehead, lies in front of a bright nebula.
Over many millions of years, stars can form inside a dark nebula. If one of these stars is very hot—much hotter than the Sun—that star could transform the dark cloud into a bright nebula. These nebulae appear red in photographs because of the energy they receive from the hot stars within them. Astronomers think that every star—including our Sun—was born in a nebula like the Swan, the Horsehead, or the Pelican.
When
Stars Die
Stars
die, too. Most die quietly, creating another kind of nebula.
The outer layers of these stars expand into thin gaseous
shells called planetary nebulae, such as the Cat’s
Eye.
At the center of these nebulae are the stars’ super-hot, shrunken cores, which are called white dwarfs. The white dwarfs cause the nebulous shells to glow with a pale light.
These nebulae are smaller than other nebulae. In fact, when they were discovered, astronomers thought the nebulae looked like planets. That’s why scientists called them planetary nebulae.
But
not all dying stars are so well-behaved. Very massive stars
end their lives in tremendous explosions called supernovae.
No event in our galaxy is more violent. For a few brilliant
moments, a supernova can out-shine a billion suns. About
nine hundred years ago, one blazed so brightly it could
be seen in the daytime. The nebula it left behind is named
the Crab.
The nebulae in the photographs above are called the Swan, Crab, Horsehead, Pelican, and Cat’s Eye nebulae. Can you see these creatures? Tip: Try looking at them from several feet away.
Answers: A. Horsehead. B. Crab. C. Swan. D. Cat’s Eye. E. Pelican.
