A New Spin on a Space Horse

This cloud in outer space is turning to face us.

Clouds in the sky can look like rabbits, bears, and lions. They may even look like people. These clouds are made of water and ice.

Much farther away, other clouds are scattered through outer space. These clouds are not made of water. But like the clouds in the air, they can also make familiar shapes.

In fact, one space cloud looks like a horse.

Now astronomers may have found out why it has that shape: the cloud is spinning.

A Giant Game Piece

This cloud is the Horsehead Nebula. (Nebula is the Latin word for “cloud.”) It looks like a black knight in a chess game. It’s eerie and haunting, one of the most famous clouds in space.

Like other space clouds, the Horsehead Nebula is made mostly of hydrogen and helium gas, the two lightest and most common elements in the universe. These gases are clear. Light goes right through them. So why is the Horsehead black? Dust particles, made mostly of heavier elements, pepper the clear gas and block visible light.

The gas and dust in the Horsehead Nebula do not make any light that your eye can see. But the gas makes radio waves, which we can detect using special telescopes called radio telescopes. You can’t look through a radio telescope the way you would look through a normal telescope. Instead, the radio telescope is a giant, dish-shaped antenna.

In 2003, astronomers in Europe used a big radio telescope in Spain to observe the gas in the Horsehead Nebula. They made a big discovery. They found that the Horsehead Nebula is slowly spinning. 

Clues in the Waves

Here’s how they did it. They observed changes in the radio waves from the Horsehead Nebula’s gas.

When an object that gives off radio waves is moving toward us, its radio waves get scrunched up a little. So they have a shorter wavelength than usual.

We call this change to shorter waves a blueshift, because blue light waves are short.

In contrast, when an object that gives off radio waves is moving away from us, its radio waves are stretched out, so they’re longer than usual. We call this change to longer waves a redshift, because red light has a longer wavelength than blue light.

You have probably heard blueshifts and redshifts, because sound waves make them, too. When a fire truck races toward you with its siren blaring, the siren’s sound waves get scrunched up, making a shorter wavelength, and you hear a high-pitched siren: a blueshift. After the fire truck speeds by, the siren is moving away from you, so the sound waves get stretched out, making a longer wavelength, and the siren’s pitch drops: a redshift.

Compared with the Horsehead’s neck, the nose has scrunched-up radio waves, a blueshift, so it’s moving toward us. Part of the horse’s mane has stretched-out radio waves, a redshift, so it’s moving away from us. Thus, the Horsehead is turning. It’s as if the horse is trying to look our way! 

Slowly Turning  

Astronomers estimate that the horse will take four million years to spin once. In contrast, the Sun takes just a month to spin, and Earth takes just a day to spin.

But the Horsehead Nebula is much bigger than the Sun or Earth.

Furthermore, the Horsehead’s spin may explain its unique shape. To see how, imagine riding a merry-go-round. Now imagine that the merry-go-round started spinning really, really fast. It would fling you off.

In the same way, the nebula’s spin may be flinging out two pieces of gas and dust from the horse’s neck. One piece has become the horse’s nose. The other piece has become the horse’s mane.

So if the Horsehead Nebula were not spinning, it probably wouldn’t look like a horse.

Then the nebula wouldn’t be famous, and you wouldn’t be reading this article, because it wouldn’t exist!