The Horsehead Nebula: A Dark Cloud With a Familiar Shape

The Milky Way Galaxy is an amazing place. It's filled with with stars and planets as far as astronomers can see. It also has these mysterious regions, clouds of gas and dust, called "nebulae". Some of these places are formed when stars die, but many others are filled with cold gases and dust particles that are the building blocks of stars and planets. Such regions are called "dark nebulae". The process of starbirth often begins in them. As stars are born in these cosmic crèches, they heat up the leftover clouds and cause them glow, forming what astronomers call "emission nebulae".

One of the most familiar and beautiful of these space places is called the Horsehead Nebula, known to astronomers as Barnard 33. It lies about 1,500 light-years from Earth and is between two and three light-years across. Due to the complex shapes of its clouds that are lit by nearby stars, it appears to us to have the shape of a horse's head. That dark head-shaped region is filled with hydrogen gas and grains of dust. It's very similar to the cosmic Pillars of Creation, where stars are also being born in clouds of gas and dust.

The Depths of the Horsehead Nebula

The Horsehead is part of a larger complex of nebulae called the Orion Molecular Cloud, which spans the constellation of Orion. Studded around the complex are little nurseries where stars are being born, forced into the birth process when the cloud materials are pressed together by shock waves from nearby stars or stellar explosions. The Horsehead itself is a very dense cloud of gas and dust that is backlit by very bright young stars. Their heat and radiation cause the clouds surrounding the Horsehead to glow, but the Horsehead blocks light from directly behind it and that's what makes it appear to glow against the backdrop of reddish clouds. The nebula itself is made up largely of cold molecular hydrogen, which gives off very little heat and no light. That's why the Horsehead appears dark. The thickness of its clouds also block the light from any stars within and behind.

Are there stars forming in the Horsehead? It's hard to tell. It would make sense that there could be some stars being born there. That's what cold clouds of hydrogen and dust do: they form stars. In this case, astronomers don't know for sure. Infrared light views of the nebula do show some parts of the cloud's interior, but in some regions, it's so thick that the IR light can't get through to reveal any star birth nurseries. So, it's possible that there could be newborn protostellar objects hidden deep inside. Perhaps a new generation of infrared-sensitive telescopes will someday be able to peer through the thickest parts of the clouds to reveal star birth créches. In any case, the Horsehead and nebulae like it give a peek at what our own solar system's birth cloud might have looked like.

Dissipating the Horsehead

The Horsehead Nebula is a short-lived object. It will last perhaps another 5 billion years, buffeted by radiation from young nearby stars and their stellar winds. Eventually, their ultraviolet radiation will erode away the dust and gas, and if there are any stars forming inside, they will use up a lot of the material, too. This is the fate of most nebulae where stars form — they get consumed by the starbirth activity going on inside. Stars that form withini the cloud and in nearby regions emit such strong radiation that whatever is left over is eaten away by a process called photodissociation. It literally means that the radiation tears apart the molecules of gas and blows away the dust. So, about the time that our own star begins to expand and consume its planets, the Horsehead Nebula will be gone, and in its place will be a sprinkling of hot, massive blue stars.

Observing the Horsehead

This nebula is a challenging target for amateur astronomers to observe. That's because it's so dark and dim and distant. However, with a good telescope and the right eyepiece, a dedicated observer can find it in the winter skies of the northern hemisphere (summer in the southern hemisphere). It appears in the eyepiece as a dim grayish fog, with bright regions surrounding the Horsehead and another bright nebulae below it.

Many observers photograph the nebula using time-exposure techniques. This allows them to gather more of the dim light and get a satisfying view that the eye just can't capture. An even better way is to explore the Hubble Space Telescope's views of the Horsehead Nebula in both visible and infrared light. They provide a level of detail that keeps the armchair astronomer gasping at the beauty of such a short-lived, but important galactic object. 

Key Takeaways

• The Horsehead Nebula is part of the Orion Molecular Cloud complex.
• The nebula is a cloud of cold gas and dust in the shape of a horse's head.
• Bright nearby stars are backlighting the nebula. Their radiation will eventually eat away at the cloud and eventually destroy it in about five billion years.
• The Horsehead lies about 1,500 light-years away from Earth.

Sources

• “Bok Globule | COSMOS.” Centre for Astrophysics and Supercomputing, astronomy.swin.edu.au/cosmos/B/Bok Globule.
• Hubble 25 Anniversary, hubble25th.org/images/4.
• “Nebulae.” NASA, NASA, www.nasa.gov/subject/6893/nebulae.