Nebulae

Gas, dust and debris from star explosions and around star-forming regions are called nebulae (plural of nebula). Nebulae are often very large and span across many light-years.

Image at right: The Horsehead Nebula, Nigel Sharp (NOAO), KPNO, AURA, NSF (courtesy of NASA).

The term nebula comes from the Latin word for "cloud." Nebulae are crucial to the life cycle of stars. The gas and dust in cool, dense nebulae can sometimes collapse to make new stars, while dying stars and supernovae create nebulae from the gases they expel. Eventually, these hot nebulae cool off and can form new generations of stars.

Features

Nebulae come in a variety of shapes — from wispy to ring-like. Their appearance is due to the type of energy source that lights them up. Most nebulae can be described as diffuse, meaning they contain no well defined boundaries. A diffuse nebula can reflect light from a nearby star (reflection nebulae) or absorb light (absorption or dark nebulae). Emission nebulae are hot enough (typically 10,000 °C) that they can emit their own light. Another type of nebulae are "planetary nebulae" (which got their name because they looked like planets in early small telescopes; they actually have nothing to do with planets).

Planetary nebulae form when a dying star begins to shed its outer layers. These outer layers slowly drift away from the star, forming such objects as the Ring Nebula and the Dumbbell Nebula. A similar fate awaits our own Sun in 4 to 5 billion years. The nebulae created from a supernova explosion (such as the Crab Nebula) can contain million degree gas-so hot that much of its light is emitted as x-rays.

The Crab Nebula

: The Crab Nebula from Hubble (Courtesy of NASA, ESA, J. Hester, A. Loll (ASU))

The Crab Nebula is centered around the location of a supernova explosion of a star observed by Chinese and Arab astronomers in 1054. Today, we know that the center contains the Crab Pulsar, a rotating neutron star. The neutron star at the center is about the size of a city, but is very dense and puts out 100,000 times the energy emitted by our Sun. We call it a "pulsar" because the star rotates about thirty times per second and therefore appears to pulse. We measure the energy the Crab Pulsar puts out by using the electromagnetic spectrum. Learn more about the electromagnetic system from NASA. Telescopes that measure different types of energy can be used to see the same object in new ways. Take a look at the NASA Fact Sheet to see what scientists have learned about the Crab Nebula by studying these different types of energy.

To observe the Crab Nebula in the Taurus constellation, you will need a dark, clear night and a pair of binoculars or a telescope. Use this sky map from DePaul University to help you point your lens in the right direction.

Explore more features of nebulae at Space.com.

Missions

: Hubble Space Telescope (Courtesy of NASA)

Launched in 1990, the Hubble Space Telescope orbits Earth at an altitude of 380 miles (612 kilometers), while traveling at 17,500 miles per hour (28,000 kilometers per hour). One orbit takes only 97 minutes at this speed. Hubble has provided one of the largest and highest resolution images ever made of the entire Crab Nebula.

NASA's Chandra X-ray Observatory was launched and deployed by Space Shuttle Columbia in July 1999. An image taken by Chandra also provides significant clues to the workings of the mighty Crab Nebula 6,000 light-years from Earth.

Find out more about Hubble at the Hubble Site.

Myths, Stories, and More

: Constellation of Orion (Courtesy of NASA)

The Orion Nebula in the constellation Orion is one of the most beautiful objects in the northern night sky. You can find it with binoculars or a telescope. Hanging from Orion's belt is a sword of three faint stars. The middle "star" is actually the Orion Nebula, a huge cloud of glowing gas 1,500 light-years from Earth.

In mythology, Orion stands by the river Eridanus with his faithful dogs, Canis Major and Canis Minor, hunting animals like Lepus, the rabbit, and Taurus, the bull.

Learn more about Orion at Suite101.com

Earth Matters

: Orion Nebula in visible light (Courtesy of NASA)

The Orion Nebula is one of the closest star-forming regions to Earth. In the 1960s, astronomers found bright infrared stars in the Orion Nebula and guessed that they could be protostars (early forming stars) buried in clouds and dust. Since then, dozens of protostars have been observed in Orion.

Once stars form, they become chemical factories. Stars fuse hydrogen and helium into dozens of other chemical elements. The oxygen, carbon, iron, and other elements in your body came from stars that lived and died billions of years ago.

The Orion Nebula is known as an emission nebula, or made up mostly of high temperature gas. Very young, hot stars embedded in the nebula heat the surrounding gas so that it emits, or sends out, light of various colors. Many of the stars in the Orion Nebula are approximately one million years old and there are many young star and planet systems forming in the stellar nursery.

The nebula appears as a part of the constellation of Orion as the middle star of Orion's sword. Use a pair of binoculars and follow these step-by-step instructions from ehow.com to see it for yourself.

Explore more about nebula from NASA and World Book