To understand the place of nebulae in the universe, it's helpful to think like an astronomer. Astronomers make sense of the universe by organizing it into a series of "nested" levels. Nebulae, which are enormous objects in their own right, occupy a level in the middle of this hierarchy. This is the sequence: Superclusters form the top level, followed by clusters, galaxies, nebulae, star systems, stars, planets and moons. Let's look briefly at each, using the illustration below as a guide.
- Superclusters consist of several clusters of galaxies. They represent the highest level of the cosmic hierarchy and are the largest objects in the universe. Some are as large as 100 million light-years across. A light-year is the distance light travels in a year. Because light travels at 300,000 kilometers per second, it can cover 9.46 trillion kilometers in a year. That's the same as 5.88 trillion miles. Examples of superclusters include the Virgo, or Local, Supercluster; the Coma Supercluster; the Hercules Supercluster; the Perseus Supercluster; and the Southern Supergalaxy.
- A cluster is a group of galaxies that travel together. They can contain two or three galaxies, or they may have thousands of galaxies. Our home galaxy, the Milky Way, is part of a cluster known as the Local Group. Our nearest galactic neighbor, the Andromeda Galaxy, also belongs to the Local Group, as do several others.
- Next on the scale come galaxies, what astronomers once called island universes. They're not individual universes, of course, but are collections of stars, gases and dust particles. They come in a variety of shapes -- spiral, elliptical and irregular -- and vary greatly in size. The Milky Way is a spiral galaxy that is 70,000 to 100,000 light-years across [source: NASA].
- Nebulae are found inside galaxies, filling the space between stars or enveloping stars like a cloak. They're made of dust and gas and can appear as either bright or dark clouds. The gas is mostly hydrogen mixed with some helium. In fact, astronomers sometimes classify nebulae based on the type of hydrogen they contain: H+ nebulae contain mostly ionized hydrogen (hydrogen atoms in which electrons have been removed); H I nebulae contain mostly neutral hydrogen; and H II nebulae contain hydrogen existing in molecular form (H2). The other principal component of nebulae -- dust -- consists of fine particles containing carbon, silicon, magnesium, aluminum and other elements.
The dust and gas of a nebula are spread very thinly. A single cloud contains fewer atoms per cubic inch than a puff of smoke. Yet because a single cloud is vast in size, stretching many light-years across, it can dim or block out other objects positioned behind it.
- Star systems, like our solar system, come next. A star system may be one to two light-years across, depending on the number of planets it contains. At the heart of such a system is a star, a celestial body that produces energy by thermonuclear reactions. A single nebula can be associated with numerous stars. For example, the Eagle Nebula is home to star cluster M16, a collection of hundreds of young, bright stars. Our sun, a medium-sized, middle-aged star, is much older than those located in the Eagle Nebula. Other well-known stars include Alpha Centauri, Proxima Centauri and Sirius.
Courtesy NASA and STScI
Finally, at a level of the cosmic hierarchy that is difficult to show on our scale, we have planets and moons -- mere specks compared to nebulae. Asteroids, comets and meteoroids are even smaller, ranging in size from small moons to large rocks.
Now that we have a scale to work with, let's examine the different type of nebulae in greater detail.