From moons to planets to suns to solar systems to galaxies, what are the largest objects in the universe?

In the grand hierarchy of the universe, clusters of galaxies are the biggest gravitationally bound objects. The key term here is "gravitationally bound." This means that the entire cluster moves together as a unit in space with its components -- hundreds to thousands of galaxies -- attracted to each other through gravity.

The Earth and Moon, bound by gravity, orbit as one unit around the Sun. Our Sun and its planets, bound by gravity into a unit called a solar system, orbit around the center of the Milky Way galaxy. Our galaxy is loosely bound to a local group of galaxies (about 30 of them); and we and our neighbors stay pretty much bunched together as we fly through the universe.

Do we belong to a cluster of galaxies? That depends on whom you ask. Most scientists call our arrangement a group of galaxies or, at best, a small cluster. The nearest cluster is the Virgo cluster with at least 2,500 galaxies.

Now, scientists love creating new terms with bold adjectives. For example, explosions on a star's surface are called novas. If the whole star explodes, this is a supernova. If the star is really, really big (50 times more massive than our Sun), this is a hypernova. So far, no one has coined the term super-duper-nova. Using this logic, though, there is a term for something bigger than a galaxy cluster. This would be... and you might have guessed it... a supercluster.

Superclusters contain several galaxy clusters, but scientists are not convinced that these individual clusters are really gravitationally bound to each other. So, for now, we'll say that the biggest things out there that behave like a single entity are galaxy clusters.

All of this is much more than simply a name game. Clusters are important. Scientists study them because they represent mini-universes. Each cluster contains trillions of stars. Scientists say that if they understand how clusters form and grow, they can better understand how the Universe itself evolves over time. Because these clusters exist at great distances, they appear small on the sky; and scientists can watch the stars and galaxies in these clusters dance about just like the action on a movie screen.

One mystery that scientists want to solve through the study of clusters is the nature of dark matter. About 80 percent of all the matter in the universe is in a form different than the matter that makes stars, planets and people. This dark matter doesn't seem to radiate or reflect light, so we cannot see it. Yet like all matter, dark matter is a source of gravitational force. And because there's so much dark matter out there, ordinary matter can't help but be captive to its gravity.

Albert Einstein described the force of gravity as the effect of matter warping space. Imagine the mass of dark matter making a gravitational well in space. Gorges of dark matter dictate the flow of the stars and gas in a galaxy cluster. So, by following the flow of visible matter in galaxy clusters, scientists can map out the location of invisible dark matter.

You may have heard the phrase "when worlds collide." Can you imagine galaxy clusters colliding? In September 2004, scientists announced that they observed a massive merger of two galaxy clusters. This is the biggest collision ever witnessed, second only to the big bang in total energy released. One cluster crashed through the other, and now it is falling back in, attracted by gravity. (See http://www.gsfc.nasa.gov/topstory/2004/0831galaxymerger.html.) This is, in essence, how the universe built its hierarchal structure -- through the merger of smaller things into bigger things.
Maybe scientists will find evidence for superclusters gravitationally bound in a hypercluster. To do so, they will need to peer further into space. They will likely need a super-duper telescope.

This week's question comes from Christopher Wanjek. Mr. Wanjek is a science writer supporting the Beyond Einstein initiative, a roadmap to understand the forces of nature beyond General Relativity and Quantum Mechanics through the study of the Universe from the Big Bang to black holes.