The universe is defined by The American Heritage Dictionary, Third Edition (Electronic Version 3.6p) as "all matter and energy, including Earth, the galaxies and all therein, and the contents of intergalactic space, regarded as a whole." It is known that the universe is expanding rapidly, and that all other galaxies are "rushing" away from us. The universe is made up of many different structures arranged in a fairly well-defined hierarchy.
Scientists do not know for sure how the universe came into being, but the accepted theory states that all the matter in the universe was once condensed into one tiny region similar to a black hole. For an unknown reason, this region suddenly began to expand at an astronomical rate of speed. At the same time, density and temperature decreased rapidly. Irregularities in the distribution of matter in this rapidly expanding universe became more pronounced as gravity became the predominant influence on matter. These irregularities condensed to become the galaxies, stars, planets, and other celestial objects that we know today. This massive expansion is termed the Big Bang and can be researched more fully in CosmoNet's Big Bang Section.
Planets, moons, asteroids, and some comets are generally held to have been formed by the slow accretion of dust and other particles at the outer edges of gravitationally collapsing nebulae. The accretions of particles, called planetesimals, collide and eventually form one of these structures, although the eventual designation may not yet be apparent. After these collisions, the planets (themselves orbited by moons) fall into distinct orbits around the sun(s). Asteroids, sometimes called "failed planets" are usually too small to survive these constant collisions (or are the fragments of collisions) and usually form a discrete belt. Comets, composed mainly of ice and dust, settle into irregular orbit around the suns(s).
The planets can take two forms, depending on their original constituents: they can become gas giants, planets with thick atmospheres and tiny cores, or hard planets such as Earth with molten cores, crusts, and lighter atmospheres. More information on the formation of planets is available in the article on Planet Formation.
Stars are the smallest significant constituent of the universe's structural hierarchy. Formed from hot nebulae, they consist mainly of hydrogen, helium, and other gases which undergo nuclear fusion in the center of the star to form heavier elements. Stars typically follow a distinct sequence in their lifetime of several billion years - they are born from nebulae, they burn off their nuclear fuel, they expand to become giants or supergiants, and they die as neutron stars, black holes, or dwarves. Some particularly massive stars undergo a spectacular explosion known as a supernova during their slow death. More information on this topic can be found in The Life and Structure of a Star.
"Solar system" is the collective term for all the planets, asteroids, comets, and other bodies locked by gravity into orbits around one or more stars, termed the sun(s). When a solar system contains two stars in mutual orbit, the system is called a binary system. In the rarer case of three stars in mutual orbit, the system is called a tertiary system. In our solar system (often capitalized), there are nine planets (five hard, four gaseous), one asteroid belt, many comets, scores of moons, and one average-sized star. Though substantial proof of solar systems besides our own has not been offered until recently, this seems to be a fairly average setup with the exception that binary systems may be proven more common. Additional information is available in Solar Systems.
Due to their many different origins and functions, it is difficult to establish the exact position of nebulae in the universal hierarchy. Nebulae are essentially clouds of hydrogen gas and dust that do not shine with their own light, but rather are lit by stars behind or within them. Stars and star clusters are often formed from the hot spots of gas within many nebulae. As mentioned above, planets are also formed from dust and particles in nebulae. Planetary nebulae are nebulae formed from the explosion of a red giant star. See also The Life and Structure of a Star and for more information.
Star clusters are exactly what their name implies: groups of stars clustered together and gravitationally attracted to each other. The members of a star cluster also move together in the slow rotation of the galaxy. Generally, stars are born in clusters out of nebulae, then are influenced by other stars and slowly drift apart. Clusters can be of varied sizes, from the still-forming 16 stars of the Rosette Nebula to the elderly Beehive Cluster of 75 stars to the dense Pleiades cluster of 250 stars. More information is available in Solar Systems.
Galaxies are large agglomerations of stars, nebulae, and other celestial objects, the majority of which are separated by vast amounts of interstellar space. Studies show that there are at least 1 billion galaxies in the known universe, all associated with a galaxy group, yet each separated from its closest neighbor by millions of light years. Galaxies come in three types: spiral, elliptical, and irregular. Our Milky Way galaxy is a spiral galaxy in which the solar system is located on one arm. Peculiar galaxies are ordinary spiral or elliptical galaxies with the exception of one feature or detail. See Galaxies, Galaxy Groups, and Galaxy Clusters for more information.
Galaxy groups are essentially small groups of galaxies that are relatively close to one another and travel together in space. In addition to galaxies, they contain hot, tenuous threads of luminous gas. The collisions of galaxy groups are the mechanisms that lead to the formation of galaxy clusters. The Milky Way, along with its closest neighbors (the Large and Small Magellanic Clouds and the Andromeda galaxy) is a member of a small group of several large and about 20 smaller galaxies. This group is accurately named the Local Group. Other groups, such as Virgo, are much larger and contain around a thousand galaxies. See Galaxies, Galaxy Groups, and Galaxy Clusters for more information.
Galaxy clusters are enormous congregations of galaxies in the same way that galaxies are congregations of stars. The average galaxy cluster is 1000 trillion times as massive as the sun and is hotter than 75 000 000 ° C. Within a galaxy cluster, the individual galaxies orbit the cluster's center of mass just as planets orbit their sun. Galaxy clusters contain, aside from the galaxies themselves, huge amounts of very hot gas stretched tenuously through the space between the galaxies. More information can be found in Galaxies, Galaxy Groups, and Galaxy Clusters.
Galaxy clusters are the highest level of organization held together by gravitational attraction. Superclusters are special aggregations of galaxies not held together by gravity. They are similar to, but smaller than, walls. Walls, the highest level of organization detected in the universe to date, are vast, thin sheets or long strings of galaxy clusters. The longest of these sheets is called the Great Wall. Voids are huge holes (100 - 400 million light years across) interspersed with the walls. They contain no galaxies, but instead many are filled with huge clouds of hydrogen gas that contain as much or more mass as an average galaxy group. These clouds have been speculated to be either primordial hydrogen left over from the Big Bang or the halos of galaxies too faint, distant, or small to be readily detected.
Created by Dan Corbett, Kate Stafford, and Patrick Wright for ThinkQuest.