Extragalactic systems are the enormous assemblages of stars found outside our Milky Way Galaxy. They may be normal galaxies,
radio galaxies, or quasars, which are believed to be extremely active nuclei of very distant galaxies. Normal galaxies have diameters ranging from 2,000 to nearly 800,000 light-years, masses ranging from 1 million (10{) to 10 trillion (10x7) solar masses, and luminosities ranging from 1 million to 100 billion times that of the Sun. (Our own Milky Way system is a large galaxy with a visible diameter of about 100,000 light-years; see Galaxy, The.) Quasars may be more than 100 times more luminous than the brightest known galaxies but are much smaller. They range in distance from 700 million light-years to perhaps 15 billion light-years, if their red shifts are proportional to their distances, as with other galaxies. HISTORICAL BACKGROUND Early observers of the heavens could not help but note that a luminous band stretches across the starry night sky. Because of its whitish appearance this band was referred to by the Greeks and Romans in terms we know now as the Milky Way, or the GalaxyÑterms derived from Latin and Greek words meaning "milk.") A few ancient scientists suspected that the band consisted of stars, but not until the time of Galileo and the invention of the telescope was it determined that this is the case. The nature of the Galaxy was first clearly recognized by astronomer William Herschel, who explored it telescopically in the 18th and early 19th centuries and concluded that it was a flat, disk-shaped gathering of many stars. Until the 20th century this model of the Galaxy was assumed by most scientists to represent the entire universe. Herschel's universe had edges, and it was thought that beyond the edges there was only empty space. Whether or not space went on infinitely was hotly debated by natural philosophers, but to Herschel and his followers the evidence seemed clear that stars, at least, did not. Apparent corroboration for the concept of a finite stellar universe came from what is called Olbers's paradox, which argues that if the system of stars went on to infinity, the sky should appear bright in all directions. In addition to the stars of the Milky Way, Herschel and his contemporaries found thousands of other objects that they called nebulae (nebulas), from the Latin word for clouds. Some of the fuzzy objects could be resolved into faint stars when examined with a large telescope, and today it is known that they are star cluster too distant to be recognized as such with the naked eye. Other objects could not be resolved by a telescope. They were mostly of two types. One type had emission-line spectra, meaning that they consisted of gas. They were recognized in the 19th century to be immense clouds of hot gas located among the stars of the Milky Way (see nebula). The other type of objects had continuous spectra, as stars do. As for these, one possibility was promoted in 1755 by the philosopher Immanuel Kant, who suggested that they were each "island universes" consisting of large star assemblages, possibly like the Milky Way. In this sense they would be outside the Milky Way, or "extragalactic." These objects remained mysteries, however, until the 1920s, when astronomer Edwin Hubble, a researcher into nebulas, used the 100-in (254-cm) telescope at California's Mount Wilson Observatory to show that the brightest of the nebulas with continuous spectra were indeed extragalactic. He demonstrated this by intensive exploration of three of them (now known as spiral galaxies M31 and M33 and the irregular galaxy NGC6822). Among the thousands of stars that his photographs revealed in each, Hubble discovered a number of CepheidsÑstars that vary in brightness in a cycle whose period is related to their intrinsic brightness. When he applied the Cepheid period-luminosity relation determined by Henrietta Leavitt at the Harvard Observatory in 1912, he found that the three "nebulas" had to be many es as far away as the most distant stars in our Galaxy. By the 1930s, Hubble had concluded that most of the continuous-spectrum objects must be very distant, and that the universe was an immense volume of space filled nearly uniformly with what Kant had called island universes but Hubble called extragalactic nebulae. The true size of our Galaxy had been determined in the years just before 1920 by another Mount Wilson astronomer, Harlow Shapley, who used star clusters and their variable stars to show that the Milky Way is ten times larger than had been thought, or about 100,000 light-years wide. Shapley subsequently moved to Harvard College Observatory, where he developed a large-scale investigation of the structure of the universe. He called the distant systems of stars galaxies, a simpler term than Hubble's and the one now used for these objects. Studying the galaxies known as the Magellanic Clouds, he showed that they were relatively quite nearby and that they could be used to understand both the distance scale and the variety of different galaxy constituents. He also showed that, contrary to Hubble's assertion, the universe is not uniformly populated but contains huge clouds and clusters of galaxies.