Stars are found in huge groups called galaxies. Scientists estimate that the larger galaxies may contain as many as a trillion stars, while the smallest may have fewer than a million. Large galaxies may be 100,000 or more light-years in diameter.
In the 1920s Edwin Hubble separated galaxies into four general types according to their appearance—elliptical, spiral, barred spiral, and irregular—and then classified each into subtypes. Encyclopædia Britannica, Inc.
Galaxies may have any of four general shapes. Elliptical galaxies show little or no structure and vary in general shape from moderately flat and round or oval to spherical. Spiral galaxies have a small, bright central region, or nucleus, and arms that come out of the nucleus and wind around, trailing off like a giant pinwheel. In barred spiral galaxies, the arms extend sideways in a short straight line before turning off into the spiral shape. Both kinds of spiral systems are flat. Irregular galaxies are usually rather small and do not have a symmetrical shape.
Galaxies were long thought to be more or less passive objects, containing stars and interstellar gas and dust and shining by the radiation that their stars give off. When astronomers became able to make accurate observations of radio frequencies coming from space, they were surprised to find that a number of galaxies emit large amounts of energy in the radio region. Ordinary stars are so hot that most of their energy is emitted in visible light, with little energy emitted at radio frequencies. Furthermore, astronomers were able to deduce that this radiation had been given off by charged particles of extremely high energy moving in magnetic fields. These galaxies are called radio galaxies.
The giant galaxy NGC 1316, classified as an elliptical galaxy, is also a radio galaxy. VLT/ESO
The spiral Whirlpool galaxy (M51), at left, is accompanied by a small, irregular companion galaxy, NGC 5195, at top right. NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team (STScI/AURA)
ENERGY AND INTERSTELLAR MATTER
Radio galaxies are usually rather peculiar in appearance. Many galaxies, and the radio galaxies in particular, show evidence of interstellar matter expanding away from their centers, as though gigantic explosions had taken place in their nuclei. The giant elliptical galaxy known as M87 has a jet of material nearby that it apparently ejected in the past. The jet itself is the size of an ordinary galaxy.
Astronomers have found that, in many galaxies, stars near the center move very rapidly, apparently orbiting some very massive unseen object. The most likely explanation is that a giant black hole, with millions or even billions of times the Sun’s mass, lurks in the center of most large galaxies. As stars and gas spiral into these black holes, much of their mass vanishes from sight. The violent heating and compression produces a huge release of energy, including high-speed jets of matter (such as in M87).
NASA scientists have discovered what they believe to be two midsize black holes near the center of the starburst galaxy M82 (the Cigar galaxy). The giant black holes at the center of large galaxies may form from the merger of such midsized black holes. NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
Very distant galaxies are sometimes found to have extremely energetic sources of light and radio waves at their centers. These objects, called quasars, are generally believed to be several billion light-years from Earth. This means that astronomers who observe quasars are actually peering several billion years into the past. Most astronomers believe that quasars represent an early phase in the life of some galaxies, when the central black holes, with plenty of fresh gas and stars to consume, were generating huge amounts of energy.
Another problem has puzzled astronomers for years. Most, if not all, galaxies occur in clusters, presumably held together by the gravity of the cluster members. When the motions of the cluster members are measured, however, it is found in almost every case that the galaxies are moving too fast to be held together only by the gravity of the matter that is visible. Astronomers believe there must be a large amount of unseen matter in these clusters—perhaps 10 times as much as can be seen. While some of this likely consists of objects such as black holes and neutron stars, most of it is believed to be dark matter, of unknown origin.
NGC 1300 is a barred spiral galaxy in the Eridanus cluster. Dark matter accounts for much of the gravitational attraction that holds such galaxy clusters together. NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
Some dark matter surely consists of well-understood objects such as undetected planets, brown dwarfs (bodies just short of having enough mass to become stars), neutron stars, and black holes. Still, these objects and visible matter together probably make up less than 5 percent of the necessary mass. Computer simulations of early galaxy formation seem to require additional matter, though, to provide enough gravitation to produce the clustering of galaxies seen today. These simulations work well only when this matter is “cold,” meaning that its particles are moving slowly relative to each other. This cold dark matter is not made of protons and neutrons like ordinary matter. It is thought to account for another 20–25 percent of the needed mass. All together, these types of matter likely provide about 25–30 percent of the needed mass and therefore seemed to leave the universe open and destined to expand forever.
Like most stars, the Sun belongs to a galaxy. Since the Sun and Earth are embedded in the galaxy, it is difficult for astronomers to obtain an overall view of this galaxy. In fact, what can be seen of its structure is a faint band of stars called the Milky Way (the word galaxy comes from the Greek word for “milk”). Because of this, the galaxy has been named the Milky Way galaxy.
The name of Earth’s galaxy comes from the visual phenomenon of the Milky Way, a band of stars seen in Earth’s night sky. This band is actually the major portion of the galaxy. Because Earth lies in the midst of the galaxy, the spiral structure is hidden by the nearest stars that lie in the plane of the galactic equator and form the Milky Way. Encyclopædia Britannica, Inc.
The visible band of the Milky Way seems to form a great circle around Earth. This indicates that the galaxy is fairly flat rather than spherical. (If it were spherical, the stars would not be concentrated in a single band.) The Sun is located on the inner edge of a spiral arm. The center, or nucleus, of the galaxy is about 27,000 light-years distant, in the direction of the constellation Sagittarius. All the stars that are visible without a telescope belong to the Milky Way galaxy.
Not all the galaxy’s stars are confined to the galactic plane. There are a few stars that occur far above or below the disk. They are usually very old stars, and they form what is called the halo of the galaxy. Evidently the galaxy was originally a roughly spherical mass of gas. Its gravity and rotation caused it to collapse into the disklike shape it has today. The stars that had been formed before the collapse remained in their old positions, but after the collapse further star formation could occur only in the flat disk.
All the stars in the galaxy move in orbits around its center. The Sun takes about 200 million years to complete an orbit. The orbits of most of these stars are nearly circular and are nearly in the same direction. This gives a sense of rotation to the galaxy as a whole, even as the entire galaxy moves through space.
Dark clouds of dust almost completely obscure astronomers’ view of the center of the Milky Way galaxy. Radio waves penetrate the dust, however, so radio telescopes can provide astronomers with a view of the galactic nucleus. In that region stars travel in very fast, tight orbits—which implies the existence of a huge mass at the center. The Earth-orbiting Chandra X-ray Observatory has detected flares of X rays lasting only a few minutes in the region, which are best explained by the existence of a black hole that is violently accelerating and compressing infalling blobs of matter. Infrared observations made at the European Southern Observatory demonstrated that this supermassive black hole has a mass about 4.3 million times that of the Sun.
