- For the automobile, see Ford Galaxy.
A galaxy is a group of many stars, along with gas, dust, and dark matter. The name ‘galaxy’ is taken from the Greek word galaxia meaning milky, a reference to our own galaxy, the Milky Way.
Gravity holds galaxies together against the general expansion of the universe. In effect, the expansion of the universe takes place between groups of galaxies, not inside those groups. This is because the mass of a galaxy holds the galaxy together, and the same applies to the groups of galaxies, such as our Local Group. The gravitation is produced by the matter and energy in a galaxy or group of galaxies. Everything in a galaxy moves around a centre of mass, which is also an effect of gravity.
There are various types of galaxies: elliptical, spiral and lenticular galaxies, which can all be with or without bars. Then there are irregular galaxies. All galaxies exist inside the universe. There are probably over 170 billion (1.7×1011) galaxies within the observable universe.
There are galaxies of different sizes. Typical galaxies range from dwarfs with as few as ten million (107) stars up to giants with a hundred trillion (1014) stars, all orbiting the galaxy’s center of mass. Galaxies may contain many multiple star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.
Star clusters are not galaxies, they are inside galaxies. Globular clusters are spherical tightly knit balls of stars which are part of the outer halo of the Milky Way. One of the largest (and oldest) known star clusters, Messier 15, has several million stars, packed closely together, with a black hole at its centre. The stars are too closely packed to get an accurate count, but it certainly has more stars than some of the smaller galaxies.
Within galaxy clusters, galaxies move relative to other galaxies. They can and do collide. When this happens, the stars generally move past each other, but gas clouds and dust interact, and can form a burst of new stars. Gravity pulls both galaxies into somewhat new shapes, forming bars, rings or tail-like structures.
Many galaxies also continue to form new generations of stars. The Milky Way, and all spiral shaped galaxies like it (see above image of NGC 2997), produce new stars at a rate of one or two stars per year. These stars are formed in the vast interstellar clouds that account for about 1% to 10% of the mass of these galaxies. Globular star clusters, on the other hand, are not currently forming stars because this activity happened billions of years ago and then stopped once all of the gas and dust clouds were used up.
Types of Galaxies
Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence. Since the Hubble sequence is entirely based upon visual morphological type, it may miss certain important characteristics of galaxies such as star formation rate in starburst galaxies and activity in the cores of active galaxies.
The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter. Consequently these galaxies also have a low portion of open clusters and a reduced rate of new star formation. Instead they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst. In this sense they have some similarity to the much smaller globular clusters.
The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the interaction of galaxies, resulting in a collision and merger. They can grow to enormous sizes (compared to spiral galaxies, for example), and giant elliptical galaxies are often found near the core of large galaxy clusters. Starburst galaxies are the result of such a galactic collision that can result in the formation of an elliptical galaxy.
Spiral galaxies resemble spiraling pinwheels. Though the stars and other visible material contained in such a galaxy lie mostly on a plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of dark matter that extends beyond the visible component, as demonstrated by the universal rotation curve concept.
Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) that indicates the degree of tightness of the spiral arms and the size of the central bulge. An Sa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, an Sc galaxy has open, well-defined arms and a small core region. A galaxy with poorly defined arms is sometimes referred to as a flocculent spiral galaxy; in contrast to the grand design spiral galaxy that has prominent and well-defined spiral arms.
It appears the reason that some spiral galaxies are fat and bulging while some are flat discs is because of how fast they rotate.
In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constant angular velocity. The spiral arms are thought to be areas of high-density matter, or “density waves”. As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a “wave” of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.
A majority of spiral galaxies, including our own Milky Way galaxy, have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure. In the Hubble classification scheme, these are designated by an SB, followed by a lower-case letter (a, b or c) that indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core, or else due to a tidal interaction with another galaxy. Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.
Our own galaxy, the Milky Way, is a large disk-shaped barred-spiral galaxy about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×1011) stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.