Galaxy Types

FIELDS OF STUDY: Astronomy; Cosmology; Extragalactic Astronomy

ABSTRACT: Galaxies are collections of dust, gas, and stars. They are usually found in clusters. Galaxies come in many shapes and sizes and are classified into four major types: elliptical, spiral, irregular, and lenticular. Other subtypes also exist. Elliptical galaxies are oval in shape, while spiral galaxies look like a pinwheel. Galaxies that do not look like either of these shapes are classified as irregular. Lenticular galaxies share qualities of both ellipticals and spirals. Edwin Powell Hubble is the American astronomer who discovered the existence of galaxies outside the Milky Way and devised a method of classifying them. The Hubble Space Telescope was named in honor of his achievements.

The Known Universe Expands

In 1923, American astronomer Edwin Hubble (1889–1953) was working at the Mount Wilson Observatory in California when he spotted a new Cepheid variable in Messier 31 (M31), then known as the Andromeda Nebula. A Cepheid is a star that can be used to calculate distance in space. At the time, most astronomers, led by Harvard College Observatory director Harlow Shapley (1885–1972), believed that the Milky Way galaxy was the extent of the universe. However, when Hubble used Shapley’s methods to calculate the distance to the Cepheid, the result placed it about a million light-years away—far beyond the Milky Way, which Shapley himself had determined to be only three hundred thousand light-years across. This meant that other galaxies outside of the Milky Way did exist, including M31, now called the Andromeda galaxy.

Hubble’s discovery expanded the universe significantly and was a key finding in the astronomical world. Many other objects once thought to be nebulae within the Milky Way were soon found to be separate galaxies as well. Hubble began to devise a classification system for galaxies. This system, now called the Hubble sequence or the Hubble tuning-fork diagram (due to its shape), forms the basis of the modern galaxy-classification system.

Hubble initially divided galaxies into three main categories: spiral, elliptical, and irregular. The earliest version of his tuning-fork diagram, published in 1936, deals primarily with spirals and ellipticals. The left side of the diagram shows three types of elliptical galaxies (E0, E3, and E7), ordered from the most circular (E0) to the most elongated (E7). The diagram then splits into two branches—the "tines" of the tuning fork—representing the two categories of spiral galaxies, barred (SBa, SBb, and SBc) and "normal," or unbarred (Sa, Sb, and Sc). Irregular galaxies were any that did not fall into one of these two categories.

In the center of the diagram, at the point where the two spiral branches meet the elliptical branch, Hubble included type S0, representing a hypothetical intermediate form. This type later came to represent lenticular galaxies.

Galaxy Finder

Edwin Powell Hubble was born on November 20, 1889, in Marshfield, Missouri. His family moved to Wheaton, Illinois, less than a year later. Hubble attended the University of Chicago on a partial scholarship and graduated in 1910 with a bachelor of science degree in mathematics and astronomy. He then won a Rhodes Scholarship to attend the University of Oxford, where he studied law.

After returning to the United States, Hubble practiced law and taught high school for a year before deciding to pursue his first love. He returned to the University of Chicago in 1914 to complete his doctorate in astronomy. Upon graduation, Hubble was offered a job at the Mount Wilson Observatory near Pasadena, California, but he enlisted in the army instead.

After World War I ended, Hubble returned and accepted the position at Mount Wilson. There, he worked with the largest and newest telescope in the world: the hundred-inch Hooker Telescope, completed in 1917. It was with this telescope that Hubble first observed the Cepheid variable that would ultimately lead him to revolutionize humanity’s view of the universe.

Elliptical Galaxies

Elliptical galaxies are among the oldest in the universe. They have a three-dimensional ellipsoid shape, ranging from nearly spherical to an elongated oval. Due to their age, they contain very little dust or gas, as they contain mostly old stars and generally do not form new ones. This gives elliptical galaxies a reddish color and makes them appear fainter than other galaxies. In the sky, they look like a blurry, rounded patch of light. While the majority of galaxies observed thus far have been spiral galaxies, scientists suspect that elliptical galaxies are in fact more common.

Of all galaxy types, ellipticals have the largest range in terms of size and mass. The smallest ones are called dwarf elliptical galaxies. These can be as small as ten thousand light-years in diameter. The largest elliptical galaxies can be a million or more light-years across and contain trillions of stars. They often contain unusually large numbers of globular clusters. Messier 87 (M87), the largest known elliptical galaxy in the universe, is believed to have as many as fifteen thousand globular clusters orbiting in its halo.

Elliptical galaxies are believed to be formed by collisions between smaller galaxies. When two spiral galaxies collide, they lose their spiral arms and merge into one large, disordered elliptical galaxy. A large number of elliptical galaxies are found in the Coma Cluster, a cluster of more than one thousand galaxies. At the center sit two massive elliptical galaxies, New General Catalog 4874 (NGC 4874) and New General Catalog 4889 (NGC 4889), surrounded by numerous other ellipticals, both dwarf and giant.

Spiral Galaxies

More than three-quarters of all known galaxies are spiral galaxies. They are one of two types of disk galaxies, so called because they are shaped like a flattened, rotating disk. In a spiral galaxy, the disk has a distinct spiral structure, typically with arms extending from the center. Approximately two-thirds are classified as "barred," meaning that a bar-shaped structure extends through the middle, with arms emerging from either end. About 60 percent of spiral galaxies have multiple arms, and 10 percent have only two arms. It is believed that the remaining 30 percent used to have arms but that they have since faded. The Milky Way is an example of a barred spiral galaxy with multiple arms. In some galaxy-classification systems, barred spirals are placed in their own category.

Spiral galaxies are younger than elliptical galaxies. They are made up of dust, gas, and mostly young stars. Because of the abundance of gas and dust, they are able to form new stars and appear brighter than ellipticals. The majority have a central bulge, a dense collection of older, dimmer stars that protrudes above and below the disk. Scientists believe that such bulges contain a supermassive black hole at their center.

The disk surrounding the bulge and is made up of both old and new stars. The spiral arms contain large numbers of young, blue stars, causing them to shine more brightly than the rest of the disk. Scientists are uncertain how these arms form. Some believe that they are formed by regions of increased density causing stars to briefly slow their orbit, creating a kind of stellar traffic jam. More recent evidence suggests that they result from the gravitational effects of giant molecular clouds.

Spiral galaxies range in size from about one billion to one trillion times the mass of the sun. Their disks are typically between ten thousand and three hundred thousand light-years across. The largest known spiral galaxy is New General Catalog 6872 (NGC 6872). It is a barred spiral about five times larger than the Milky Way. The oldest one is called BRI 1335-0417, which is thought to be more than 12.4 billion years old.

As spiral galaxies spin, they burn up their gas, which slows down new star formation. Scientists believe that as this happens, they lose their spirals and evolve into elliptical galaxies.

Lenticular Galaxies

Lenticular galaxies are the second type of disk galaxy. They have properties of both elliptical and spiral galaxies. Some scientists believe that this class is a transitional form between the two. The term "lenticular" means "lens-like." Lenticular galaxies are so named because when viewed from the side, the shape of their disks resembles a convex lens.

While lenticular galaxies appear visually similar to fading spiral galaxies, their compositions are closer to those of elliptical galaxies. Like spiral galaxies, lenticulars have a disk and bulge, but unlike spirals, they do not have arms. Like ellipticals, lenticulars contain little gas and mostly consist of old, reddish stars, with few bluish new stars. Lenticulars, however, contain a large amount of dust. Because of these characteristics, some scientists believe that lenticulars are very old, faded spiral galaxies. Others believe that they are formed by the merging of two old spiral galaxies.

Irregular Galaxies

Irregular galaxies are ones that cannot be categorized by shape. Their age, shape, and structure vary.

They do not have spiral arms or bright centers. Some may have been spiral or elliptical at one time, but a collision or merger caused them to take on an irregular shape. Irregular galaxies contain large amounts of gas and dust and a mix of old and new stars. They are also smaller than both elliptical and spiral galaxies.

Two classifications of irregular galaxies exist. Type I irregular galaxies have some sort of structure but are not considered elliptical, spiral, or lenticular galaxies. Type II galaxies do not have any structure.

PRINCIPAL TERMS

• bulge: a dense, spheroidal group of stars found in the center of many spiral and lenticular galaxies.
• disk: the roughly flat, circular component of a spiral or lenticular galaxy that contains the majority of the galaxy’s stars, most of which orbit in the same direction; also spelled "disc."
• elliptical galaxy: a roughly spherical or ellipsoid galaxy with no bulge or disk, consisting mainly of old stars that follow random orbits.
• globular cluster: a dense, spheroidal collection of old stars of the same age and origin, bound together by gravity and orbiting within the halo of a large galaxy.
• halo: a roughly spherical region that extends beyond the visible reaches of a galaxy, containing old, dim stars, dark matter, gas, and globular clusters.
• irregular galaxy: a galaxy without a distinct shape or structure.
• lenticular galaxy: a disk galaxy with a prominent central bulge, no arms, and little interstellar medium; so named because its shape is similar to that of a lens.
• spiral galaxy: a disk galaxy with spiral arms that extend into the disk from either a central bulge or a bar that passes through the bulge.

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