Galaxy

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Galaxy

Outside of the galaxy

Classification of galaxies

Formation and evolution

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A galaxy is a very large collection of stars, atomic hydrogen gas, cosmic rays, molecular hydrogen, and complex molecules of carbon, nitrogen, hydrogen, silicon, and others. The word galaxy comes from the Greek word galaxias, or milky circle. The Milky Way galaxy in which the Earth’s solar system is located is one such galaxy. Astronomers generally classify galaxies according to their shape as either spiral, elliptical, or irregular. Spiral galaxies are further subdivided into normal and barred spirals. Elliptical galaxies can be either giant or dwarf ellipticals, depending on their size. In the 2000s, astronomers using the Hubble Deep Field, part of the Hubble Space Telescope, estimated that about 175 billion galaxies exist in the universe.

Galaxies can contain anywhere from a few million stars, for dwarf ellipticals, to a few trillion stars, for giant ellipticals or spirals. Galaxies emitting far more energy than can easily be explained by a collection of stars are classified as active galaxies. The study of other galaxies, in addition to being intrinsically interesting, helps scientists understand the Milky Way galaxy and gives clues as to the understanding of the universe as a whole.

Outside of the galaxy

Astronomers did not recognize galaxies as separate from the Milky Way until the early part of the twentieth century. The Andromeda galaxy, which is the nearest spiral galaxy to the Milky Way and the Large and Small Magellanic Clouds, which are the nearest irregular galaxies to the Milky Way, are visible to the naked eye, and have therefore been observed since antiquity. Their nature was, however, unknown.

With the development of the telescope, astronomers were able to discern the whorled shape of spiral galaxies, which were called spiral nebulae at the time. Until the 1920s, there was a controversy: Were these spiral nebulae part of the Milky Way galaxy, or were they external galaxies similar to the Milky Way? In April 1920, there was a debate on this topic between American astronomers Harlow Shapley (1885–1972) and Heber Doust Curtis (1872–1942) before the National Academy of Sciences. Curtis argued that spiral nebulae were external galaxies, Shapley that they were part of the Milky Way. Curtis did not win the debate, but astronomy has since proven him right—spiral nebulae are external galaxies similar to the Milky Way.

To settle the controversy, scientists needed an accurate method to gauge the distance to galaxies. Working at

Harvard College Observatory in the early twentieth century, American astronomer Henrietta Leavitt (1868–1921) found the required celestial yardstick. Leavitt was studying a type of star in the Magellanic Clouds known as a Cepheid variable, when she discovered a way to measure the distance to any Cepheid variable by comparing the star’s apparent and absolute magnitudes. The distance to the variable star gave the distance to the galaxy or cluster of stars containing the Cepheid variable. Cepheid variables have since become a fundamental yardstick for measuring the distance scale of the universe.

In 1924, the American astronomer Edwin Hubble (1889–1953) used Leavitt’s Cepheid variable technique to measure the distance to the Andromeda galaxy. Hubble’s original distance estimates have since been refined; the modern distance to the Andromeda galaxy is about 2.2 million light years. (A light year is the distance light travels in one year within the vacuum of space, about 6 trillion miles, or 9.654 trillion kilometers). The Milky Way galaxy is however only a little over 100,000 light years in diameter. Hubble therefore conclusively proved that the Andromeda galaxy must be outside the Milky Way. Other galaxies are more distant.

With his work, Hubble launched the science of extragalactic astronomy—the study of galaxies outside the Milky Way. Hubble devised the classification scheme for galaxies that astronomers still use today. More importantly, Hubble found that more distant galaxies are moving away from the Milky Way at a faster rate. From this observation, known as Hubble’s law, he deduced that the universe is expanding. Hubble used his study of galaxies to uncover a fundamental fact about the nature of the universe. Fittingly, one of the scientific goals of the Hubble’s namesake, the Hubble Space Telescope, is to continue this work.

Classification of galaxies

Hubble classified the galaxies he observed according to their shape. His scheme is still in use today. The basic regular shapes are elliptical and spiral. He classified galaxies with no regular shape as irregular galaxies. Galaxies that, basically, look like either elliptical or spiral galaxies but have some unusual feature are classified as peculiar galaxies. They are classified according to the closest match in the classification scheme. They are, then, given the added designation peculiar (pec). Hubble initially thought that his classification scheme represented an evolutionary sequence for galaxies; they started as one type and gradually evolved into another type.

Modern astronomers have supplemented Hubble’s original scheme with luminosity classes. The luminosity of a galaxy is its total energy output each second. Note that the luminosity refers to the intrinsic energy output of the galaxy corrected for the distance of the galaxy. Therefore a high luminosity but distant galaxy might appear fainter than a nearby low luminosity galaxy. The luminosity classes are the roman numerals I, II, III, IV, and V. The most luminous galaxies are class I, and the least luminous are V. As one might guess, the more luminous galaxies are generally larger in size and contain more stars.

How common are the various types of galaxies? In a given volume of space, about one-third of all the galaxies (34%) are spirals, a little over half (54%) are irregulars, and the rest (12%) are ellipticals. However, irregular and elliptical galaxies tend to be smaller and fainter on the average than spiral galaxies. They are therefore harder to find. Of the galaxies that astronomers can observe the overwhelming majority (77%) are spirals and only 3% are irregular galaxies. The remaining 20% of observed galaxies are ellipticals.

Elliptical galaxies

Elliptical galaxies have a three-dimensional ellipsoidal shape, so they appear in their two dimensional projections on the sky as ellipses. In his scheme, Hubble denoted elliptical galaxies with the letter E. He further subdivided ellipticals according to the amount of elongation of the ellipse, using numbers from 0 to 7. An E0 galaxy appears spherical. The most elongated elliptical galaxies are E7. The E1 through E6 galaxies are intermediate.

Note that this classification is based on the appearance of a galaxy, which may be different from its true shape owing to projection effects on Earth. Since Hubble’s time, astronomers have learned that some ellipticals are relatively small and others are large. Astronomers now have the additional classification of either dwarf ellipticals or giant ellipticals. For finer divisions astronomers use the luminosity classes I, for the supergiant ellipticals, down to V for the smallest dwarf ellipticals.

Dwarf elliptical galaxies tend to be fairly small. They average about 30,000 light years in diameter, but can be as small as about 10,000 light years. The diameters of galaxies are a little uncertain because galaxies do not end sharply. Instead, they tend to gradually fade out with increasing distance from the center. By contrast, giant elliptical galaxies average about 150,000 light years in diameter. The largest supergiant ellipticals are a few million light years in diameter.

The dwarf ellipticals have masses ranging from 100,000 to 10 million times the mass of the sun, suggesting that they have about that many stars. Giant ellipticals on the other hand will typically have 10 trillion times the mass of the sun and, therefore, roughly that many stars. Both giant and dwarf elliptical galaxies have only old stars and very small amounts of the interstellar gas and dust that is the raw material for forming new stars, probably due to the loss of gas clouds to star formation during the collisions that formed the elliptical shape.

Spiral galaxies

Spiral galaxies have a disk shape with a bulging central nucleus, so that they look like an astronaut’s pancake floating in midair with a fried egg in the center on both sides. Surrounding the disk is a spherical halo consisting of globular clusters—spherical clusters of roughly 100,000 stars each. The astronaut’s breakfast has drops of syrup floating in a spherical distribution around the pancake.

The disk of a spiral galaxy contains the spiral arms that give this class of galaxy its name. There are usually two spiral arms that wind around each other several times in a whorl shape—from the nucleus to the edge of the disk. A few spiral galaxies have more than two spiral arms.

There are two types of spiral galaxies, normal spirals and barred spirals. In the normal spiral galaxies, the spiral arms wind out ward from the nucleus. In barred spirals, there is a central bar structure extending out on either side of the nucleus. The spiral arms wind outward from the edge of this bar structure.

In his classification scheme, Hubble denoted normal spiral galaxies by S and barred spiral galaxies by SB. He, then, subclassified spirals according to how tightly the spiral arms wind around the nucleus, using a, b, and c. Galaxies denoted Sa are the most tightly wound and, therefore, have a relatively small disk compared to the spiral arms. Sc galaxies are the most loosely wound. They, therefore, extend well beyond the nucleus and have a relatively larger disk compared to the nucleus. Sb galaxies are intermediate between the Sa and Sc galaxies. Hubble used a similar scheme for barred spirals, producing the classifications SBa, SBb, and SBc.

Some galaxies have a disk surrounding a nucleus, but do not have spiral arms in the disk. Hubble classified these galaxies as SO. They are now also called lenticular galaxies. As for elliptical galaxies, modern astronomers also add luminosity classes (I, II, III, IV, V) to Hubble’s classification scheme. Luminosity class I galaxies are the most luminous and are referred to as supergiant spirals. Luminosity class V galaxies are the least luminous.

The luminosity classes of spiral galaxies do not have as wide a range as elliptical galaxies, so there are no dwarf spiral galaxies. Spiral galaxies are typically about the size of the Milky Way, roughly 100,000 light years in diameter. They will typically have a mass of about 100 billion times the mass of the Sum, so will contain roughly 100 billion stars. The largest super-giant spirals can have as much as several trillion times the mass of the sun.

Spiral galaxies contain fairly young stars in their disks and spiral arms and older stars in their nuclei and halos. The disks and spiral arms also contain interstellar gas and dust, which are the raw materials for forming new stars. The halos-like elliptical galaxies contain very little gas and dust. This difference in the distribution of the contents of spiral galaxies tells astronomers that they were originally spherical in shape. The rotation of these galaxies caused them to flatten out and form their disks.

Irregular galaxies

Hubble classified galaxies that do not fit neatly into his scheme of ellipticals and spirals as irregular (Irr) galaxies. Irregular galaxies as a class have no particular shape, and have no spherical or circular symmetries as the ellipticals and spirals do. There is a range of sizes, but irregulars tend to be small. They average about 20,000 light years in diameter. The smallest irregulars, dwarf irregulars, are only about 1,000 light years in diameter.

Because they are relatively small, irregular galaxies have small masses (typically about one million times the mass of the sun) and therefore relatively few stars. Astronomers now classify irregular galaxies into two groups, Irr I and Irr II. In Irr I galaxies, astronomers can resolve young stars and evidence of ongoing star formation. In Irr II galaxies, astronomers cannot resolve individual stars. They also have no distinct shape. Both types of irregular galaxies contain a large percentage of young stars and interstellar gas and dust.

Active galaxies

Many galaxies look almost like one of the Hubble classifications, but with some unusual feature. For example, imagine an elliptical galaxy that looks like someone sliced it through the center, pulled it apart a little bit, and displaced each half sideways. Hubble called these galaxies peculiar and added the designation, pec, to the classification. The galaxy described above might be an E0 pec galaxy. Whatever causes a galaxy to look as if it were ripped apart as described above would require large amounts of energy. Peculiar galaxies are therefore interesting because they often tend to be the active galaxies that emit large amounts of energy.

Active galaxies are galaxies that emit far more energy than normal galaxies. A galaxy is considered an active galaxy if it emits more than 100 times the energy of the Milky Way galaxy. Active galaxies often have a very compact central source of energy, much of which is emitted as radio waves rather than optical light. These radio waves are emitted by electrons moving in a helical path in a strong magnetic field at speeds near the speed of light. Active galaxies also often have a peculiar photographic appearance, which can include jets of material streaming out from the nucleus or the appearance of either explosions or implosions. They also tend to vary erratically in brightness on rapid time scales. There are a number of varieties of active galaxies, including: compact radio galaxies, extended radio galaxies, Seyfert galaxies, BL Lacertac objects, and quasars.

Compact radio galaxies appear photographically as ordinary giant elliptical galaxies. Radio telescopes, however, reveal a very energetic compact nucleus at the center. This radio nucleus is the source of most of the energy emitted by the galaxy.

Perhaps the best known compact radio galaxy is M87. This giant elliptical galaxy has both a very compact energetic radio source in the nucleus and a jet consisting of globs of material shooting out from the nucleus. Recent observations from the Hubble Space Telescope provide strong evidence that this core contains a supermassive black hole.

Extended radio galaxies consist of two giant lobes emitting radio waves. These lobes are on either side of a peculiar elliptical galaxy. The lobes can appear straight or curved as if the galaxy is moving through space. These lobes are the largest known galaxies and can stretch for millions of light years.

Seyfert galaxies look like spiral galaxies with a hyperactive nucleus. The spiral arms appear normal photographically, but they surround an abnormally bright nucleus. Seyfert galaxies also have evidence for hot turbulent interstellar gas.

BL Lacertae objects look like stars. In reality, they are most likely to be very active nuclei of elliptical galaxies. However, BL Lacertae objects have sufficiently unusual behavior, including extremely rapid and erratic variations in observed properties, that their exact nature is not known for certain.

Quasars also look like stars, but they are perhaps the most distant and energetic objects in the universe known so far. Most astronomers consider them the very active nuclei of distant galaxies in the early stages of evolution. As for the other types of active galaxies, they produce large amounts of energy in a very small volume. Most astronomers currently think that the energy source is a supermassive black hole. Astronomers found evidence in 1998 that the center of the Milky Way galaxy, which is about 28,000 light years from Earth, contains a black hole that is several million times the mass of the sun. Other discoveries in the next few years lent credence that other black holes were at the center of other galaxies. Then, in June 2004, a

KEY TERMS

Active galaxy— A galaxy that emits more energy than can easily be explained, usually at least 100 times the energy output of the Milky Way.

Barred spiral galaxy— A spiral galaxy in which the spiral arms start at the end of a central bar structure rather than the nucleus.

Cepheid variable star— A type of star that varies in brightness as the star pulsates in size. Cepheid variables are important distance yardsticks in establishing the distance to nearby galaxies.

Disk— The flat disk-shaped part of a spiral galaxy that contains the spiral arms.

Elliptical galaxy— A galaxy having an elliptical shape.

Galaxy— A large collection of stars and clusters of stars, containing anywhere from a few million to a few trillion stars.

Halo— A spherical distribution of older stars and clusters of stars surrounding the nucleus and disk of a spiral galaxy.

Irregular galaxy— A galaxy that does not fit into Hubble’s classification scheme of elliptical and spiral galaxies.

Light year— The distance light travels in one year, roughly 6 trillion miles, or 9,654 trillion kilometers.

Milky Way— The galaxy in which Earth is located.

Nucleus— The central core of a galaxy.

Spiral arms— The regions where stars are concentrated that spiral out from the center of a spiral galaxy.

Spiral galaxy— A galaxy in which spiral arms wind outward from the nucleus.

black hole was discovered, which scientists believe helps to confirm that gigantic black holes were created early in the formation of the universe. In 2005, a black hole was discovered to be traveling at twice the escape velocity of the galaxy as it exited the Milky Way. Scientists think that such a black hole may help to support the theory that a black hole exists in the center of the Milky Way galaxy and other galaxies within the universe.

Formation and evolution

For many years scientists had no ideas how galaxies formed. According to all observations at the time, galaxies formed during a single epoch very far back in the history of the universe. In the absence of direct evidence, astronomers formed two theories: the theory of accretion, in which blobs of stars came together to form galaxies; and the theory of collapse, in which galaxies were formed in the collapse of an enormous gas cloud.

In late 1996, scientists got their first view of galaxy formation, looking back in time 11 billion years to see clumps of young star clusters gradually banding together into a galaxy. It is too early to fully dismiss the gas collapse theory, however; there may be more than one way to form a galaxy.

When Hubble first devised his classification scheme, he thought that the different types of galaxies represented different evolutionary stages; they started as one type and gradually evolved into another type. Astronomers now know that his theory was true, though the phrase gradual evolution is something of a misnomer.

Elliptical galaxies are formed by the collision of two spiral galaxies. The process is slow—scientists estimate that it takes nearly one-half billion years for the merging spiral galaxies to smooth into an elliptical galaxy—but can be quite violent. Although galaxies are mostly empty space, gravitational interaction between stars can cause them to explode into supernovas. More important, gravitationally induced collisions between clouds of interstellar hydrogen gas can create intense heat and pressure that can trigger the formation of new stars.

One clue to the evolution of galaxies is the distribution of different types of galaxies at different distances from the Milky Way. Because light travels at a finite speed, when astronomers look at a distant galaxy, they are seeing the galaxy as it appeared in the distant past when the light left it. Some types of active galaxies, such as quasars and BL Lacertae objects, occur only at great distances from the Earth. They existed when the universe was much younger, but no longer exist. Many astronomers, therefore, think that active galaxies are an early stage in the evolution of galaxies. If this idea is correct, an astronomer living now on a distant quasar might see the quasar as a normal galaxy, and the Milky Way in its earlier active stage as a quasar.

However galaxies formed and evolved, the process must have occurred quickly very early in the history of the universe. The age of the oldest galaxies appears to be not much younger than the age of the universe. Though astronomers now have some support for theories of galactic formation and evolution, they are still searching for more evidence and trying to understand the details.

See also Radio astronomy.