Galaxies are collections of stars, gas, and dust, combined with some unknown form of dark matter , all bound together by gravity. The visible parts come in a variety of sizes, ranging from a few thousand light years with a billion stars, to 100,000 light-years with a trillion stars. Our own Milky Way galaxy contains about 200 billion stars.
Types of Galaxies
The invisible parts of galaxies are known to exist only because of their influence on the motions of the visible parts. Stars and gas rotate around galaxy centers too fast to be gravitationally bound by their own mass, so dark matter has to be present to hold it together. Scientists do not yet know the size of the dark matter halos of galaxies; they might extend over ten times the extent of the visible galaxy. What we see in our telescopes as a giant galaxy of stars may be likened to the glowing hearth in the center of a big dark house.
Imagine viewing a galaxy through a small telescope, as pioneering astronomers William and Caroline Herschel and Charles Messier did in the late eighteenth century. You would see mostly a dull yellow color from countless stars similar to the Sun, all blurred together by the shimmering Earth atmosphere. This light comes from stars that formed when the universe was only a tenth of its present age, several billion years before Earth existed.
American astronomer Edwin P. Hubble used a larger telescope starting in the 1920s and saw a wide variety of galaxy shapes. He classified them into elliptical, with a smooth texture; disk-like with spirals; and everything else, which he called irregular.
Elliptical galaxies are three-dimensional objects that range from spheres to elongated spheroids like footballs. Some may have developed from slowly rotating hydrogen clouds that formed stars in their first billion years. Others may have formed from the merger of two or more smaller galaxies. Most ellipticals have very little gas left that can form new stars, although in some there is a small amount of star formation within gas acquired during recent mergers with other galaxies.
Spiral galaxies, which include the Milky Way, formed from faster-spinning clouds of hydrogen gas. Theoretical models suggest they got this spin by interacting with neighboring galaxies early in the universe. The center of a spiral galaxy is a three-dimensional bulge of old stars, surrounded by a spinning disk flattened to a pancake shape.
Hubble classified spiral galaxies according to the tightness of the spirals that wind around the center, and the relative size of the disk and bulge. Galaxies with big bulges tend to have more tightly wrapped spirals; they are designated type Sa. Galaxies with progressively smaller bulges and more open arms are designated Sb and Sc. Barred galaxies are similar but have long central barlike patterns of stars; they are designated SBa, SBb, and Sbc, while intermediate bar strengths are designated SAB.
Type Sa galaxies rotate at a nearly constant speed of some 300 kilometers per second (186 miles per second) from the edge of the bulge to the far outer disk. Sc galaxies have a rotation speed that increases more gradually from center to edge, to typically 150 kilometers per second (93 miles per second). The rotation rate and the star formation rate depend only on the average density. Sa galaxies, which are high density, converted their gas into stars so quickly that they have very little gas left for star formation today. Sc galaxies have more gas left over and still form an average of a few new stars each year. Some galaxies have extremely concentrated gas near their centers, sometimes in a ring. Here the star formation rate may be higher, so these galaxies are called starbursts.
The pinwheel structures of spiral galaxies result from a concentration of stars and gas in wavelike patterns that are driven by gravity and rotation. Bright stars form in the concentrated gas, highlighting spiral arms with a bluish color. Theoretical models and computer simulations match the observed spiral properties. Some galaxies have two long symmetric arms that give them a "grand design." These arms are waves of compression and rarefaction that ripple through a disk and organize the stars and gas into the spiral shape. These galaxies change shape slowly, on a timescale of perhaps ten rotations, which is a few billion years. Other galaxies have more chaotic, patchy arms that look like fleece on a sheep; these are called flocculent galaxies. The patchy arms are regions of star formation with no concentration of old stars. Computer simulations suggest that each flocculent arm lasts only about 100 million years.
Irregular galaxies are the most common type. They are typically less than one-tenth the mass of the Milky Way and have irregular shapes because their small sizes make it difficult for spiral patterns to develop. They also have large reservoirs of gas, leading to new star formation. The varied ages of current stars indicate that their past star formation rates were highly nonuniform. The dynamical processes affecting irregulars are not easily understood. Their low densities and small sizes may make them susceptible to environmental effects such as collisions with larger galaxies or intergalactic gas clouds. Some irregulars are found in the debris of interacting galaxies and may have formed there.
Some small galaxies have elliptical shapes, contain very little gas, and do not see any new star formation. It is not clear how they formed. The internal structures of irregulars and dwarf ellipticals are quite different, as are their locations inside clusters of galaxies (the irregulars tend to be in the outer parts). Thus it is not likely that irregulars simply evolve into dwarf ellipticals as they age.
Active Galaxies, Black Holes, and Quasars
In the 1960s, Dutch astronomer Maarten Schmidt made spectroscopic observations of an object that appeared to be a star but emitted strong radio radiation, which is uncharacteristic of stars. He found that the normal spectral lines emitted by atoms were shifted to much longer wavelengths than they have on Earth. He proposed that this redshift was the result of rapid motion away from Earth, caused by the cosmological expansion of the universe discovered in the 1920s by Hubble. The velocity was so large that the object had to be very far away. Such objects were dubbed quasi-stellar objects, now called quasars . Several thousand have been found.
With the Hubble Space Telescope, astronomers have recently discovered that many quasars are the bright centers of galaxies, some of which are interacting. They are so far away that their spatial extents cannot be resolved through the shimmering atmosphere. Other galaxies also show the unusually strong radio and infrared emissions seen in quasars; these are called active galaxies.
The energy sources for quasars and active galaxies are most likely black holes with masses of a billion suns. Observers sometimes note that black holes are surrounded by rapidly spinning disks of gas. Theory predicts that these disks accrete onto the holes because of friction. Friction also heats up the disk so much that it emits X rays . Near the black hole, magnetic and hydrodynamic processes can accelerate some of the gas in the perpendicular direction, forming jets of matter that race far out into intergalactic space at nearly the speed of light. Nearby galaxies, including the Milky Way, have black holes in their centers too, but they tend to be only one thousand to one million times as massive as the Sun.
Active spiral galaxies are called Seyferts, named after American astronomer Carl Seyfert. Their spectral lines differ depending on their orientation, and so are divided into types I and II. The lines tend to be broader, indicating more rapid motions, if Seyfert galaxies are viewed nearly face-on. Active elliptical galaxies are called BL Lac objects (blazars) if their jets are viewed end-on; they look very different, having giant radio lobes , if their jets are viewed from the side. These radio lobes can extend for hundreds of millions of light-years from the galaxy centers.
Galaxies generally formed in groups and clusters, so most galaxies have neighbors. The Milky Way is in a small group with another large spiral galaxy (Andromeda, or Messier 31), a smaller spiral (Messier 33), two prominent irregulars (the Large and Small Magellanic Clouds), and two dozen tiny galaxies. In contrast, the spiral galaxy Messier 100 is in a very large cluster, Virgo, which has at least 1,000 galaxies. With so many neighbors, galaxies regularly pass by each other and sometimes merge together, leading to violent gas compression and star formation. In dense cluster centers, galaxies merge into giant ellipticals that can be 10 to 100 times as massive as the Milky Way. There is a higher proportion of elliptical and fast-rotating spiral galaxies in dense clusters than in small groups. Presumably the dense environments of clusters led to the formation of denser galaxies.
The Milky Way Galaxy
In the 1700s the philosophers Thomas Wright, Immanuel Kant, and Johann Heinrich Lambert speculated that our galaxy has a flattened shape that makes the bright band of stars called the Milky Way. Because English physicist and mathematician Isaac Newton (1642-1727) showed that objects with mass will attract each other by gravity, they supposed that our galaxy disk must be spinning in order to avoid collapse. In the early 1800s William Herschel counted stars in different directions. The extent of the Milky Way seemed to be about the same in all directions, so the Sun appeared to be near the center.
In the 1900s American astronomer Harlow Shapley studied the distribution of globular clusters in our galaxy. Globular clusters are dense clusters of stars with masses of around 100,000 Suns. These stars are mostly lower in mass than the Sun and formed when the Milky Way was young. Other galaxies have globular clusters too. The Milky Way has about 100 globular clusters, whereas giant elliptical galaxies are surrounded by thousands of globulars.
Shapley's observations led to an unexpected result because he saw that the clusters appear mostly in one part of the sky, in a spherical distribution around some distant point. He inferred that the Sun is near the edge of the Milky Way—not near its center as Herschel had thought. Shapley estimated the distance to clusters using variable stars. Stars that have finished converting hydrogen into helium in their cores change their internal structures as the helium begins to ignite. For a short time, they become unstable and oscillate, changing their size and brightness periodically; they are then known as variable stars. American astronomer Henrietta Leavitt (1868-1921) discovered that less massive, intrinsically fainter stars vary their light faster than higher mass, intrinsically brighter stars. This discovery was very important because it enabled astronomers to determine the distance to a star based on its period and apparent brightness. Much of what we know today about the size and age of the universe comes from observations of variable stars.
Shapley applied Leavitt's law to the variable stars in globular clusters. He estimated that the Milky Way was more than 100,000 light years across, several times the previously accepted value. He made an understandable mistake in doing this because no one realized at the time that there are two different types of variable stars with different period-brightness relations: the so-called RR Lyrae stars in globular clusters are fainter for a given period than the younger Cepheid variables .
The Discovery of Galaxies
In the 1920s astronomers could not agree on the size of the Milky Way or on the existence of other galaxies beyond. Several lines of conflicting evidence emerged. Shapley noted that nebulous objects tended to be everywhere except in the Milky Way plane. He reasoned that there should be no special arrangement around our disk if the objects were all far from it, so this peculiar distribution made him think they were close. Actually the objects are distant galaxies, and dust in the Milky Way obscures them. The distance uncertainty was finally settled in the 1930s when Hubble discovered a Cepheid variable star in the Andromeda galaxy. He showed from the period-brightness relationship that Andromeda is far outside our own galaxy.
Galaxy investigations will continue to be exciting in the coming decades, as new space observatories, such as the Next Generation Space Telescope , and new ground-based observatories with flexible mirrors that compensate for the shimmering atmosphere, probe the most distant regions of the universe. Scientists will see galaxies in the process of formation by observing light that left them when the universe was young. We should also see quasars and other peculiar objects with much greater clarity, leading to some understanding of the formation of nuclear black holes .
see also Age of the Universe (volume 2); Black Holes (volume 2); Cosmology (volume 2); Gravity (volume 2); Herschel Family (volume 2); Hubble Constant (volume 2); Hubble, Edwin P. (volume 2); Hubble Space Telescope (volume 2); Shapley, Harlow (volume 2).
Debra Meloy Elmegreen and Bruce G. Elmegreen
Bothun, Gregory. "Beyond the Hubble Sequence: What Physical Processes Shape Galaxies." Sky and Telescope 99, no. 5 (2000):36-43.
Elmegreen, Debra Meloy. Galaxies and Galactic Structure. Upper Saddle River, NJ:Prentice Hall, 1998.
Elmegreen, Debra Meloy, and Bruce G. Elmegreen. "What Puts the Spiral in Spiral Galaxies?" Astronomy Vol. 21, No. 9 (1993):34-39.
Ferris, Timothy Coming of Age in the Milky Way. New York: Anchor Books, 1989.
Sandage, Allan. The Hubble Atlas of Galaxies. Washington, DC: Carnegie Institution of Washington, 1961.
Sawyer, Kathy. "Unveiling the Universe." National Geographic 196 [supplement](1999):8-41.
When Galaxies Collide
WHEN GALAXIES COLLIDE
Collisions between galaxies can form spectacular distortions and bursts of star formation. Sometimes bridges of gas and stars get pulled out between two galaxies. In head-on collisions, one galaxy can penetrate another and form a ring. Interactions can create bars in galaxy centers and initiate spiral waves that make grand design structure. Close encounters can also strip gas from disks, which then streams through the cluster and interacts with other gas to make X rays.