Galaxies: Their Structure and Distances

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay

Deep-Sky Objects

There are many kinds of objects in the sky besides stars: star clusters, nebulae or clouds of interstellar dust and gas, and galaxies. Because most of them are far away and visible only in telescopes, astronomers use the term deep-sky object for them. These objects, like stars, are catalogued. The most famous catalog is the Messier Catalog. Charles Messier was a French comet-hunter who became frustrated by objects in the sky that looked like faint comets but were not, so he compiled a list of 110 of them. Messier's list is almost a tour guide to the Universe, with many of the brightest and most spectacular deep-sky objects on it. (Ironically, Messier sought fame by seeking comets, but his list is his monument and his 21 comet discoveries are largely forgotten. Moral: seek knowledge, and you may find fame; seek fame, and you may be forgotten.) Objects on the list are denoted by M plus a catalog number; for example, the nearest spiral galaxy to us, The Andromeda Galaxy, is M31. A far more extensive catalog, the New General Catalog, is a standard reference for astronomers, and objects in this catalog are denoted by NGC. For example, M31 is also known as NGC 224.

Star Clusters

Stars tend to form in batches, and to remain together for long periods of time. Tightly-bunched groupings of stars are called star clusters. Star clusters are very useful to astronomers. The cluster method is valuable in determining the distance scale in the Universe, and star clusters provide batches of stars of the same ages that are valuable for testing theories of stellar evolution.

The nearest star cluster to the Earth is one almost everyone knows, but very few recognize as a cluster. Five of the stars in the Big Dipper, plus another dozen or so faint stars nearby, are moving through space as a group. We do not recognize this cluster easily because it is so close by and has so few stars. A few other star clusters are close enough for us to see their individual stars without a telescope. There are two such clusters in the constellation Taurus: the Plieades, a young, tightly-packed cluster about 410 light years away, still with remnants of its ancestral gas cloud, and the Hyades (HY-a-deez) an older, looser cluster about 130 light years away.

Nebulae

There are many types of gas and dust clouds in space, collectively called nebulae.

Bright and dark nebulae are basically the same, differing mostly in how they are illuminated. It is common for one part of a nebula to be bright, while another part of the same nebula is dark and obscures the bright nebula behind it.

Our Galaxy

The Milky Way

The Milky Way is a band of light that encircles the sky. It is faintly visible, if at all, in brigthly-lit city skies, but shows an astonishing wealth of detail in clear skies. It is only in the 20th Century that we have fully understood what the Milky Way actually is.

Our Galaxy, the Milky Way Galaxy, is a vast, disk-shaped aggregate of stars about 100,000 light years across. The disk thickens at the center into a hub about 10,000 light years in diameter. Our Solar System lies far on the outskirts of the Galaxy, about two-thirds of the way from the center to the edge. When we see the Milky Way, we are looking along the plane of the disk. (We are not the only culture to connect the Milky Way with milk. The word galaxy itself comes from the Greek word form milk.)

To get an idea of the size of the galaxy, let us shrink the Solar System, defined by the orbit of Pluto, to a quarter. The Sun becomes a microscopic speck .0001 inch across, the size of a bacterium. The Earth is the size of a virus, travelling in an orbit about the size of the period at the end of this sentence. The nearest star is still 300 feet away. Many people think the stars are not far beyond Pluto. Have a friend hold up a quarter 300 feet away to see just how much beyond the Solar System the stars really are. On this scale we can finally show the Galaxy. It is 1300 miles across, comparable to the United States east of the Mississippi. In the central hub of the galaxy, comparable to Ohio perhaps, the stars are dust specks a few feet apart. Our Solar System, somewhere around Green Bay, Wisconsin or Cape Hatteras, North Carolina, is in a sparsely populated region where the dust specks are a few hundred feet apart.

Many people confuse galaxies and solar systems. This size coparison ought to help place the two in their proper perspective.

In the summer, when the Milky Way is wide and bright, we are looking toward the center of the Galaxy. We cannot see the center in visible light because of stars, dust, and gas in the way, but we can detect its radio emissions. The center of the Galaxy lies in (actually far behind) the constellation Sagittarius. In the winter, when the Milky Way is narrow and faint, we are looking toward the sparse outer edges of the disk. When we look toward the Big Dipper, or in the southern sky in autumn, we are looking out of the plane of the Galaxy into intergalactic space.

Structure Of Our Galaxy

It is not too hard to picture the Milky Way as a disk of stars, and even to reason out from its brightness variations that we are not at its center, but how can we know its diameter if we cannot even see all the way to the center?

For many years, astronomers had known of disk-like, spiral objects in the sky. To the eye, they were fuzzy swirls of light, perhaps whirlpools of glowing gas. It was not until photography was applied to astronomy, and great telescopes like the 100-inch diameter telescope at Mount Wilson were erected early in this century, that it became possible to take very long time exposures of these objects. To the astonishment of everyone, they were made of stars. In a stroke, the size of the Universe increased unimaginably. The Universe was teeming with "island universes", of which our own Milky Way was only one.

Nearby galaxies also have remarkable objects around them called globular star clusters, great spherical swarms of 100,000 or more stars in a ball perhaps 100 light years across. These globular star clusters form a spherical halo around the center of each galaxy. But in our own sky, the great majority of globular clusters are tightly bunched in one part of the sky. Clearly, if our own Galaxy is like the millions of others we can detect, we must lie far off to one side, and outside the halo of globular clusters.

The final piece in the puzzle of our Galaxy was the discovery of Cepheid variables and their use as a cosmic yardstick. Because the globular clusters contain Cepheids, it is possible to determine their distances. The center of the Globular cluster halo is also the center of our Galaxy.

The Nature of Galaxies

Spiral Galaxies

When most people hear the word "galaxy", they picture one of the great spiral objects on observatory photographs, spiral galaxies. Spiral galaxies are enormous disks of stars like our own Milky Way, all held together by their mutual gravitational attraction, and all orbiting around their common center of gravity, the central hub. Our own Sun takes about 200 million years to complete one orbit of the Galaxy. At the center of the hub is the galactic nucleus, perhaps 50 light years in diameter, where stars are packed a fraction of a light year apart. The sky would be spectacular on a planet in a galactic nucleus, but even here the stars are so far apart they would appear as points of light.

Astronomers classify spiral galaxies on the basis of their shape.

There is, as yet, no definite map of our own galaxy. Figure 21.xx shows one attempt to map our galaxy, using radio emissions from distant gas clouds to map the regions we cannot see directly.

The Puzzle Of The Spiral Arms

It is no surprise that galaxies are often spirals. Just like planets orbiting the Sun, stars close to the galactic hub orbit faster. Any system that rotates faster at the center, whether water in a bathtub drain, clouds around a hurricane, or stars in a spiral galaxy, will naturally develop a spiral structure. The problem lies in the fact that spiral galaxies are not nearly spiral enough. In the estimated 15 billion years since the Universe formed, most stars have made 50-100 orbits around their galaxies, yet in most galaxies the spiral arms have only two or three turns, not 50 or 100.

Population I and Population II

The spiral arms are much younger than the galaxies. Astronomers have found two major groups of stars in galaxies. Population II stars are old stars, mostly red giants and supergiants, that are poor in heavy elements. These stars dominate the central hubs of galaxies and are spread smoothly out in the disk of the galaxy. These stars have made many orbits of the central hub and whatever spiral pattern they once had has long since been smeared out.

Population I stars, on the other hand, are young blue-white stars rich in heavy elements. It is these stars that mark the spiral arms of galaxies. One theory holds that the rotation of the galaxy compresses interstellar gas to start the process of star formation. Another theory suggests that supernovae compress interstellar gas and triggers star formation, and rotation of the galaxy smears the young star groups into a spiral pattern. Probably both mechanisms are at work.

The disks of galaxies also contain enormous clouds of gas and dust. Galaxies seen edge-on often have a prominent dark line across them where dust and gas obscure stars in the disk. In our own Galaxy, there are many dark clouds, visible even to the unaided eye, that obscure parts of the Milky Way. One such cloud against a bright part of the Milky Way is so prominent for Southern Hemisphere viewers that it is nicknamed the Coal Sack. If a galaxy uses up all its interstellar gas, it will have no more material to form stars and will lose its spiral form. Many astronomers are convinced that S0 galaxies formed this way; they were once ordinary spiral galaxies whose spiral arms were smeared into a smooth disk shape.

Barred Spirals

Some spiral galaxies have a central bar from which the spiral arms originate. These galaxies are called barred spirals. They are classified SBa, SBb and SBc on the basis of their arm structure just like normal spirals. In some cases the bar forms when the gravitational attraction of a passing galaxy pulls stars out into a long streamer. In other cases, the bar may form spontaneously when stars in the galaxy bunch together and in turn attract other stars. In any event, the bar is a temporary structure that eventually dissipates as the galaxy rotates.

Elliptical Galaxies

Though not as famous as spiral galaxies, elliptical galaxies are the largest, some containing a trillion or more stars. These vast systems long ago used up most of their gas for star formation and consist entirely of Population II stars. These galaxies are not disks, but flattened spheres called ellipsoids. Astronomers classify them as E0 for nearly spherical galaxies through E7 for very elongated ones. Elliptical galaxies tend to be smooth and featureless, offering few clues to their origin or evolution.

Irregular Galaxies

Many galaxies have no regular structure, and often have bizarre forms. In many cases, it is clear that the galaxies have been disrupted by close encounters or collisions with other galaxies. Even in the case of a collision between galaxies, the individual stars are so far apart that they do not collide, but the gravitational attraction of the galaxies distorts the forms of the galaxies.

Active Galaxies

Some galaxies emit titanic amounts of energy in the form of radio emissions and X-rays. In many cases the galaxy is a spiral, with an energy source at the center that emits great jets of gas at right angles to the plane of the galaxy. The central problem of such galaxies is explaining their energy output. The best explanation appears to be a massive black hole in the center of the galaxy. As material falls into the black hole, it gathers speed and collides with other infalling matter, emitting radiation in the process. Other galaxies, called starburst galaxies, are experiencing great bursts of star formation.

Distances Of Galaxies

Cepheid variables, pulsating yellow stars of known absolute brightness, are of incalculable value in determining distances in the Universe, but even the brightest Cepheid cannot be seen more than a few million light years away, even with the Hubble Space Telescope. We can see Cepheids in many nearby galaxies, but not in very distant ones.

An object of known absolute brightness is called a standard candle by astronomers. Cepheids are one standard candle, but there are others. For example, the brightest galaxies in large clusters of galaxies all appear to be about the same in absolute brightness. So do Type I and Type II Supernovae. Using different standard candles allows astronomers to cross-check their distance estimates and assess the reliability of different methods.

There is one last distance estimator astronomers use. All over the Universe, galaxies appear to be receding from us. As nearly as we can determine, the farther away galaxies are, the faster they are receding, at a rate of about 50 miles per second for every million light years. For the most distant objects known, billions of light years away, the only clue we have to their distance is the speed at which they are moving away from us.


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Created 26 March 1998, Last Update 10 April 1998

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