Galaxies: Their Structure and Distances

Steven Dutch, Professor Emeritus, 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 forlong periods of time. Tightly-bunched groupings of stars arecalled star clusters. Star clusters are very useful toastronomers. The cluster method is valuable in determining thedistance scale in the Universe, and star clusters provide batchesof stars of the same ages that are valuable for testing theoriesof stellar evolution.

The nearest star cluster to the Earth is one almost everyoneknows, but very few recognize as a cluster. Five of the stars inthe Big Dipper, plus another dozen or so faint stars nearby, aremoving through space as a group. We do not recognize this clustereasily because it is so close by and has so few stars. A fewother star clusters are close enough for us to see theirindividual stars without a telescope. There are two such clustersin 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. Itis faintly visible, if at all, in brigthly-lit city skies, butshows an astonishing wealth of detail in clear skies. It is onlyin the 20th Century that we have fully understood what the MilkyWay actually is.

Our Galaxy, the Milky Way Galaxy, is a vast, disk-shapedaggregate of stars about 100,000 light years across. The diskthickens at the center into a hub about 10,000 light years indiameter. Our Solar System lies far on the outskirts of theGalaxy, 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 thedisk. (We are not the only culture to connect the Milky Way withmilk. The word galaxy itself comes from the Greek word formmilk.)

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. Manypeople 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 ofstars, and even to reason out from its brightness variations thatwe are not at its center, but how can we know its diameter if wecannot even see all the way to the center?

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

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

The final piece in the puzzle of our Galaxy was thediscovery of Cepheid variables and their use as a cosmicyardstick. Because the globular clusters contain Cepheids, it ispossible to determine their distances. The center of the Globularcluster halo is also the center of our Galaxy.

The Nature of Galaxies

Spiral Galaxies

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

Astronomers classify spiral galaxies on the basis of theirshape.

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

The Puzzle Of The Spiral Arms

It is no surprise that galaxies are often spirals. Just likeplanets orbiting the Sun, stars close to the galactic hub orbitfaster. Any system that rotates faster at the center, whetherwater in a bathtub drain, clouds around a hurricane, or stars ina spiral galaxy, will naturally develop a spiral structure. Theproblem lies in the fact that spiral galaxies are not nearlyspiral enough. In the estimated 15 billion years since theUniverse formed, most stars have made 50-100 orbits around theirgalaxies, yet in most galaxies the spiral arms have only two orthree 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 andsupergiants, that are poor in heavy elements. These starsdominate the central hubs of galaxies and are spread smoothly outin the disk of the galaxy. These stars have made many orbits ofthe central hub and whatever spiral pattern they once had haslong 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 thegalaxy 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 thespiral arms originate. These galaxies are called barred spirals.They are classified SBa, SBb and SBc on the basis of their armstructure just like normal spirals. In some cases the bar formswhen the gravitational attraction of a passing galaxy pulls starsout into a long streamer. In other cases, the bar may formspontaneously when stars in the galaxy bunch together and in turnattract other stars. In any event, the bar is a temporarystructure that eventually dissipates as the galaxy rotates.

Elliptical Galaxies

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

Irregular Galaxies

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

Active Galaxies

Some galaxies emit titanic amounts of energy in the form ofradio emissions and X-rays. In many cases the galaxy is a spiral,with an energy source at the center that emits great jets of gasat right angles to the plane of the galaxy. The central problemof such galaxies is explaining their energy output. The bestexplanation appears to be a massive black hole in the center ofthe galaxy. As material falls into the black hole, it gathersspeed and collides with other infalling matter, emittingradiation 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 standardcandle by astronomers. Cepheids are one standard candle, butthere are others. For example, the brightest galaxies in largeclusters of galaxies all appear to be about the same in absolutebrightness. So do Type I and Type II Supernovae. Using differentstandard candles allows astronomers to cross-check their distanceestimates and assess the reliability of different methods.

There is one last distance estimator astronomers use. Allover the Universe, galaxies appear to be receding from us. Asnearly as we can determine, the farther away galaxies are, thefaster they are receding, at a rate of about 50 miles per secondfor every million light years. For the most distant objectsknown, billions of light years away, the only clue we have totheir distance is the speed at which they are moving away fromus.


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Created 26 March 1998, Last Update 10 April 1998