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Galaxies What do other galaxies look like? Galaxies come in many sizes and shapes, but can generally be classified into three types: elliptical (E), the spiral (S), and “none of the above”: irregular (I). There are sub-classes for the elliptical and spiral galaxies. (For example, E0, E1, E2, and S0, S1, S2). This system of galactic classifications was first worked out by the astronomer Edwin Hubble. Galaxy types according to Hubble’s classification system. Note that this chart is not an evolutionary track for galaxies; we do not know much about galactic evolution at the moment.

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Spiral Galaxies

Spiral galaxies contain both younger, bluish stars and older, reddish stars. As with our own Milky Way, the center of a typical spiral galaxy is a bulge called the nucleus, with a radius of a few tens of thousands of light years. The galaxy’s spiral arms are contained in a flat disk. Usually the disk is only a few thousand light years thick, but up to several hundred thousand light years across. Galactic nuclei are often the source of intense radio or X-ray emissions. We believe that tremendous black holes, with thousands of times the mass of our Sun, are the source of these emissions. Such black holes are thought to exist at the center of most spiral galaxies, including our own Milky Way. The famous “edge-on” spiral galaxy NGC 4565 in Coma Berenices. (Jim Misti.) The most prominent feature a spiral galaxy is the spiral arms. These consist of young, bluish stars, extending from the center to the edge of the disk. However, the laws of orbital mechanics should not allow the arms to exist at all, because the stars near the edge of the disk will orbit the nucleus much slower than the stars near the center. Hence the arms should “wrap around” each other be unable to survive. The face-on spiral galaxy M 101 in Ursa Major (left), whose typical spiral structure is explained by the density wave theory (right). (Robert Gendler.) The most widely-accepted theory to explain the existence of spiral arms is the density wave theory. This says that the stars do indeed orbit at different speeds, but that the density of the interstellar medium forms a wave. At the wave front, where the density is higher, star formation is triggered. That is why we find young stars along the spiral arms and old stars elsewhere. Spiral galaxies are sub-divided into two classes: normal spirals and barred spirals. The most famous example of a “normal” spiral galaxy is the Andromeda Galaxy (M 31), about 2,000,000 ly away in the constellation Andromeda. M 31 is visible to naked eye on dark nights as an elongated, fuzzy patch. It is, in fact, the most distant object that can be seen with the naked eye. Hubble Space Telescope image of barred spiral galaxy NGC 1300 The image above shows a barred spiral spiral galaxy, NGC 1300 in Eridanus. The core of a barred spiral is elongated, thus giving the name. There is evidence that our own Milky Way galaxy is also a barred spiral.

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Elliptical Galaxies

The galaxy in the photo is M87, a typical elliptical galaxy. Ellipticals usually consist of old stars. In general, they are fainter than spiral galaxies. Some dwarf ellipticals have as few as 10 million stars, making them not very different from a large globular cluster. Elliptical galaxy M 87. (Jim Misti.) Up to a few years ago, we believed ellipticals were the most common type of galaxies. But now, we believe the irregular galaxies are more common.

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Irregular Galaxies

These galaxies have an shape that cannot be classified into either of the categories above. The two satellite galaxies of our own Milky Way, the Large Magellanic Cloud and the Small Magellanic Cloud, are both irregular galaxies. They are clearly visible to the naked eye from the southern hemisphere. The Large Magellanic Cloud, including the Tarantula Nebula (NGC 2070). (Robert Gendler). Many irregular galaxies are thought to be formed by galactic collisions. When galaxies collide, the stars inside them never collide with each other. But the distribution of the stars is distorted by their mutual gravity. Streams of stars may be ejected\to form something like antennae, and/or the two galactic cores may merge. Some galaxies we see with multiple nuclei may be the end result of the merger of two galaxies a long time ago. Galaxy collisions may also trigger star births in their intersecting regions. These colliding galaxies, NGC 4038 and NGC 4039 in Corvus, are known as the “Antennae” galaxies. (Jim Misti.) Quasars Another kind of interesting object is the quasar, which is an abbreviation of the term “quasi-stellar object”. When observed in visible light, quasars are small and dim, appearing just like faint stars. However, in the infrared and radio parts of the spectrum, they are quite bright. Their spectra show large amount of redshift, indicating that they are very far away from us. Thus, we conclude that they must radiate enormous amounts of energy - even more than active galaxies nearby. A quasar is probably a super-massive black hole at the center of a young galaxy.

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Distances to Galaxies, Red Shift, and the Expanding Universe

Before 1920s, we did not know that galaxies were far away. We thought they were located inside our galaxy, and were called spiral nebulae. Today, we mainly use two methods to measure the distances to other galaxies. The first method is the Cepheid variables. The second method is Type I supernovae. Recall that a Type I supernova is the a strong explosion of a nova that destroys a white dwarf. We have found that the luminosities of Type I supernovae are all about the same. Therefore, if we can see a Type I supernova in a galaxy, we can compute its distance by measuring its apparent brightness. Type I supernovae are much brighter than cepheid variables. As a result, the supernovae allow us to measure the distances to galaxies much further away. But they happens much less frequently. Each method has its advantages and disadvantages. Both are distance indicators. The redshift of a galaxy (often referred to by astronomers as ‘z’) is the shift in the galaxy’s spectral lines due to the galaxy’s motion away from the Earth. A galaxy with a redshift of z = 0.01 will be receding at 3,000 kilometers per second, which is 1% of the speed of light. A few nearby galaxies actually have blue shifts (i.e. they are moving towards us), but these are just the small random motions of nearby objects. Galaxies at large distances in space uniformly have redshifts in their spectral lines, indicating motion away from us. In the 1920s, Edwin Hubble discovered that galaxies at increasing distances had proportionately larger redshifts, and therefore higher radial velocities. The farther the galaxy, the faster is was moving away from us. The universe was expanding. The “Hubble Constant” is the numerical factor that relates a galaxy’s radial velocity to its distance: radial velocity = Hubble Constant x distance Hubble’s Constant is currently estimated to be about 75 kilometers per second per megaparsec. This means that for each 75 kilometers/second of radial velocity, a galaxy will be 1 megaparsec away (3.26 million light years). If you know the radial velocity, you can find the distance. A galaxy receding at about 7,500 kilometers/second is at a distance of 100 megaparsecs (i.e. 7500 = 100 x 75). A galaxy receding at 15,000 km/second will be at a distance of about 200 megaparsecs (i.e. 15,000/75 = 200). The farthest known galaxy, Abell 1835 IR1916, has a redshift of about 10, and hence a distance of over 13 billion light years.