A 103 Notes, Week 14, Kaufmann-Comins Chapter 15
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1 NEARBY GALAXIES I. Brief History A 103 Notes, Week 14, Kaufmann-Comins Chapter 15 A. Kant B. Curtis-Shapley debate C. Distance to Andromeda II. Classification of nearby galaxies: Spirals, Ellipticals, Irregulars III. Clusters of galaxies A. Rich and poor clusters B. Superclusters: walls, filaments and voids C. Galactic Cannibalism D. Missing mass IV. The Hubble law and the distance ladder 15-1
2 I. Brief History A. Kant By 1750 astronomers speculated that the Milky-was a disk of stars containing the Sun, and that diffuse-looking nebulae (the word means clouds) seen scattered across the sky were other similar collections of stars seen at large distances. The speculation appears in 1775 in the book Universal Natural History and Theory of the Heavens, by the German philosopher Kant. Messier s catalog: 25 years after Kant suggested that nebulae were galaxies, the reappearance of Halley s comet galvanized the astronomical community, and Messier, to help comet hunters, made a catalog of fuzzy objects that didn t move: These aren t comets don t look at them. Some of Messier s objects were clouds of gas, like the Crab Nebula, M1 or the Ring Nebula in Lyra, M57; some were open clusters of stars in the disk of our own galaxy, like the Beehive cluster in Cancer, M67; some were globular clusters, like M13 in Hercules. But a number of Messier s objects were what Kant and Thomas Wright had already speculated were island universes, giant collections of stars like the Milky Way. B. Curtis-Shapley debate 145 years later the question was still undecided, because no single star of a galaxy other than the Milky Way had been seen clearly enough to find its distance. In a famous debate in 1920, Harlow Shapley opposed Heber Curtis; Shapley argued that the spiral nebulae were small objects in the Milky Way, like the globular clusters Shapley had recently used to find the Milky Way s size. Curtis correctly argued that they were distinct from the Milky Way. Part of the problem was that both Curtis and Shapley thought that supernovae in nebulae (other galaxies) were really novae. Since novae are much dimmer, the confusion made both Curtis and Shapley think that the nebulae were much closer to the Milky Way than they really are. C. Distance to Andromeda The distance to other galaxies was finally measured when Edwin Hubble, using the new 100 inch telescope at Mt. Wilson, found a Cepheid variable in the Andromeda nebula. Henrietta Leavitt had found a relation between period and luminosity for Cepheid variables, and by the 1920 s the intrinsic brightness of Cepheids was known. This meant that if you saw a Cepheid variable, then from its period of oscillation you would know how bright it really is. By knowing its real brightness and observing its apparent brightness, you can figure out its distance. The Cepheid Hubble found in Andromeda was nearly times dimmer than a Cepheid with the same period 10 pc away from us. So the Andromeda Cepheid has to be 10 5 times further, or nearly 10 6 pc away. Hubble used a his discovery of a Cepheid in Andromeda, together with the period-luminosity relation for Cepheids, to find the distance to Andromeda. The Andromeda Galaxy is 2 million LY away. ( LY) Hubble s subsequent observations gave distances to a set of nearby galaxies. 15-2
3 II. Classification of nearby galaxies. Galaxies age slowly. Because typical stars live for billions of years, galaxies, which are large collections of stars, take billions of years to change their appearance. When you look at the Andromeda galaxy through the roof telescope, you are seeing it the way it looked 2 million years ago and it would look the same today. But if you look at the most distant galaxies, billions of LY away, you see them as they were billions of years ago, and they were different. We begin by classifying the galaxies as they are today: A classification of nearby galaxies is equivalent to a classification of the Universe s galaxies as they appear today. New stars form from the gas and dust of spirals and irregulars. Elliptical Spiral Irregular Rotate slower Rotate faster No ordered rotation No gas, dust Gas and dust in disk Gas and dust throughout No new stars New stars in spiral arms New stars throughout Redder - no blue O and B stars Bluer Bluer Wide range of sizes ( M) Smaller ( M) The obvious spiral structure seen is spiral galaxies is partly an illusion. The galaxy s gas and stars fill a disk. The spiral structure is a wave of slightly higher density that moves through the disk. When the wave passes a cold cloud of gas, it triggers the collapse of the cloud, which then forms a cluster with many young, bright, blue (type O and B) stars. The brightest stars evolve fastest, and they are gone by the time the spiral wave has passed. As a result the spiral arms are much brighter than the rest of the disk, but they do not contain most of the disk s matter. The spiral arms contain most of the O and B stars of spiral galaxies Only about 5% more stars are in the spiral arms than in the rest of the disk (according to Kaufmann- Comins), and more stars are in the nucleus + rest of disk than are in the spiral arms Elliptical galaxies look like giant globular clusters, having little gas outside their stars. As a result, the stars one sees in elliptical galaxies all formed about 10 billion years ago, at about the time of formation of the galaxy. There are no longer any massive main sequence stars, because massive stars evolve quickly. The hot blue stars that light up spiral and irregular galaxies died long ago. Elliptical galaxies are redder than spirals or irregulars, because they have no new stars and therefore no blue main-sequence stars. Elliptical galaxies have only old stars that formed at about the time the galaxy formed. Why is there so little gas in ellipticals from the stars that have evolved and died? The most popular 15-3
4 theory is that the gas is stripped away by collisions of galaxies in clusters. It is possible that elliptical galaxies are former spirals that once collided with another, larger, galaxy and their gas was pulled off, ending up mostly in the space between galaxies. In large clusters of galaxies, there are always giant ellipticals which appear to have grown huge by eating other galaxies. III. Clusters of galaxies A. Rich and poor clusters The Milky Way and the Andromeda Galaxy are members of a small cluster of about 30 galaxies, called the Local Group. These include a couple of small elliptical companions to Andromeda; and a couple of small irregular companions to the Milky Way, the Large and Small Magellanic Clouds (LMC and SMC). Most galaxies in the universe are members of larger clusters, containing hundreds or thousands of galaxies. The nearest fairly rich cluster with about 1,000 galaxies, all about 50 million LY away in the direction of the constellation Virgo. Rich clusters have giant elliptical galaxies at their centers, which formed by collisions of galaxies. B. Galactic Cannibalism A giant elliptical grows by swallowing nearby galaxies that pass it: the process is called galactic cannibalism. If a smaller spiral galaxy passes not too closely, it can lose its gas and dust, and become an elliptical, while a closer collision will merge the two galaxies. The distance between galaxies in a cluster is only about ten times the size of a galaxy, about 1/2 Mly. [500 km/s = c. To travel 1/2 Mly takes about yrs]. More than half of all galaxies are likely to have collided since they first formed. 15-4
5 C. Superclusters: walls, filaments and voids Clusters of galaxies are themselves grouped in larger structures that look like flat walls or long filaments where the walls intersect. Between the walls are large, nearly empty regions called voids, and the universe looks something like suds a collection of soap bubbles, with clusters of galaxies largely confined to the walls of the bubbles. D. Missing mass The orbits of matter around large galaxies show that the mass of galaxies is mostly invisible and mostly in their haloes. Similarly, the orbits of galaxies in a cluster show that most of the mass of every cluster of galaxies is invisible. In fact, without 5 10 times more mass than can be seen, clusters of galaxies would fly apart. Where is the mass? (1) Jupiter-sized objects would be too small and too dim to see. They would increase the density of ordinary matter at the time of the Big Bang and would imply more helium was produced than we observe. Another objection: Why should the gas that contracts to form clusters of galaxies not have formed the usual larger stars at the same place it formed Jupiter-sized objects? (2) Black holes. Black holes of a few solar mass or larger would give too many gravitational lenses. Smaller black holes might work, but how would they have formed? (3) Exotic particles. If neutrinos have small masses, they could account for all the missing matter. Or, there may be undiscoverd particles more massive than neutrinos that fill the universe. We do not yet know what the dark matter is, so we do not know what constitutes most of the universe s mass. 15-5
6 IV. The Hubble law Slipher, in 1914 noticed that the spectral lines of most galaxies ( spiral nebulae ) were redshifted: they seemed to be moving away from us at speeds larger than those of most stars. Once Hubble measured distances to galaxies, he found that all but the closest galaxies are moving away, and the speed at which the galaxies are receding is proportional to their distance from us: v = H 0 r; where v is the speed at which the galaxy is moving away in km/s, r is its distance from us in millions of light years (Mly), and H 0 is a constant. The best current measurements give H 0 between 20 and 23 km/s/mly. To make life easier, we ll use H 0 =20 km=sec=mly: Thus a galaxy that is 100 Mly away, is receding from us at a speed of about v = 100 Mly 2 20 km=s=mly=2; 000 km=s: Once one has measured Hubble s constant, H 0, one can use the law to find the distance to other galaxies: If a galaxy is seen (from its redshift) to be moving away from us at about 10,000 km/s, then its distance is about r = v 10; 000 km=s = H 0 20 km=s=mly = 500 Mly As we will see, Hubble s law does not mean that we are at the center of the universe and that everything is moving away from us. The universe is uniformly expanding, and every galaxy sees all the other galaxies moving away from it. Kaufmann Comins use Mpc instead of Mly. Because 1 Mpc = 3.26 Mly, we can write H 0 =20 km=sec=mly 2 3:26Mly 1Mpc H 0 =65km=s=Mpc (The Kaufmann-Comins value of 75 km/s/mpc is somewhat higher than the current best value). 15-6
7 The Distance Ladder Here is a summary of how one finds distances in the universe, starting from the solar system and going out the most distant objects that can be seen. Objects at successively greater distances are found by using our knowledge of the distance to closer objects. It is like climbing a ladder: To reach each new rung, one needs the one below Begin with 1AU Distances to planets found by radar bouncing The distance to the closest stars is found by parallax, the difference in a star s apparent position when seen from positions of the Earth that are 1 AU apart. Knowing the distance to the closest stars, most of which are on the main sequence one can find their luminosity: main sequence stars become standard candles. This allows one to find distances to stars throughout the galaxy. Distances to stars in the galaxy farther than 500 ly away are found using main sequence stars as standard candles ( spectroscopic parallax ) Knowing the distance to clusters of stars in our galaxy that contain Cepheid variables, one can find the luminosity of Cepheids, which then become standard candles. They are brighter than main sequence stars, allowing one to find distances to galaxies up to 50 million ly away. The distances to galaxies closer than 50 million ly are found using Cepheids as standard candles Knowing the distance to galaxies 50 million ly away (from Cepheids), one can find Hubble s constant. Using Hubble s constant, one can find the distance to any galaxy that can be observed (Several standard candles not mentioned in this list are also used. The most important are supernovae from the collapse of white dwarfs. Far brighter than Cepheids, their luminosity can be found when they are seen in the Virgo cluster of galaxies, known to be 50 million ly away by Cepheids; like Hubble s law, they give distances everywhere in the visible universe.) Distance Indicator Radar bouncing Parallax Main sequence stars (spectroscopic parallax) Cepheid variables Hubble s law Objects whose distances are determined Planets in the solar system (used to find 1 AU) Distances to closest stars in our galaxy, out to a few hundred ly Distance to stars anywhere in the galaxy where they can be seen Distances to closest galaxies, out to about 50 million ly Distances to the most distant galaxies, out to over 10 billion ly 15-7
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