Week #12 Notes:

1. Week #12 Notes: A Universe of Galaxies

2. The Discovery of Galaxies • In the 1770s, the French astronomer Charles Messier was interested in discovering comets. • To do so, he had to be able to recognize whenever a new fuzzy object (a candidate comet) appeared in the sky. • To minimize possible confusion, he thus compiled a list of about 100 diffuse objects that could always be seen, as long as the appropriate constellation was above the horizon. • Some of them are nebulae, and others are star clusters, which can appear fuzzy through a small telescope such as that used by Messier.

3. The Discovery of Galaxies • To this day, the objects in Messier’s list are commonly known by their Messier numbers (see figures). • They are among the brightest and most beautiful objects in the sky visible from mid-northern latitudes. • A set of photographs of all the Messier objects appears as an Appendix.

4. The Discovery of Galaxies • Other astronomers subsequently compiled additional lists of nebulae and star clusters. • By the early part of the 20th century, several thousand nebulae and clusters were known. • The nebulae were especially intriguing: Although some of them, such as the Orion Nebula (see figure, right), seemed clearly associated with bright stars in our Milky Way Galaxy, the nature of others was more controversial. • When examined with the largest telescopes then available, many of them showed traces of spiral structure, like pinwheels, but no obvious stars (see figure, left).

5. The Shapley-Curtis Debate • Some astronomers thought that these so-called spiral nebulae were merely in our own Galaxy, while others suggested that they were very far away—“island universes” in their own right, so distant that the individual stars appeared blurred together. • The distance and nature of the spiral nebulae was the subject of the well-publicized “Shapley-Curtis debate,” held in 1920 between the astronomers Harlow Shapley and Heber Curtis. • Shapley argued that the Milky Way Galaxy is larger than had been thought, and could contain such spiral-shaped clouds of gas. • Curtis, in contrast, believed that they are separate entities, far beyond the outskirts of our Galaxy. • This famous debate is an interesting example of the scientific process at work.

6. The Shapley-Curtis Debate • Shapley’s wrong conclusion was based on rather sound reasoning but erroneous measurements and assumptions. • For example, one distinguished astronomer thought he had detected the slight angular rotation of a spiral nebula, and Shapley correctly argued that this change would require a preposterously high physical rotation speed if the nebula were very distant. • It turns out, however, that the measurement was faulty.

7. The Shapley-Curtis Debate • Shapley also argued that a bright nova that had appeared in 1885 in the Andromeda Nebula, M31, the largest spiral nebula (see figures), would be far more powerful than any previously known nova if it were very distant. • Unfortunately, the existence of supernovae (this object turned out to be one), which are indeed more powerful than any known nova, was not yet known. • On the other hand, Curtis’s conclusion that the spiral nebulae were external to our Galaxy was based largely on an incorrect notion of our Galaxy’s size; his preferred value was much too small. • Moreover, he treated the nova in Andromeda as an anomaly.

8. Galaxies: “Island Universes” • The matter was dramatically settled in the mid-1920s, when observations made by Edwin Hubble (see figure) at the Mount Wilson Observatory in California proved that the spiral nebulae were indeed “island universes” (now called galaxies) well outside the Milky Way Galaxy. • Using the 100-inch (2.5-m) telescope, Hubble discovered very faint Cepheid variable stars in several objects, including the Andromeda Nebula. • As we saw in Chapter 11, Cepheids are named after their prototype, the variable star d (the Greek letter “delta”) Cephei. • Their light curves (brightness vs. time) have a distinctive, easily recognized shape. • Cepheids are intrinsically very luminous stars, 500 to 10,000 times as powerful as the Sun, so they can be seen at large distances, out to a few million light-years, with the 100-inch telescope used by Hubble.

9. Galaxies: “Island Universes” • Cepheids are very special to astronomers because measuring the period of a Cepheid’s brightness variation (using what we are calling Leavitt’s law, after Henrietta Leavitt) gives you its average luminosity. • And comparing its average luminosity with its average apparent brightness tells you its distance, using the inverse-square law of light. • The Cepheids in the spiral nebulae observed by Hubble turned out to be exceedingly distant. • The Andromeda Nebula, for example, was found to be over 1 million light-years away (the value is now known to be about 2.4 million light-years)—far beyond the measured distance of any known stars in the Milky Way Galaxy.

10. Galaxies: “Island Universes” • From the distance and the measured angular size of the Andromeda Nebula, its physical size was found to be enormous. • Clearly, the “spiral nebulae” were actually huge, gravitationally bound stellar systems like our own Milky Way Galaxy, not relatively small clouds of gas like the Orion Nebula (and so the Andromeda Nebula was renamed the Andromeda Galaxy). • The effective size of the Universe, as perceived by humans, increased enormously with this realization. • In essence, Hubble brought the Copernican revolution to a new level; not only is the Earth just one planet orbiting a typical star among over 100 billion stars in the Milky Way Galaxy, but also ours is just one galaxy among the myriads in the Universe! • Indeed, it is humbling to consider that the Milky Way is one of roughly 50 to 100 billion galaxies within the grasp of the world’s best telescopes such as the Keck twins and the Hubble Space Telescope.

11. Types of Galaxies • Galaxies come in a variety of shapes. • In 1925, Edwin Hubble set up a system of classification of galaxies, and we still use a modified form of it.

12. Spiral Galaxies • There are two main “Hubble types” of galaxies. • We are already familiar with the first kind—the spiral galaxies. • The Milky Way Galaxy and its near-twin, the Andromeda Galaxy (M31; see figure, left), are relatively large examples containing several hundred billion stars. (Most spiral galaxies contain a billion to a trillion stars.) • Another near-twin is NGC 7331 (see figure, below). • Spiral galaxies consist of a bulge in the center, a halo around it, and a thin rotating disk with embedded spiral arms. • There are usually two main arms, with considerable structure such as smaller appendages. • Doppler shifts indicate that spiral galaxies rotate in the sense that the arms trail.

13. Spiral Galaxies • Spiral galaxies viewed along, or nearly along, the plane of the disk (that is, “edge-on”) often exhibit a dark dust lane that appears to divide the disk into two halves (see figures).

14. Spiral Galaxies • In nearly one half of all spirals, the arms unwind not from the nucleus, but rather from a relatively straight bar of stars, gas, and dust that extends to both sides of the nucleus (see figure, top).

15. Spiral Galaxies • Because they contain many massive young stars, the spiral arms appear bluish in color photographs. • Between the spiral arms, the whitish-yellow disks of spiral galaxies contain both old and relatively young stars, but not the hot, massive, blue main-sequence stars, which have already died. • Very old stars dominate the bulge, and especially the faint halo (which is difficult to see), and so the bulge is somewhat yellow/orange or even reddish in photographs (see figures).

16. Spiral Galaxies • Since young, massive stars heat the dusty clouds from which they formed, resulting in the emission of much infrared radiation, the current rate of star formation in a galaxy can be estimated by measuring its infrared power. • Space telescopes such as the Infrared Astronomical Satellite (IRAS, in the mid-1980s) and the Infrared Space Observatory (ISO, in the mid-1990s) were very useful for this kind of work, and it is being continued with the infrared camera on the Hubble Space Telescope and, at even longer infrared wavelengths, with the Spitzer Space Telescope.

17. Spiral Galaxies • About half of the energy emitted by our own Milky Way Galaxy is in the infrared, indicating that a lot of stars are being formed. • But we don’t know why the Andromeda Galaxy, which in optical radiation resembles the Milky Way, emits only 3 per cent of its energy in the infrared. • This galaxy and the Sombrero Galaxy emit infrared mostly in a ring rather than in spiral arms (see figure).

18. Elliptical Galaxies • Hubble recognized a second major galactic category: elliptical galaxies (see figures). • These objects have no disk and no arms, and generally very little gas and dust. • Unlike spiral galaxies, they do not rotate very much. • At the present time, nearly all of them consist almost entirely of old stars, so they appear yellow/orange or even reddish in true-color photographs. • The dearth of gas and dust is consistent with this composition: There is insufficient raw material from which new stars can form. • In many ways, then, an elliptical galaxy resembles the bulge of a spiral galaxy.

19. Elliptical Galaxies • Elliptical galaxies can be roughly circular in shape (which Hubble called type E0), but are usually elongated (from E1 to E7, in order of increasing elongation). • Since the classification depends on the observed appearance, rather than on the intrinsic shape, some E0 galaxies must actually be elongated, but are seen end-on (like a cigar viewed from one end).

20. Elliptical Galaxies • Most ellipticals are dwarfs, like the two main companions of the Andromeda Galaxy (see figure below and (b)), containing only a few million solar masses—a few per cent of the mass of our Milky Way Galaxy. • Some, however, are enormous, consisting of a few trillion stars in a volume several hundred thousand light-years in diameter (see figure (a)). • Many ellipticals may have resulted from two or more spiral galaxies colliding and merging, as we will discuss later.

21. Other Galaxy Types • “Lenticular” galaxies (also known as “S0” [pronounced “ess-zero”] galaxies) have a shape resembling an optical lens; they combine some of the features of spiral and elliptical galaxies. • They have a disk, like spiral galaxies. • On the other hand, they lack spiral arms, and generally contain very little gas and dust, like elliptical galaxies.

22. Other Galaxy Types

23. Other Galaxy Types • A few per cent of galaxies at the present time in the Universe show no clear regularity. • Examples of these “irregular galaxies” include the Small and Large Magellanic Clouds, small satellite galaxies that orbit the much larger Milky Way Galaxy (see figures). • Sometimes traces of regularity—perhaps a bar—can be seen. • Irregular galaxies generally have lots of gas and dust, and are rapidly forming stars. • Indeed, some of them emit 10 to 100 times as much infrared as optical energy, probably because the rate of star formation is greatly elevated.

24. Other Galaxy Types • Some galaxies are called “peculiar.” • These often look roughly like spiral or elliptical galaxies, but have one or more abnormalities. • For example, some peculiar galaxies look like interacting spirals (see figure, above), or like spirals without a nucleus (that is, like rings, (see figure, left)), or like ellipticals with a dark lane of dust and gas. • The ring galaxies are the result of collisions of galaxies.

25. Habitats of Galaxies • Most galaxies are not solitary; instead, they are generally found in gravitationally bound binary pairs, small groups, or larger clusters of galaxies. • Binary and multiple galaxies consist of several members. • An example is the Milky Way Galaxy with its two main companions (the Magellanic Clouds (see figure, top)), or Andromeda and its two main satellites (see figure, bottom). • Both Andromeda and the Milky Way have several even smaller companions.

26. Habitats of Galaxies • Galaxies and clusters of galaxies all over the Universe are studied with the Hubble Space Telescope in the ultraviolet, visible, and near-infrared, the Spitzer Space Telescope in the infrared, and the Chandra X-ray Observatory in x-rays. • NASA’s Galaxy Evolution Explorer (GALEX), launched in 2003, is a small satellite that is studying galaxies and surveying the sky in the ultraviolet.

27. Clusters of Galaxies • The Local Group is a small cluster of about 30 galaxies, some of which are binary or multiple galaxies. • Its two dominant members are the Andromeda (M31) and Milky Way Galaxies. • M33, the Triangulum Galaxy (see figures, top), is a smaller spiral. • M31 and M33, at respective distances of 2.4 and 2.6 million light-years, are the farthest objects you can see with your unaided eye. • The Local Group also contains four irregular galaxies, at least a dozen dwarf irregulars (see figure, bottom), four regular ellipticals, and the rest are dwarf ellipticals or the related “dwarf spheroidals.” • The diameter of the Local Group is about 3 million light-years.

28. Clusters of Galaxies • The Virgo Cluster (in the direction of the constellation Virgo, but far beyond the stars that make up the constellation), at a distance of about 50 million light-years, is the largest relatively nearby cluster (see figure, top). • It consists of at least 2000 galaxies spanning the full range of Hubble types, covering a region in the sky over 15° in diameter—about 15 million light-years. • The Coma Cluster of galaxies (in the direction of the constellation Coma Berenices) is very rich, consisting of over 10,000 galaxies at a distance of about 300 million light-years (see figure, bottom).

29. Clusters of Galaxies • A majority of the galaxies in rich clusters are ellipticals, not spirals. • There is often a single, very large central elliptical galaxy (sometimes two) that is cannibalizing other galaxies in its vicinity, growing bigger with time (see figure, top). • X-ray observations of rich clusters reveal a hot intergalactic gas (10 million to 100 million K) within them, containing as much (or more) mass as the galaxies themselves (see figure, bottom).

30. Superclusters of Galaxies • Clusters are seen to vast distances, in a few cases up to 8 billion light-years away. • When we survey their spatial distribution, we find that they form clusters of clusters of galaxies, appropriately called superclusters. • These vary in size, but a typical diameter is about 100 million light-years. • The Local Group, dozens of similar groupings nearby, and the Virgo Cluster form the Local Supercluster.

31. Superclusters of Galaxies • Superclusters often appear to be elongated and flattened. • The thickness of the Local Supercluster, for example, is only about 10 million light-years, or one tenth of its diameter. • Superclusters tend to form a network of bubbles, like the suds in a kitchen sink (see figures). • Large concentrations of galaxies (that is, several adjacent superclusters) surround relatively empty regions of the Universe, called voids, that have typical diameters of about 100 million light-years (but sometimes up to 300 million light-years).

32. Superclusters of Galaxies • Does the clustering continue in scope? • Are there clusters of clusters of clusters, and so on? • The present evidence suggests that this is not so. • Surveys of the Universe to very large distances do not reveal many obvious super-superclusters. • There are, however, a few giant structures such as the “Great Wall” that crosses the center of the slices shown in the figures. • We will discuss in Chapter 19 how the “seeds” from which these objects formed were visible within 400,000 years after the birth of the Universe.

33. The Dark Side of Matter • There are now strong indications that much of the matter in the Universe does not emit any detectable electromagnetic radiation, but nevertheless has a gravitational influence on its surroundings.

34. Dark Matter Everywhere • We conclude that the Milky Way Galaxy contains large quantities of “dark matter”—material that exerts a gravitational force, but is invisible or at least very difficult to see! • This material has sometimes been called the “missing mass,” especially in older texts, but the term is not appropriate because the mass is present. • Instead, it is the light that’s missing. • Estimates suggest that 80 to 90 per cent of the mass of a typical spiral galaxy consists of dark matter. • However, it has been shown that the amount of matter in the disk cannot exceed what is visible by more than a factor of 2. • Instead, the dark matter is probably concentrated in an extended, spherical, outer halo of material that extends to perhaps 200,000 light-years from the galactic center.

35. Dark Matter Everywhere • Decades ago, the Caltech astronomer Fritz Zwicky was the first to point out that clusters of galaxies could not remain gravitationally bound if they contain only visible matter. • He postulated the existence of some form of dark matter. • However, this idea was largely ignored or dismissed—it was too far ahead of its time.

36. What Is Dark Matter? • What is the physical nature of the dark matter in single and binary galaxies, groups, and clusters? • We just don’t know—this is one of the outstanding unsolved problems in astrophysics. • A tremendous number of very faint normal stars (or even brown dwarfs) is a possibility, though it seems unlikely, extrapolating from the numbers of the faintest stars that we can study. • There is some evidence that part of the dark matter consists of old white dwarfs. • If these and other corpses of dead stars (neutron stars, black holes) accounted for most of the dark matter, however, then where is the chemically enriched gas that the stars must have ejected near the ends of their lives? • Other candidates for the dark matter are small black holes, massive planets (“Jupiters”), and neutrinos.

37. What Is Dark Matter? • Certain kinds of measurements indicate that only a small fraction of the dark matter can consist of “normal” particles such as protons, neutrons, and electrons; the rest must be exotic particles. • Most of the normal dark matter consists of tenuous, million-degree gas in galactic halos. • This gas was recently detected by the absorption spectra it produced in the radiation from background objects, and also from its emission at relatively long x-ray wavelengths. • Though no longer technically “dark” (because we have seen it!), such matter is still generally considered to be part of the “dark matter” that pervades the Universe; it is difficult to detect.

38. What Is Dark Matter? • Probably the most likely candidate for a majority of the dark matter (the “abnormal” part) is undiscovered subatomic particles with unusual properties, left over from the big bang, such as WIMPs—“weakly interacting massive particles.” • Physicists studying the fundamental forces of nature suggest that many WIMPs exist, though it is disconcerting that none has been unambiguously detected in a laboratory experiment.

39. The Birth and Life of Galaxies • It is difficult or impossible to determine what any given nearby galaxy (or our own Milky Way Galaxy) used to look like, since we can’t view it as it was long ago. • However, as we discussed in Week 1, the finite speed of light effectively allows us to view the past history of the Universe: We see different objects at different times in the past, depending on how long the light has been travelling toward us. • At least in a statistical manner we can explore galactic evolution by examining galaxies at progressively larger distances or lookback times, and hence progressively farther back in the past.

40. The Expanding Universe • Early in the 20th century, Vesto Slipher of the Lowell Observatory in Arizona noticed that the optical spectra of “spiral nebulae” (later recognized by Edwin Hubble to be separate galaxies) almost always show a redshift. • The absorption or emission lines seen in the spectra have the same patterns as in the spectra of normal stars or emission nebulae, but these patterns are displaced (that is, shifted) to longer (redder) wavelengths (see figure). • Under the assumption that the redshift results from the Doppler effect, we can conclude that most galaxies are moving away from us, regardless of their direction in the sky. (In Chapter 18, we will see that the redshift is actually caused by the stretching of space, but the equation is the same as that for the Doppler effect, at least at low redshifts.)

41. The Expanding Universe • In 1929, using newly derived distances to some of these galaxies (from Cepheid variable stars), Hubble discovered that the displacement of a given line (that is, the redshift) is proportional to the galaxy’s distance. (In other words, when the redshift we observe is greater by a certain factor, the distance is greater by the same factor.) • Thus, under the Doppler-shift interpretation, the recession speed, v, of a given galaxy must be proportional to its current distance, d (see figure). • This relation is known as Hubble’s law, v=H0d, where H0 (pronounced “H naught”) is the present-day value of the constant of proportionality, H (the factor by which you multiply d to get v). H0 is known as Hubble’s constant.

42. The Expanding Universe • For various reasons, Edwin Hubble’s original data were suggestive but not conclusive. • Subsequently, Hubble’s assistant and disciple Milton Humason joined Hubble in very convincingly showing the relationship (see figure).

43. The Expanding Universe • This behavior is similar to that produced by an explosion: Bits of shrapnel are given a wide range of speeds, and those that are moving fastest travel the largest distance in a given amount of time. • Although Edwin Hubble himself initially resisted this idea, the implication of Hubble’s law is that the Universe is expanding! • However, there is no unique center to the expansion, so in this way it is not like an explosion. • Moreover, the expansion of the Universe marks the creation of space itself, unlike the explosion of a bomb in a preexisting space.

44. The Expanding Universe • Note that Hubble’s law cannot be used to find the distances of stars in our own Galaxy, or of galaxies in our Local Group; these objects are gravitationally bound to us, and hence the expansion of the intervening space is overcome. • Moreover, Hubble’s law does not imply that objects in the Solar System or in our Galaxy are themselves expanding; they are bound together by forces strong enough to overcome the tendency for empty space to expand. • Hubble’s law applies to distant galaxies and clusters of galaxies; the space between us and them is expanding.

45. The Search for the Most Distant Galaxies • With the Hubble Space Telescope, we obtained relatively clear images of faint galaxies that are suspected to be very distant. • The main imaging camera of the time (the Wide Field/Planetary Camera 2) exposed on a small area of the northern sky for 10 days in December 1995. • Though it covers only about one 30-millionth of the area of the sky (roughly the apparent size of a grain of sand held at arm’s length), this Hubble Deep Field contains several thousand extremely faint galaxies (see figure). • If we could photograph the entire sky with such depth and clarity, we would see about 50 to 100 billion galaxies, each of which contains billions of stars!

46. The Search for the Most Distant Galaxies • Three years later, the Hubble Space Telescope got very deep images of another region, this time in the southern celestial hemisphere: the Hubble Deep Field—South. • It looks similar to the northern field, even though it is nearly in the opposite direction in the sky, providing some justification for our assumption that the Universe is reasonably uniform over large scales. • Later, after the Advanced Camera for Surveys was installed on Hubble, it was used to make a Hubble Ultra Deep Field (see figure). • These regions of the deep fields and ultra deep field have since been observed by many other telescopes on the ground and in space, notably the Chandra X-ray Observatory.

47. The Search for the Most Distant Galaxies • Other deep surveys further strengthen our conclusion that we live in a rather typical place in the Universe. • Spectra obtained with large telescopes, especially the two Keck telescopes in Hawaii, confirm that many galaxies in the Hubble Deep/Ultra Deep Fields and other deep surveys have large redshifts and hence are very far away. • Though a few of the galaxies have relatively low redshifts, typical redshifts of the faintest objects are between 1 and 4 (see figure).

48. The Search for the Most Distant Galaxies • Light that we observe at visible wavelengths actually corresponds to ultraviolet radiation emitted by the galaxy, but shifted redward by 100 per cent to 400 per cent! • If we convert these redshifts to “distances” (or, more precisely, lookback times—see Table 16 –1), we find that the galaxies are billions of light-years away. • We see them as they were billions of years in the past, when the Universe was much younger than it is now. • Note that when astronomers say that light from a high-redshift galaxy comes from “the distant universe,” what they really mean is “distant parts of our Universe.” • We do not receive light from other universes, even if they exist!