A Diversity of Galaxies study guide

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A Diversity of Galaxies study guide

CHAPTER 17
A Diversity of Galaxies

CHAPTER OUTLINE

17-1 The Hubble Classification

1. In 1924, Hubble found Cepheid variables in three spiral nebulae, showing that they were actually spiral galaxies. The evidence that galaxies existed outside the Milky Way expanded our appreciation of the size of the universe.

2. Hubble divided galaxies into three basic types: spiral, elliptical, irregular. Each major classification contains subdivisions.

3. An elliptical galaxy is one of a class of galaxies that have smooth spheroidal shapes. An irregular galaxy is a galaxy of irregular shape that cannot be classified as spiral or elliptical.

Spiral Galaxies

1. Hubble divided spiral galaxies into two groups: ordinary spirals and barred spirals.

2. Ordinary spirals are designated with an S; barred spirals are designated with an SB.

3. A barred spiral galaxy is a spiral galaxy in which the spiral arms come from the ends of a bar through the nucleus rather than from the nucleus itself.

4. Each type of spiral galaxy is then further subdivided into categories a, b, c, depending on how tightly the spiral arms are wound around the nucleus. Galaxies with the most tightly wound arms are type a; they also have the most prominent nuclear bulges.

5. Up to 2/3 of all spirals contain bars. The bar system provides an efficient mechanism for fueling star birth at the center of an SB galaxy.

6. Galaxies that seem to have the nuclear bulge and disk of a spiral, but no arms, are called lenticular (or S0) galaxies.

7. Type c spirals contain more gas and dust than type a, resulting in a larger proportion of their mass being involved in star formation.

8. Most spiral galaxies are from 50,000 to 2,000,000 million light-years across and contain from 10⁹ to 10¹² stars.

Elliptical Galaxies

1. Elliptical galaxies are ellipsoids; they are classified from round (E0) to very elongated (E7).

2. Most of the galaxies in existence are ellipticals, but most of these are smaller than spiral galaxies.

3. A few giant elliptical galaxies have 2´10¹³ stars and are thus larger than any spiral galaxy.

Irregular Galaxies

1. Fewer than 20% of all galaxies fall in the category of irregulars, and they are all small, normally having fewer than 25% of the number of stars in the Milky Way.

2. Collisions between galaxies are not unusual because on average galaxies are separated by distances only about 20 times their diameter. On the other hand, stars in a galaxy rarely collide since they are separated by distances that are millions of time their diameter.

3. Because of their great distances, galaxies exhibit no proper motion. Evidence of past collisions has to come from present appearance.

4. Computer simulations show that colliding galaxies actually pass through one another with few collisions between individual stars. However, the large dust and gas clouds in the galaxies make them more likely targets, resulting in increased star formation rates.

5. Bursts of star formation may also occur as a result of tidal interactions among neighboring galaxies.

6. Galactic cannibalism often occurs as a result of collisions.

Hubble’s Tuning Fork Diagram

1. Hubble’s tuning fork diagram relates the various types of galaxies. In his plan, S0 galaxies form the connecting link, because they have characteristics of both elliptical and spiral galaxies.

2. Astronomers once also thought the diagram represented an evolutionary sequence, but this interpretation has been discarded as old stars have been found in all three types.

17-2 Measuring Galaxies

1. Most important properties of a galaxy that we can measure are its distance, mass, and motion.

Distances Measured by Various Indicators

1. Cepheid variables are excellent distance indicators but can be seen in only relatively nearby galaxies, out to perhaps 200 million light-years.

2. Bright stars (giants, supergiants, novae) can also be used as distance indicators.

3. Large globular clusters and supernovae are of consistent brightness so they, too, can be used to determine distances to more distant galaxies.

4. These objects allow astronomers to determine distances out to about 1000 million light-years.

5. Starting with the period-luminosity relationship of Cepheids, astronomers are able to follow a chain of reasoning and observation that allows them to determine the distances to galaxies too far away for their Cepheids to be visible.

6. As one distance measurement builds on another in a series of steps, constant checks are always being made as new data arrive. Otherwise, an error in the first step will propagate up through the chain of steps and lead to wrong conclusions.

7. In this analysis we are assuming that galaxies in our neighborhood are basically the same as those farther away. This may seem reasonable but keep in mind that we are seeing distant galaxies as they were in the past, not as they are today.

The Hubble Law

1. In 1912, Slipher found that spiral nebulae had redshifted spectra indicating that they were moving away from us at tremendous velocity.

2. In 1920s, Hubble and Humason showed that there is a relationship between the recessional velocities of galaxies and their distances.

3. Hubble showed that the universe is expanding, and his work is the foundation for today’s theories of cosmology—the study of the nature and evolution of the universe as a whole.

4. The redshift that Hubble observed is not due to the Doppler effect.

5. The Hubble law states that a galaxy’s recessional speed (u) is directly proportional to its distance (d): u = H₀d, where H₀ is the Hubble constant (the proportionality constant in the Hubble law; the ratio of recessional velocities of galaxies to their distances).

6. Modern day measurements of the Hubble constant place it between 15 and 25 km/s per million light-years (Mly) or between 50 and 80 km/s per megaparsec (Mpc).

7. The latest observations of the radiation left over from the hot big bang indicate a value of H₀ = 73 ± 3 (km/s)/Mpc or about 22.5 (km/s)/Mly.

8. Determining a precise value for the Hubble constant is difficult because accurate measurements of distances to galaxies far away are hard to obtain.

9. The value of H₀ changes with time. It is simply the slope of the line in the graph of recessional velocity of galaxies versus their distance as measured during this period of time in the universe’s life.

The Hubble Law Used to Measure Distance

1. For the most distant galaxies, most of our distance indicators can be seen. Therefore, the Hubble law can be used to determine their distances.

The Tully-Fisher Relation

1. The Tully-Fisher relation holds that the wider the 21-centimeter spectral line, the greater the absolute luminosity of a spiral galaxy.

2. Using the Tully-Fisher relation, astronomers can determine the absolute magnitude of a galaxy and use it as a distance indicator.

17-3 The Masses of Galaxies

1. A galaxy’s mass can be determined by observing the rotation periods of some parts of it (using Doppler shift data) and then applying Kepler’s third law.

2. Another method is to use a pair of galaxies revolving around each other. The problem with this method is that it is difficult to determine the angle of the plane of revolution to our line of sight.

Clusters of Galaxies; Missing Mass

1. Most galaxies are part of clusters. A cluster of galaxies is a gravitationally linked assemblage.

2. The local group of galaxies is a cluster of 20 or so galaxies that includes the Milky Way Galaxy, the Andromeda galaxy, and the two Magellanic Clouds.

3. A third method of measuring the masses of galaxies takes advantage of their clustering. It uses the Doppler effect to find the speed (and thus period) of a galaxy at the outskirts of a cluster.

4. The cluster method gives mass values for clusters that are much greater than is accounted for by the visible stars within the galaxies in the cluster.

5. For the Milky Way we can account for as little as 1/10 of the total mass of the Galaxy.

6. Missing mass is the difference between the mass of clusters of galaxies as calculated from Keplerian motions and the amount of visible mass.

7. Several possibilities have been proposed for the nonluminous matter.

(i) Ordinary “nonluminous” matter; composed of ordinary matter but not easily observed (e.g., planets, brown dwarfs, very old white dwarfs, etc.)
(ii) Hot dark matter; neutrinos and other exotic particles (introduced by theories but not observed yet) moving at very high speeds
(iii) Black holes
(iv) Cold dark matter; an exotic form of matter, moving at relatively slow speed, which can be detected only by its gravitational interactions; it appears to be quite abundant throughout the universe.

8. It seems that the universe is only about 4% normal matter and 20% dark matter, the remaining 76% being dark energy (discussed in Chapter 18).

9. Dark matter is distributed in galaxies and clusters of galaxies in a way similar to visible matter, as shown by the rotation curves of galaxies.

10. Galactic halos may contain much of the missing matter.

11. A supercluster is a group of clusters of galaxies. Our local supercluster contains the local group and the Virgo cluster. Between superclusters are great voids with no galaxies.

12. It seems that matter in the universe forms a cosmic web in which galaxies are formed along filaments of normal and dark matter, and clusters are formed at the intersections of these filaments.

17-4 The Origin of Galactic Types

1. Two modern theories—the cloud density theory and the merger theory—purport to explain why galaxies exist in various types.

The Cloud Density Theory

1. Elliptical galaxies formed from the densest gas/dust clouds. Rapid star formation then used up the gas/dust before a disk had a chance to form.

2. Clouds with lower density would have formed stars less frequently, and the dust and gas would have collapsed into a disk before star formation used it all up.

The Merger Theory

1. According to this theory, spiral galaxies formed before elliptical galaxies, and ellipticals are the result of mergers of spirals.

2. In clusters where galaxies are packed close together, ellipticals dominate, supporting the notion of frequent mergers. In loosely packed clusters of galaxies, ellipticals are fairly rare.

3. At this point neither theory explains irregular galaxies well. Some irregulars are seen to be pairs of galaxies in collision.

Look-Back Time

1. We have observed objects that may be as far away as 13 billion light-years. This means that the light we see left these objects 13 billion light-years ago.

2. Look-back time is the time light from a distant object has traveled to reach us.

3. The look-back time complicates our interpretation of galaxies because the farther out we look, the earlier in time we are seeing them. Our assumption that distant clusters are similar to nearby clusters may not be valid, since we have observed galactic cannibalism in large clusters of galaxies.

17-5 Active Galaxies

1. All galaxies emit radio waves; for a normal galaxy, radio waves constitute only about 1% of the galaxy’s total luminosity.

2. A radio galaxy is a galaxy having greatest luminosity at radio wavelengths. A typical radio galaxy emits millions of times more energy in radio waves than does a normal galaxy.

3. Cygnus A, the first radio galaxy, was discovered in 1951 and has a double-lobed radio source associated with the visible light image.

4. Most of the galaxies associated with double-lobed radio sources are either giant ellipticals or spirals.

5. The radio lobes are enormous and mark the positions where the outflows (jets) start interacting with the intergalactic medium.

6. Radio galaxies often appear unusual when viewed in visible light.

7. Radio galaxies are one type of a group of high-energy galaxies called active galaxies. An active galaxy is a galaxy with an unusually luminous nucleus. Because the energy of an active galaxy comes from its nucleus, astronomers often refer to active galactic nuclei (AGNs) rather than active galaxies.

8. Jets seem to be a universal phenomenon. They are the natural byproducts of accretion onto a compact objects, emanating at right angles to the disk that surrounds the object. They are mostly well-collimated and transfer energy, matter, momentum, and magnetic fields from the central region to the surrounding environment.

Quasars

1. In 1960 an unusual star-like object—3C 273—was discovered that emitted intense radio waves. The object appeared to be very small, it had a small jet protruding from it, and the radio waves were emanating from the jet and the main body of the object.

2. The spectra of 3C 273 and 3C 48 (the second unusual object discovered in 1960) showed emission lines, which could not be identified. Because of their star-like appearance and strong radio emission, the objects were named quasars.

3. A quasar (quasi-stellar radio source) is a small, intense celestial source of radiation with a very large redshift.

4. In 1963, the unusual spectral lines found in 3C 273 and 3C 48 were shown to be highly redshifted hydrogen lines. If the redshifts are caused by the Doppler effect, the quasars are moving at 15% and 30% of the speed of light, respectively.

5. Since the early 1960s, more than 23,000 quasars have been discovered. Unlike 3C 273 and 3C 48, most quasars are not sources of radio waves. Most are blue-white objects and X-ray emitters.

6. Many quasars vary in intensity in an irregular way, changing intensity in weeks or months. This observation confirms their small size.

7. Redshifts for quasars range from 0.06 up to 6.41 for the farthest known quasar. The latter quasar is receding at about 96.4% the speed of light.

8. At such great distances, a bright quasar must be 1000 times more luminous than a galaxy like ours, while being much smaller than a typical galaxy.

Competing Theories for the Quasar Redshift

1. The local hypothesis is a proposal stating that quasars are much nearer than a cosmological interpretation of their redshifts would indicate.

2. If quasars were local, then we would see at least some highly blueshifted ones, but we don’t.

3. Most astronomers now agree that quasars’ redshifts do fit the Hubble law.

Seyfert Galaxies

1. Observations of quasars suggest that there are many similarities between quasars and active galaxies.

2. A Seyfert galaxy is one of a class of spiral galaxies having active nuclei and spectra containing emission lines.

3. It now appears that quasars may be at the nuclei of some type(s) of galaxies.

Quasars and Gravitational Lenses

1. Twin quasars were discovered in 1979, having the same luminosity, the same redshift, and identical spectra.

2. According to the general theory of relativity, a gravitational lens is the phenomenon in which the gravity due to a massive body between a distant object and the viewer bends light from the distant object and causes it to be seen as two or more objects.

3. Since 1979 many examples of gravitational lensing have been found.

4. When the alignment between the viewer, distant object and massive body is perfect, we observe a ring (called an Einstein ring).

5. Gravitational lenses are important not only because they provide another confirmation of the general theory of relativity but also because they indicate that quasars are indeed very distant.

6. A graph of the density of quasars as a function of distance shows that most quasars appear at a fairly specific distance from us. Since distance is proportional to time, this indicates that quasars existed during a relatively short periodof time in the distant past.

Quasars, Blazars, and Superluminal Motion

1. In the early 1970s, astronomers discovered blazars (BL Lac objects). Blazars are especially luminous AGNs that vary in luminosity by a factor of up to 100 in just a few months.

2. Radio observations of blazers indicated that they are double radio sources oriented in such a way that one jet is coming straight (or nearly so) at us. This was supported by observations of superluminal motion, motion that appears to occur faster than the speed of light.

3. Superluminal motion is also observed in some quasars. It is simply a projection effect, but does indicate very high speeds for the material in the jets.

17-6 The Nature of Active Galactic Nuclei

1. According to present theory, the tremendous energy that comes from an AGN is caused by an immense black hole at the nucleus of the galaxy. The black hole is surrounded by an accretion disk heated by infalling material.

2. The leading theory on the nature of AGNs holds that the different observed types of AGNs are basically the same, and that they appear different depending upon their orientation with respect to us.

3. According to this unification theory, when an AGN is viewed edge-on we see it as a radio galaxy (radio lobes and jets). When an AGN is viewed at a small angle, we see it as a quasar. When the jet is aimed directly toward us, we see the AGN as a blazar.

4. In order to test this theory we must detect radiation from AGNs that is not blocked and is not affected by the orientation of the dust torus surrounding the accretion disk around the central black hole. Such requirement is fulfilled by far-infrared radiation.

5. In 2001, ESA’s Infrared Space Observatory showed that very hot and luminous quasar cores are found even in weak radio galaxies at large distances.

6. AGNs are not found in our neighborhood because previous AGNs are the ancestors of today’s galaxies.

7. A census of many nearby galaxies suggests that nearly all of them harbor supermassive black holes that once powered quasars. The mass of the black hole is proportional to the mass of the host galaxy and the number and masses of the black holes are consistent with what would have been required to power the quasars.

8. It seems that galaxies have progressed from having quasars or blazars at their centers, to Seyferts or radio galaxies, to normal spiral or elliptical galaxies.

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