Copyright © 2010 Pearson Education, Inc. Chapter 15 Normal and Active Galaxies

Copyright © 2010 Pearson Education, Inc. Chapter 15 Normal and Active Galaxies

Copyright © 2010 Pearson Education, Inc. Units of Chapter 15 Hubble’s Galaxy Classification The Distribution of Galaxies in Space Hubble’s Law Active Galactic Nuclei The Central Engine of an Active Galaxy

Copyright © 2010 Pearson Education, Inc.

a) giant globular clusters in the halo. b) small irregular galaxies that orbit the Milky Way. c) large molecular clouds in the disk of our Galaxy. d) the brightest ionized hydrogen regions in our Galaxy. e) spiral nebulae originally discovered by Herschel. Question 1 The Magellanic Clouds are

Copyright © 2010 Pearson Education, Inc. a) giant globular clusters in the halo. b) small irregular galaxies that orbit the Milky Way. c) large molecular clouds in the disk of our Galaxy. d) the brightest ionized hydrogen regions in our Galaxy. e) spiral nebulae originally discovered by Herschel. Question 1 The Magellanic Clouds are

Copyright © 2010 Pearson Education, Inc. Spiral galaxies are classified according to the size of their central bulge. Hubble’s Galaxy Classification

Copyright © 2010 Pearson Education, Inc. Type Sa has the largest central bulge, Type Sb is smaller, and Type Sc is the smallest. Type Sa tends to have the most tightly bound spiral arms, with Types Sb and Sc progressively less tight, although the correlation is not perfect. Hubble’s Galaxy Classification

Copyright © 2010 Pearson Education, Inc. a) ongoing star formation b) a disk, bulge, and halo c) globular clusters in the halo d) open clusters in the disk e) all of the above Question 2 What property is shared by spiral galaxies?

Copyright © 2010 Pearson Education, Inc. a) ongoing star formation b) a disk, bulge, and halo c) globular clusters in the halo d) open clusters in the disk e) all of the above Question 2 What property is shared by spiral galaxies?

Copyright © 2010 Pearson Education, Inc. Similar to the spiral galaxies are the barred spirals. Hubble’s Galaxy Classification

Copyright © 2010 Pearson Education, Inc. Elliptical galaxies have no spiral arms and no disk. They come in many sizes, from giant ellipticals of trillions of stars, down to dwarf ellipticals of fewer than a million stars. Ellipticals also contain very little, if any, cool gas and dust, and show no evidence of ongoing star formation. Many do, however, have large clouds of hot gas, extending far beyond the visible boundaries of the galaxy. Hubble’s Galaxy Classification

Copyright © 2010 Pearson Education, Inc. Ellipticals are classified according to their shape, from E0 (almost spherical) to E7 (the most elongated). Hubble’s Galaxy Classification

Copyright © 2010 Pearson Education, Inc. a) disk and spiral arms. b) halo. c) central bulge. d) open clusters. e) companion galaxies, the Magellanic Clouds. Question 3 Based on their shapes and stars, elliptical galaxies are most like the Milky Way’s

Copyright © 2010 Pearson Education, Inc. a) disk and spiral arms. b) halo. c) central bulge. d) open clusters. e) companion galaxies, the Magellanic Clouds. Question 3 Based on their shapes and stars, elliptical galaxies are most like the Milky Way’s Like the stars and globular clusters in our halo, elliptical galaxies contain little or no gas and dust to make new stars.

Copyright © 2010 Pearson Education, Inc. S0 (lenticular) and SB0 galaxies have a disk and bulge, but no spiral arms and no interstellar gas. Hubble’s Galaxy Classification

Copyright © 2010 Pearson Education, Inc. The irregular galaxies have a wide variety of shapes. These galaxies appear to be undergoing interactions with other galaxies. Hubble’s Galaxy Classification

Copyright © 2010 Pearson Education, Inc. A summary of galaxy properties by type Hubble’s Galaxy Classification

Copyright © 2010 Pearson Education, Inc. Hubble’s “tuning fork” is a convenient way to remember the galaxy classifications, although it has no deeper meaning. Hubble’s Galaxy Classification

Copyright © 2010 Pearson Education, Inc. Cepheid variables allow measurement of galaxies to about 25 Mpc away. However, most galaxies are farther away then 25 Mpc. New distance measures are needed. Type I supernovae Tully-Fisher The Distribution of Galaxies in Space

Copyright © 2010 Pearson Education, Inc. Type I supernovae all have about the same luminosity, as the process by which they happen doesn’t allow for much variation. Type I Supernovae

Copyright © 2010 Pearson Education, Inc. Apparent Vs. Absolute

Copyright © 2010 Pearson Education, Inc. The rotation of a galaxy results in Doppler broadening of its spectral lines. The Distribution of Galaxies in Space

Copyright © 2010 Pearson Education, Inc. Tully-Fisher relation correlates a galaxy’s rotation speed (which can be measured using the Doppler effect) to its luminosity. Tully-Fisher

Copyright © 2010 Pearson Education, Inc. With these additions, the cosmic distance ladder has been extended to about 1 Gpc. The Distribution of Galaxies in Space

Copyright © 2010 Pearson Education, Inc. Here is the distribution of galaxies within about 1 Mpc of the Milky Way. The Distribution of Galaxies in Space

Copyright © 2010 Pearson Education, Inc. There are three spirals in this group – the Milky Way, Andromeda, and M33. These and their satellites – about 45 galaxies in all – form the Local Group. Such a group of galaxies, held together by its own gravity, is called a galaxy cluster. The Distribution of Galaxies in Space

Copyright © 2010 Pearson Education, Inc. A nearby galaxy cluster is the Virgo Cluster; it is much larger than the Local Group, containing about 3500 galaxies. The Distribution of Galaxies in Space

Copyright © 2010 Pearson Education, Inc. Question 4 Hubble took spectra of galaxies in the 1930s. What did he find? a) Most galaxies showed redshifts. b) All galaxies showed blueshifts. c) Galaxies showed about half redshifts and half blueshifts. d) Galaxies showed no line shifts at all. e) Some galaxies showed a redshift that changed into a blueshift at other times.

Copyright © 2010 Pearson Education, Inc. Question 4 Hubble took spectra of galaxies in the 1930s. What did he find? Redshifts of galaxies indicate they are moving away from us. a) Most galaxies showed redshifts. b) All galaxies showed blueshifts. c) Galaxies showed about half redshifts and half blueshifts. d) Galaxies showed no line shifts at all. e) Some galaxies showed a redshift that changed into a blueshift at other times.

Copyright © 2010 Pearson Education, Inc. Universal recession: All galaxies (with a couple of nearby exceptions) seem to be moving away from us, with the redshift of their motion correlated with their distance. Hubble’s Law

Copyright © 2010 Pearson Education, Inc. These plots show the relation between distance and recessional velocity for the five galaxies in the previous figure, and then for a larger sample. Hubble’s Law

Copyright © 2010 Pearson Education, Inc. The relationship (slope of the line) is characterized by Hubble’s constant H 0 : recessional velocity = H 0  distance The value of Hubble’s constant is currently uncertain, with most estimates ranging from 50 to 80 km/s/Mpc. Measuring distances using Hubble’s law actually works better the farther away the object is; random motions are overwhelmed by the recessional velocity. Hubble’s Law

Copyright © 2010 Pearson Education, Inc. a) more distant galaxies showing greater blueshifts. b) distant quasars appearing proportionally dimmer. c) more distant galaxies showing greater redshifts. d) slowly varying Cepheid variables appearing brighter. e) more distant galaxies appearing younger. Question 5 Hubble’s law is based on

Copyright © 2010 Pearson Education, Inc. a) more distant galaxies showing greater blueshifts. b) distant quasars appearing proportionally dimmer. c) more distant galaxies showing greater redshifts. d) slowly varying Cepheid variables appearing brighter. e) more distant galaxies appearing younger. Question 5 Hubble’s law is based on

Copyright © 2010 Pearson Education, Inc. Question 6 Hubble’s constant measures a) the density of galaxies in the universe. b) the luminosity of distant galaxies. c) the reddening of light from dust clouds. d) the speed of a galaxy. e) the rate of expansion of the universe.

Copyright © 2010 Pearson Education, Inc. a) the density of galaxies in the universe. b) the luminosity of distant galaxies. c) the reddening of light from dust clouds. d) the speed of a galaxy. e) the rate of expansion of the universe. Question 6 Hubble’s constant measures Hubble’s law relates how fast galaxies are moving away from us at different distances. A larger value for H 0 implies a faster expansion rate. Velocity = H 0 x Distance

Copyright © 2010 Pearson Education, Inc. a) the size of the universe. b) distances to galaxies. c) the speed of recession of galaxies. d) the density of matter in the universe. e) the temperature of the Big Bang. Question 7 To calibrate Hubble’s constant, astronomers must determine

Copyright © 2010 Pearson Education, Inc. a) the size of the universe. b) distances to galaxies. c) the speed of recession of galaxies. d) the density of matter in the universe. e) the temperature of the Big Bang. Question 7 To calibrate Hubble’s constant, astronomers must determine Distances to galaxies are determined using a variety of “standard candles,” including Cepheid variables, supernova explosions, model galaxies, and model clusters.

Copyright © 2010 Pearson Education, Inc. This puts the final step on our distance ladder. Hubble’s Law

Copyright © 2010 Pearson Education, Inc. About 20–25 percent of galaxies don’t fit well into the Hubble scheme – they are far too luminous. Such galaxies are called active galaxies. They differ from normal galaxies in both the luminosity and type of radiation they emit. Active Galactic Nuclei

Copyright © 2010 Pearson Education, Inc. The radiation from these galaxies is called nonstellar radiation. Many luminous galaxies are experiencing an outburst of star formation, probably due to interactions with a neighbor. These galaxies are called starburst galaxies, and we will discuss them later. The galaxies we will discuss now are those whose activity is due to events occurring in and around the galactic center. Active Galactic Nuclei

Copyright © 2010 Pearson Education, Inc. Active galaxies are classified into three types: Seyfert galaxies, radio galaxies, and quasars. Seyfert galaxies resemble normal spiral galaxies, but their cores are thousands of times more luminous. Active Galactic Nuclei

Copyright © 2010 Pearson Education, Inc. a) supermassive black holes at their cores. b) dark matter. c) self-sustaining star formation. d) spiral density waves. e) hypernova explosions. Question 8 Seyfert and radio galaxies could be powered by

Copyright © 2010 Pearson Education, Inc. a) supermassive black holes at their cores. b) dark matter. c) self-sustaining star formation. d) spiral density waves. e) hypernova explosions. Question 8 Seyfert and radio galaxies could be powered by The Circinus Galaxy, a Seyfert galaxy about 4 Mpc away

Copyright © 2010 Pearson Education, Inc. The rapid variations in the luminosity of Seyfert galaxies indicate that the core must be extremely compact. Active Galactic Nuclei

Copyright © 2010 Pearson Education, Inc. They may have enormous lobes, invisible to optical telescopes, perpendicular to the plane of the galaxy. Radio galaxies emit very strongly in the radio portion of the spectrum. Active Galactic Nuclei

Copyright © 2010 Pearson Education, Inc. Radio galaxies may also be core dominated. Active Galactic Nuclei

Copyright © 2010 Pearson Education, Inc. Core-dominated and radio-lobe galaxies are probably the same phenomenon viewed from different angles. Active Galactic Nuclei

Copyright © 2010 Pearson Education, Inc. Many active galaxies have jets, and most show signs of interactions with other galaxies. Active Galactic Nuclei

Copyright © 2010 Pearson Education, Inc. Quasars – quasi-stellar objects – are starlike in appearance, but have very unusual spectral lines. Active Galactic Nuclei

Copyright © 2010 Pearson Education, Inc. Eventually it was realized that quasar spectra were normal, but enormously redshifted. Active Galactic Nuclei

Copyright © 2010 Pearson Education, Inc. a) they generate energy partly through H to He fusion like stars. b) they show spectra similar to extremely bright O stars. c) their luminosity varies like eclipsing binary stars. d) in short exposure photographs, their images appear stellar. e) they are dense concentrations of millions of stars. Question 9 Quasars are “quasi-stellar” because

Copyright © 2010 Pearson Education, Inc. a) they generate energy partly through H to He fusion like stars. b) they show spectra similar to extremely bright O stars. c) their luminosity varies like eclipsing binary stars. d) in short exposure photographs, their images appear stellar. e) they are dense concentrations of millions of stars. Question 9 Quasars are “quasi-stellar” because Although short-exposure images can appear starlike, many quasars show jets or other signs of intense activity.

Copyright © 2010 Pearson Education, Inc. Solving the spectral problem introduces a new problem – quasars must be among the most luminous objects in the galaxy, to be visible over such enormous distances. Active Galactic Nuclei

Copyright © 2010 Pearson Education, Inc. Active galactic nuclei have some or all of the following properties: High luminosity Nonstellar energy emission Variable energy output, indicating small nucleus Jets and other signs of explosive activity Broad emission lines, indicating rapid rotation The Central Engine of an Active Galaxy

Copyright © 2010 Pearson Education, Inc. This is the leading theory for the energy source in an active galactic nucleus: a black hole, surrounded by an accretion disk. The strong magnetic field lines around the black hole channel particles into jets perpendicular to the magnetic axis.jets The Central Engine of an Active Galaxy

Copyright © 2010 Pearson Education, Inc. In an active galaxy, the central black hole may be billions of solar masses. The accretion disk is whole clouds of interstellar gas and dust; they may radiate away as much as 10–20 percent of their mass before disappearing. The Central Engine of an Active Galaxy

Copyright © 2010 Pearson Education, Inc. The Central Engine of an Active Galaxy The jets emerging from an active galaxy can be quite spectacular.

Copyright © 2010 Pearson Education, Inc. Measurements of the core of the galaxy M87 indicate that it is rotating very rapidly.galaxy M87 The Central Engine of an Active Galaxy

Copyright © 2010 Pearson Education, Inc. One might expect the radiation to be mostly X- and gamma-rays, but apparently it is often “reprocessed” in the dense clouds around the black hole and re-emitted at longer wavelengths. The Central Engine of an Active Galaxy

Copyright © 2010 Pearson Education, Inc. a) sudden bursts of star formation. b) supernova chain reactions in the core. c) the collapse of the core into a larger black hole. d) close encounters with a nearby galaxy. e) dark matter becoming visible and emitting light. Question 10 In active galaxies, the central engine can be “fed” by

Copyright © 2010 Pearson Education, Inc. a) sudden bursts of star formation. b) supernova chain reactions in the core. c) the collapse of the core into a larger black hole. d) close encounters with a nearby galaxy. e) dark matter becoming visible and emitting light. Question 10 In active galaxies, the central engine can be “fed” by Collisions or tidal interaction between galaxies can provide new fuel to power the supermassive black hole engines of active galaxies.

Copyright © 2010 Pearson Education, Inc. Particles will emit synchrotron radiation as they spiral along the magnetic field lines; this radiation is decidedly nonstellar. The Central Engine of an Active Galaxy

Copyright © 2010 Pearson Education, Inc. a) the universe is static. b) the universe is collapsing. c) the universe is expanding. d) the Milky Way is the center of the universe. e) There is no accepted interpretation. Question 11 Hubble’s discovery of galaxy redshifts means

Copyright © 2010 Pearson Education, Inc. a) the universe is static. b) the universe is collapsing. c) the universe is expanding. d) the Milky Way is the center of the universe. e) There is no accepted interpretation. Question 11 Hubble’s discovery of galaxy redshifts means

Copyright © 2010 Pearson Education, Inc. Possible Final Questions to Ponder 1. 1.A solar eclipse can only happen during a new moon. 2.Polaris will not always be the pole star due to precession shifting the celestial pole. 3.A tropical day is not the same length as a sidereal year. 4.The larger the parallax shift, the closer you are to an object. 5.A fatal flaw with Ptolemy’s model is its inability to predict phases of Mercury and Venus. 6.The force of gravity varies with the product of masses and inverse square of the separation distance.

Copyright © 2010 Pearson Education, Inc. Hubble classification organizes galaxies according to shape. Galaxy types include spiral, barred spiral, elliptical, irregular. Objects of relatively uniform luminosities are called “standard candles”; examples include RR Lyrae stars and Type I supernovae. The Milky Way lies within a small cluster of galaxies called the Local Group. Other galaxy clusters may contain thousands of galaxies. Summary of Chapter 15

Copyright © 2010 Pearson Education, Inc. Hubble’s law: Galaxies recede from us faster the farther away they are. Active galaxies are far more luminous than normal galaxies, and their radiation is nonstellar. Seyfert galaxies, radio galaxies, and quasars all have very small cores; many emit high-speed jets. Active galaxies are thought to contain supermassive black holes in their centers; infalling matter converts to energy, powering the galaxy. Summary of Chapter 15, cont.