Discovering the Universe Eighth Edition Discovering the Universe Eighth Edition Neil F. Comins William J. Kaufmann III CHAPTER 16 Galaxies Galaxies

Hubble Deep Field Image

Hubble’s Tuning Fork Diagram Hubble summarized his classification scheme for galaxies with this tuning fork diagram. Elliptical galaxies are classified by how oval they appear, whereas spirals and barred spirals are classified by the sizes of their central bulges and the correlated winding of their spiral arms. An S0 or SB0 galaxy, also called lenticular galaxy, is an intermediate type between ellipticals and spirals. It has a disk but no spiral arms.

Spiral Galaxies (Nearly Face-on Views) Edwin Hubble classified spiral galaxies according to the tightness of their spiral arms and the sizes of their nuclear bulges. Sa galaxies have the largest nuclear bulges and the most tightly wound spiral arms, whereas Sc galaxies have the smallest nuclear bulges and the least tightly wound arms. The images are different colors because they were taken through filters that pass different colors.

Andromeda (M31) Andromeda is a beautiful spiral galaxy and the only galaxy visible to the naked eye from Earth’s northern hemisphereLocated only 2.5 Mly (0.77 Mpc) from us, Andromeda is gravitationally bound to the Milky Way, and it covers an area in the sky roughly 5 times as large as the full Moon. Two other galaxies, M32 and M110, are also labeled on this photograph.

Spiral Galaxies Seen Nearly Edge-on from the Milky Way a) Because of its large nuclear bulge, this galaxy (called the Sombrero Galaxy) is classified as an Sa. If we could see it face-on, the spiral arms would be tightly wound around a voluminous bulge. (b) Note the smaller nuclear bulge in this Sb galaxy. (c) At visible wavelengths, interstellar dust obscures the relatively insignificant nuclear bulge of this Sc galaxy.

Ripples in Water The usual circular ripples expanding from the place where a rock was thrown into the water. Ripples in rotating water creating spiral arms, as do ripples in the gas and dust of a disk galaxy.

Dynamics of a Grand-Design Spiral Galaxy

Barred Spiral Galaxies As with spiral galaxies, Edwin Hubble classified barred spirals according to the tightness of their spiral arms (which correlates with the sizes of their nuclear bulges). SBa galaxies have the most tightly wound spirals and largest nuclear bulges, SBb have moderately tight spirals and medium- sized nuclear bulges, and SBc galaxies have the least tightly wound spirals and the smallest nuclear bulges.

Giant Elliptical Galaxies The Virgo cluster is a rich, sprawling collection of more than 2000 galaxies about 50 million light-years from Earth. Only the center of this huge cluster appears in this photograph. The two largest galaxies in the cluster are the giant elliptical galaxies M84 and M86.

Elliptical Galaxies Hubble classified elliptical galaxies according to how round or elongated they appear. An E0 galaxy is round; a very elongated elliptical galaxy is an E7. Three examples are shown.

Irregular Galaxies (a) At a distance of only 179,000 light-years, the Large Magellanic Cloud (LMC), an Irr I irregular galaxy, is the third closest known companion of our Milky Way Galaxy. About 62,000 light-years across, the LMC spans 22° across the sky, about 44 times the angular size of the full Moon.

Galaxy Distribution Galaxies group into clusters rather than being randomly scattered through the universe. Galaxies group into clusters rather than being randomly scattered through the universe. A rich cluster contains at least a thousand galaxies A rich cluster contains at least a thousand galaxies A poor cluster may contain only a few dozen up to a thousand galaxies. A poor cluster may contain only a few dozen up to a thousand galaxies. A regular cluster has a nearly spherical shape with a central concentration of galaxies; in an irregular cluster, the distribution of galaxies is asymmetrical. A regular cluster has a nearly spherical shape with a central concentration of galaxies; in an irregular cluster, the distribution of galaxies is asymmetrical. Our Galaxy is a member of a poor, irregular cluster, called the Local Group. Our Galaxy is a member of a poor, irregular cluster, called the Local Group.

Superclusters in Our Neighborhood This diagram shows the distances and relative positions of superclusters within 950 million light-years of Earth. Note also the labeling of some of the voids, which are large, relatively empty regions between superclusters.

Structure in the Universe This map shows the distribution of 62,559 galaxies in two wedges extending in opposite directions from Earth out to distances of 2 billion light-years. Note the prominent voids surrounded by thin areas full of galaxies.

Foamy Structure of the Universe A sponge that recreates the distribution of bright clusters of galaxies throughout the universe. The empty spaces in the foam are analogous to the voids found throughout the universe. The spongy regions are analogous to the locations of most of the galaxies.

The Local Group Our Galaxy belongs to a poor, irregular cluster that consists of about 40 galaxies, called the Local Group. This map shows the distribution of about three-quarters of the galaxies. The Andromeda Galaxy (M31) is the largest and most massive galaxy in the Local Group. The second largest is the Milky Way itself. M31 and the Milky Way are each surrounded by a dozen satellite galaxies. The recently discovered Canis Major Dwarf Galaxy is the Milky Way’s nearest known neighbor.

Interacting and Colliding Galaxies Pairs of colliding galaxies often exhibit long “antennae” of stars ejected by the collision. This particular system is known as NGC 4676 or “the Mice” (because of its tails of stars and gas). It is 300 million light-years from Earth in the constellation Coma Berenices. The collision has stimulated a firestorm of new star formation, as can be seen in the bright blue regions. Mass can also be seen flowing between the two galaxies, which will eventually merge.

Interacting and Colliding Galaxies These two galaxies, NGC 2207 (right) and IC 2163, are orbiting and tidally distorting each other. Their most recent close encounter occurred 40 million years ago when the two were perpendicular to each other and about 1 galactic diameter apart. Computer simulations indicate that they should eventually coalesce.

Simulated Galactic Cannibalism This computer simulation shows a small galaxy (yellow stars) being devoured by a larger, disk-shaped galaxy (blue stars, white gas). Note how spiral arms are generated in the disk galaxy by its interaction with the satellite galaxy.

The Rotation Curves of Four Spiral Galaxies This graph shows how the orbital speed of material in the disks of four spiral galaxies varies with the distance from the center of each galaxy. If most of each galaxy’s mass were concentrated near the center of the galaxy, these curves would fall off at large distances. But these and many other galaxies have flat rotation curves that do not fall off. This indicates the presence of extended halos of dark matter.

Gravitational Lensing of Extremely Distant Galaxies Schematic of how a gravitational lens works. Light from the distant object changes direction due to the gravitational attraction of the intervening galaxy and underlying dark matter. The more distant galaxy appears in two places to the observer on the right.

Gravitational Lensing of Extremely Distant Galaxies Three examples of gravitational lensing: (1) The bluer arc is a galaxy that has been lensed by the redder elliptical galaxy. (2) A pair of bluish images of the same object lensed symmetrically by the brighter, redder galaxy between them. (3) The lensed object appears as a blue arc under the gravitational influence of the group of four galaxies.

Gravitational Lensing of Extremely Distant Galaxies Superimposed in blue on this image of the galaxy cluster CL is the location of dark matter that is gravitationally lensing the galaxies behind it.

Five Galaxies and Their Spectra The photographs of these five elliptical galaxies were all taken at the same magnification. They are labeled according to the constellation in which each galaxy is located. The spectrum of each galaxy is the hazy band between the comparison spectra at the top and bottom of each plate. In all five cases, the so-called H and K lines of calcium are seen. The recessional velocity (calculated from the Doppler shifts of the H and K lines) appears below each spectrum. Note that the fainter— and thus more distant—a galaxy is, the greater is its redshift.

The Hubble Law The distances and recessional velocities of distant galaxies are plotted on this graph. The straight line is the “best fit” for the data. This linear relationship between distance and speed is called the Hubble law. For historical reasons, distances between galaxies, clusters of galaxies, and superclusters of galaxies are usually given in megaparsecs, Mpc, rather than millions of light-years.

Two Supernovae in NGC 664 In 1997 the rare occurrence of two supernovae in the same galaxy at the same time was observed in the spiral galaxy NGC 664, located about 300 Mly (90 Mpc) from Earth. Supernovae observed in remote galaxies are important standard candles used by astronomers to determine the distances to these faraway objects.

Techniques for Measuring Cosmological Distances Astronomers use different methods to determine different distances in the universe. All of the methods shown here are discussed in the text.

Distant Galaxies (a) The young cluster of galaxies MS , shown on the left, contains many orbiting pairs of galaxies, as well as remnants of recent galaxy collisions. Several of these systems are shown at the right.This cluster is located 8 billion light-years away from Earth. (b) This image of more than 300 spiral, elliptical, and irregular galaxies contains several that are an estimated 12 billion light- years from Earth. Two of the most distant galaxies are shown in the images on the right, in red,at the centers of the pictures.

The Expanding Chocolate Chip Cake Analogy The expanding universe can be compared to a chocolate chip cake baking and expanding in the International Space Station. Just as all of the chocolate chips move apart as the cake rises, all of the superclusters of galaxies recede from each other as the universe expands.

Types of Galaxies The Hubble classification system groups galaxies into four major types: spiral, barred spiral, elliptical, and irregular. The Hubble classification system groups galaxies into four major types: spiral, barred spiral, elliptical, and irregular. The arms of spiral and barred spiral galaxies are sites of active star formation. The arms of spiral and barred spiral galaxies are sites of active star formation. According to the theory of self-propagating star formation, spiral arms of flocculent galaxies are caused by the births and deaths of stars over extended regions of a galaxy. Differential rotation of a galaxy stretches the star-forming regions into elongated arches of stars and nebulae that we see as spiral arms. According to the theory of self-propagating star formation, spiral arms of flocculent galaxies are caused by the births and deaths of stars over extended regions of a galaxy. Differential rotation of a galaxy stretches the star-forming regions into elongated arches of stars and nebulae that we see as spiral arms.

Types of Galaxies According to the spiral density wave theory, spiral arms of grand-design galaxies are caused by density waves. The gravitational field of a spiral density wave compresses the interstellar clouds that pass through it, thereby triggering the formation of stars, including OB associations, which highlight the arms. According to the spiral density wave theory, spiral arms of grand-design galaxies are caused by density waves. The gravitational field of a spiral density wave compresses the interstellar clouds that pass through it, thereby triggering the formation of stars, including OB associations, which highlight the arms. Elliptical galaxies contain much less interstellar gas and dust than do spiral galaxies; little star formation occurs in elliptical galaxies. Elliptical galaxies contain much less interstellar gas and dust than do spiral galaxies; little star formation occurs in elliptical galaxies.

Clusters and Superclusters Galaxies group into clusters rather than being randomly scattered through the universe. Galaxies group into clusters rather than being randomly scattered through the universe. A rich cluster contains at least a thousand galaxies; a poor cluster may contain only a few dozen up to a thousand galaxies. A regular cluster has a nearly spherical shape with a central concentration of galaxies; in an irregular cluster, the distribution of galaxies is asymmetrical. A rich cluster contains at least a thousand galaxies; a poor cluster may contain only a few dozen up to a thousand galaxies. A regular cluster has a nearly spherical shape with a central concentration of galaxies; in an irregular cluster, the distribution of galaxies is asymmetrical. Our Galaxy is a member of a poor, irregular cluster, called the Local Group. Our Galaxy is a member of a poor, irregular cluster, called the Local Group. Rich, regular clusters contain mostly elliptical and lenticular galaxies; irregular clusters contain more spiral and irregular galaxies. Giant elliptical galaxies are often found near the centers of rich clusters. Rich, regular clusters contain mostly elliptical and lenticular galaxies; irregular clusters contain more spiral and irregular galaxies. Giant elliptical galaxies are often found near the centers of rich clusters.

Clusters and Superclusters No cluster of galaxies has an observable mass large enough to account for the observed motions of its galaxies; a large amount of unobserved mass must be present between the galaxies. Hot intergalactic gases emit X rays in rich clusters. When two galaxies collide, their stars initially pass each other, but their interstellar gas and dust collide violently, either stripping the gas and dust from the galaxies or triggering prolific star formation. The gravitational effects of a galactic collision can cast stars out of their galaxies into intergalactic space. Galactic mergers occur; a large galaxy in a rich cluster may grow steadily through galactic cannibalism, sometimes producing a giant elliptical galaxy.

Superclusters in Motion A simple linear relationship exists between the distance from Earth to galaxies in other superclusters and the redshifts of those galaxies (a measure of the speed at which they are receding from us). This relationship is the Hubble law: recessional velocity = H o x distance, where H o is the Hubble constant. A simple linear relationship exists between the distance from Earth to galaxies in other superclusters and the redshifts of those galaxies (a measure of the speed at which they are receding from us). This relationship is the Hubble law: recessional velocity = H o x distance, where H o is the Hubble constant. Astronomers use standard candles—Cepheid variables, the brightest supergiants, globular clusters, H II regions, supernovae in a galaxy, and the Tully-Fisher relation— to calculate intergalactic distances. Because of difficulties in measuring the distances to remote galaxies, the value of the Hubble constant, H o, is not known with complete certainty. Astronomers use standard candles—Cepheid variables, the brightest supergiants, globular clusters, H II regions, supernovae in a galaxy, and the Tully-Fisher relation— to calculate intergalactic distances. Because of difficulties in measuring the distances to remote galaxies, the value of the Hubble constant, H o, is not known with complete certainty.

Key Terms barred spiral galaxy cluster (of galaxies) elliptical galaxy galactic merger gravitational lensing Hubble classification Hubble constant Hubble flow Hubble law intergalactic gas irregular cluster (of galaxies) irregular galaxy lenticular galaxy Local Group poor cluster (of galaxies) regular cluster (of galaxies) rich cluster (of galaxies) spiral density wave spiral galaxy standard candle starburst galaxy supercluster (of galaxies) trailing-arm spiral galaxy Tully-Fisher relation

WHAT DID YOU THINK? Are most of the stars in spiral galaxies located in their spiral arms? No. The spiral arms contain only 5% more stars than the regions between the arms.

WHAT DID YOU THINK? Do all galaxies have spiral arms? No. Galaxies may be either spiral, barred spiral, elliptical, or irregular. Only spirals and barred spirals have arms.

WHAT DID YOU THINK? Are galaxies isolated objects? No. Galaxies are grouped in clusters, and clusters are grouped in superclusters.

WHAT DID YOU THINK? Is the universe contracting, unchanging in size, or expanding? The universe is expanding.