Neil F. Comins • William J. Kaufmann III
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Neil F. Comins • William J. Kaufmann III. Discovering the Universe Ninth Edition. CHAPTER 16 Galaxies. WHAT DO YOU THINK?. Are most of the stars in spiral galaxies located in their spiral arms? Do all galaxies have spiral arms? Are galaxies isolated objects?

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Discovering the Universe Ninth Edition

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Discovering the universe ninth edition

Neil F. Comins • William J. Kaufmann III

Discovering the Universe

Ninth Edition



What do you think


  • Are most of the stars in spiral galaxies located in their spiral arms?

  • Do all galaxies have spiral arms?

  • Are galaxies isolated objects?

  • Is the universe contracting, unchanging in size, or expanding?

In this chapter you will discover

In this chapter you will discover…

  • how galaxies are categorized by their shapes

  • the processes that produce galaxies of different shapes

  • that galaxies are found in clusters that contain huge amounts of dark matter

  • why clusters of galaxies form in superclusters

  • how some galaxies merge and others devour their neighbors

  • that the universe is changing size

Spiral galaxies nearly face on views

Spiral Galaxies (Nearly Face-on Views)

Edwin Hubble classified spiral galaxies according to the tightness of their spiral arms and the size of their central bulges. Sa galaxies have the largest central bulges and the most tightly wound spiral arms, whereas Sc galaxies have the smallest central bulges and the least tightly wound arms. The images are different colors because they were taken through filters that pass different colors.

Discovering the universe ninth edition

Andromeda (M31)

Andromeda is a beautiful spiral galaxy and the only galaxy visible to the naked eye from Earth’s northern hemisphere. It has dim, red-giant stars (not visible here) extending half a million light-years from its nucleus. Without a telescope, Andromeda appears to be a fuzzy blob in the constellation of the same name. Located 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. The points of light that pepper the image are stars in our Galaxy.

Spiral galaxies seen nearly edge on from the milky way

Spiral Galaxies Seen Nearly Edge On from the Milky Way

(a) Because of its large central 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.

Discovering the universe ninth edition

Variety in Spiral Arms

The differences in spiral galaxies suggest that at least two mechanisms create spiral arms. (a) This flocculent spiral galaxy has fuzzy, poorly defined spiral arms. (b) This grand-design spiral galaxy has well-defined spiral arms.

Discovering the universe ninth edition

The Winding Dilemma

The rotation curve of the disk stars in our Galaxy indicates that most of them have the same linear (straight-line) speed. Those farther from the center take longer to go around because they have a greater distance to travel at the same speed than stars closer to the center of the Galaxy, which orbit in smaller circles. All four dots in these drawings have circular orbits at the same linear speed. Think of them as (a) a few stars initially in a straight line. As time goes on, the outer stars are left behind, creating (b–d) a spiral shape that becomes more and more tightly wound. Such tightening is not observed in our Galaxy or in other galaxies.

Discovering the universe ninth edition

Ripples in Water

(a) The usual circular ripples expanding from the place where a rock was thrown into the water.

(b) Ripples in rotating water creating spiral arms, as do ripples in the gas and dust of a disk galaxy.

Discovering the universe ninth edition

Compression Wave in Traffic Flow

When normal traffic flow is slowed down, cars bunch together. In a grand-design galaxy, a density wave moves through the stars and gas. The wave is merely a region of slightly denser matter, which, in turn, creates more gravitational force. This force compresses the gas and enhances star formation, which highlights the spiral density wave.

Discovering the universe ninth edition

Dynamics of a Grand-Design Spiral Galaxy

This figure summarizes the activities taking place in a grand-design spiral galaxy.

Barred spiral galaxies

Barred Spiral Galaxies

As he did with spiral galaxies, Edwin Hubble classified barred spirals according to the tightness of their spiral arms (which correlates with the size of their nuclear bulges). (a) SBa galaxies have the most tightly wound spirals and largest central bulges; (b) SBb galaxies have moderately tight spirals and medium-sized central bulges; and (c) SBc galaxies have the least tightly wound spirals and the smallest central bulges.

Discovering the universe ninth edition

Giant Elliptical Galaxies

The Virgo cluster is a rich, sprawling collection of more than 2000 galaxies about 50 million ly 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.

Discovering the universe ninth edition

A Dwarf Elliptical Galaxy

This nearby E4 galaxy, called Leo I, is about 600,000 ly from Earth. It is only 3000 ly in diameter and is so sparsely populated with stars that you can see right through its center. It is a satellite of the Milky Way.

Discovering the universe ninth edition

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.

Discovering the universe ninth edition

Irregular Galaxies

(a) At a distance of only 179,000 ly, the Large Magellanic Cloud (LMC), an Irr I irregular galaxy, is passing close to our Milky Way Galaxy. About 62,000 ly across, the LMC spans 22° across the sky, about 44 times the angular size of the full Moon. Note the huge H II region (called the Tarantula Nebula or 30 Doradus). Its diameter of 800 ly and mass of 5 million solar masses make it the largest known H II region. (b) The small irregular (Irr II) galaxy NGC 4485 interacts with the highly distorted Sc galaxy NGC 4490, also called the Cocoon Galaxy. This pair is located in the constellation Canes Venatici.

Discovering the universe ninth edition

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 size of their central bulges and the correlated winding of their spiral arms. An S0 or SB0 galaxy, also called a lenticular galaxy, is an intermediate type between ellipticals and spirals. It has a disk but no spiral arms.

Discovering the universe ninth edition

A Cluster of Galaxies

This group of galaxies, called the Hercules cluster, is about 650 million ly from Earth. Both elliptical and spiral galaxies within this cluster can be easily identified.

Discovering the universe ninth edition

Superclusters in Our Neighborhood

This diagram shows the distances and relative positions of superclusters within 950 million ly of Earth. Note also the labeling of some of the voids, which are large, relatively empty regions between superclusters.

Discovering the universe ninth edition

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 33.25 billion ly. Note the prominent voids surrounded by thin areas full of galaxies.

Discovering the universe ninth edition

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.

Discovering the universe ninth edition

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 Milky Way and Andromeda galaxies are the largest and most massive galaxies in the Local Group. Andromeda (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.

Discovering the universe ninth edition

A Recently Discovered Member of the Local Group

The galaxy Antlia was first detected in 1997. It lies about 3 million ly away, outside the region depicted in Figure 16-19. This galaxy contains only about a million stars.

Discovering the universe ninth edition

The Coma Cluster

(a) This rich, regular cluster that contains thousands of galaxies is about 300 million ly from Earth. (b) Regular clusters are composed mostly of elliptical and lenticular galaxies and are sources of X rays. This Chandra image shows Coma’s central region, which is 1.5 million ly across. The gas cloud emitting most of these X rays is 100 million K.

Discovering the universe ninth edition

Galaxies with Rings

A composite image of the Cartwheel Galaxy. This ring-shaped assemblage 500 million ly from Earth is the likely result of one galaxy, probably the blue-white one below it at the eight o’clock position, having passed through the middle of the larger one. Astronomers suspect that the passage created a circular density wave in the Cartwheel that stimulated a burst of star formation, creating many bright blue and white stars. Ultraviolet is in blue, visible light in green, infrared in red, and X ray in violet.

Discovering the universe ninth edition

Galaxies with Rings

This is an infrared image of the Andromeda Galaxy. The ring of hot dust indicates star formation, probably caused by the passage of another galaxy through Andromeda. The fact that the ring is disturbed suggests that yet another galaxy had a close interaction with Andromeda.

Discovering the universe ninth edition

A Starburst Galaxy

NGC 1512, located 30 Mly away in the constellation Horologium, is 70,000 ly across. (Inset) A ring of vigorous star formation 2400 ly wide highlights this ultraviolet, visible light, and infrared composite image of the core of this galaxy. NGC 1512 may have recently passed close to its companion NGC 1510, thereby stimulating the starburst. Such rings of star formation are common in starburst galaxies.

Discovering the universe ninth edition

The M81 Group

The Irr II starburst galaxy M82 is in a nearby cluster of about a dozen galaxies, including the spectacular spiral M81. Several of the galaxies in this cluster are connected by streamers of hydrogen gas. (a) The three brightest galaxies at visual wavelengths. The inset on the left shows large volumes of hydrogen gas, in red, being ejected from M82. (b) This radio image, created from data taken by the Very Large Array, shows the streamers of hydrogen gas that connect the bright galaxies and also several dim ones, seen as regions of bright orange here.

Discovering the universe ninth edition

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 ly 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.

Discovering the universe ninth edition

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.

Discovering the universe ninth edition

Merging Galaxies

This contorted object, NGC 6240, in the constellation Ophiuchus, is the result of two spiral galaxies in the process of merging. The widespread blue-green area reveals that the collision between the two galaxies has triggered an immense burst of star formation. (Inset) The Chandra X­ray Observatory shows that at the heart of this system are two supermassive black holes, one from each of the original galaxies. Within a few hundred million years, these black holes are expected to merge into a single more-massive black hole.

Discovering the universe ninth edition

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 the disk galaxy’s interaction with the satellite galaxy.

Discovering the universe ninth edition

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 its center, 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.

Discovering the universe ninth edition

Gravitational Lensing of Extremely Distant Galaxies

This is a 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 different places than it actually is.

Discovering the universe ninth edition

Gravitational Lensing of Extremely Distant Galaxies

Here are three examples of gravitational lensing: (1) The blue ring 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; and (3) the lensed object appears as a blue arc under the gravitational influence of the group of four galaxies.

Discovering the universe ninth edition

Gravitational Lensing of Extremely Distant Galaxies

(c) Composite image of galaxy cluster 1E0657-56 showing visible galaxies, X-ray emitting gas (red) and dark matter (blue).

(d) A model of how the gas and dark matter in 1E0657-56 could have become separated.

Discovering the universe ninth edition

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.

Discovering the universe ninth edition

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 in millions of light-years.

The expanding chocolate chip cake analogy

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.

Discovering the universe ninth edition

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.

Discovering the universe ninth edition

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. The two supernovae overlap each other, as shown. The upper, yellow-orange supernova was observed to occur 2 months before the hotter, blue one, which was observed to occur less than 2 weeks before this image was made and had not yet achieved maximum brightness.

Distant galaxies

Distant Galaxies

(a) The young cluster of galaxies MS1054-03, 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 ly away from Earth. (b) This image of more than 300 spiral, elliptical, and irregular galaxies contains several galaxies that are an estimated 12 billion ly from Earth. Two of the most distant galaxies are shown in the images on the right, in red, at the centers of the pictures.

Summary of key ideas

Summary of Key Ideas

Types of galaxies

Types of Galaxies

  • The Hubble classification system groups galaxies by their shapes into four major types: spiral, barred spiral, elliptical, and irregular.

  • 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.

Types of galaxies1

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.

  • Elliptical galaxies contain much less interstellar gas and dust than do spiral galaxies; little star formation occurs in elliptical galaxies.

  • Irregular galaxies are rich in gas and dust, and star formation occurs in them.

  • Lenticular galaxies are disk galaxies without spiral arms.

Clusters and superclusters

Clusters and Superclusters

  • 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 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.

  • 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.

  • Each galaxy is held together with the aid of dark matter.

Clusters and superclusters1

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 causing gas and dust to be stripped 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.

Superclusters in motion

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 = Ho x distance, where Ho 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, Ho, is not known with complete certainty.

Key terms

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 think1


  • 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 think2


  • Are galaxies isolated objects?

  • No. Galaxies are grouped in clusters, and clusters are grouped in superclusters.

What did you think3


  • Is the universe contracting, unchanging in size, or expanding?

  • The universe is expanding.

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