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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Neil F. Comins • William J. Kaufmann III
Discovering the Universe
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.
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.
(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.
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.
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.
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.
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.
Dynamics of a Grand-Design Spiral Galaxy
This figure summarizes the activities taking place in a grand-design spiral galaxy.
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.
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.
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.
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.
(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.
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.
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.
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.
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.
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 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 in millions of light-years.
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.
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.
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.
(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.
barred spiral galaxy
cluster (of galaxies)
irregular cluster (of galaxies)
poor cluster (of galaxies)
regular cluster (of galaxies)
rich cluster (of galaxies)
spiral density wave
supercluster (of galaxies)
trailing-arm spiral galaxy