CONTENT:CHAP24:FIGURES:FG24_001.PCT
Coma Cluster Of Galaxies - (a) A collection of many galaxies, each consisting of hundreds of billions of stars. Called the Coma Cluster, this group of galaxies lies over 100 million pc from Earth. (The blue spiked object at top right is a nearby star; virtually every other object visible is a genuine galaxy.) (b) A recent Hubble image of part of the cluster.
##
CONTENT:CHAP24:FIGURES:FG24_002.PCT
Shape Variations Among Spiral Galaxies - Variation in shape among different spiral galaxies. As we progress from type Sa to Sb to Sc, the bulges tend to get smaller, while the spiral arms become less tightly wound.
##
CONTENT:CHAP24:FIGURES:FG24_003.PCT
Galaxy NGC3814, Young Spiral Arms - The type Sc galaxy NGC 3184 clearly shows young (blue) stars spread along its spiral arms. 
##
CONTENT:CHAP24:FIGURES:FG24_004.PCT
Sombrero Galaxy M104 - The Sombrero Galaxy, a spiral system seen edge-on. Officially cataloged as M104, this galaxy has a dark band composed of interstellar gas and dust. The large size of this galaxy's central bulge marks it as type Sa, even though its spiral arms cannot be seen.
##
CONTENT:CHAP24:FIGURES:FG24_005.PCT
Shape Variations Among Barred-Spiral Galaxies - Variation in shape among different barred-spiral galaxies. The variation from SBa to SBc is similar to that for the spirals in Figure 24.2, except that now the spiral arms begin at either end of a bar through the galactic center.
##
CONTENT:CHAP24:FIGURES:FG24_006.PCT
Shape Variations Among Elliptical Galaxies - Variation in shape among different elliptical galaxies. (a) The E1 galaxy M49 is nearly circular in appearance. (b) M84 is a slightly more elongated elliptical galaxy. It is classified as E3. Both these galaxies lack spiral structure, and neither shows evidence of interstellar matter.
##
CONTENT:CHAP24:FIGURES:FG24_007.PCT
Shape Variations Among S0 Galaxies - (a) S0 galaxies contain a disk and a bulge but no interstellar gas or spiral arms. They are in many respects intermediate between E7 ellipticals and Sa spirals in their properties. (b) SB0 galaxies are similar, except for a bar of stellar material extending beyond the central bulge.
##
CONTENT:CHAP24:FIGURES:FG24_008.PCT
Sample Of Irregular Galaxies - Photographs of some irregular (Irr II) galaxies. (a) The oddly shaped galaxies NGC 4485 and NGC 4490 may actually be physically close and be interacting with one another gravitationally. (b) The galaxy M82 likewise seems to show an explosive appearance, although interpretations remain uncertain. (c) Many irregular galaxies are small and dim, but this one, NGC 4449, is comparable in both size and luminosity to the Milky Way.
##
CONTENT:CHAP24:FIGURES:FG24_009.PCT
Hubble's "Tuning Fork" Diagram Of Galaxies - Hubble's "tuning fork" diagram, showing his basic galaxy classification scheme. The placement of the four basic galaxy types-ellipticals, spirals, barred spirals, and irregulars-in the diagram is suggestive, but no "evolutionary track" along the sequence (in either direction) is proven.
##
CONTENT:CHAP24:FIGURES:FG24_010.PCT
Distance Ladder For Various Cosmic Scale Measurement Techniques - The inverted pyramid summarizes the various distance techniques used to study different realms of the universe. Radar-ranging, stellar parallax, spectroscopic parallax, and variable stars take us as far as the nearest galaxies in our study of the universe. To go farther, new techniques must be employed, each based on distances known by techniques at lower levels. The top level shown here will be described later in this chapter.
##
CONTENT:CHAP24:FIGURES:FG24_011.PCT
Cepheid Variable Stars In Galaxy M100 - This sequence of six snapshots chronicles the rhythmic changes in a Cepheid variable star in the spiral galaxy M100. The Cepheid appears at the center of each inset, taken at the different times indicated in 1994. The star looks like a square because of the high magnification by the digital CCD camera-we are seeing individual pixels of the image. The 24th-magnitude star periodically doubles in brightness every 7 weeks. (Compare the image of M100 shown in Interlude 5-1. The bright, boxed core is reproduced as Inset A of the chapter opener on page 512.)
##
CONTENT:CHAP24:FIGURES:FG24_012.PCT
The Local Group Of Galaxies - Diagram of the Local Group of some 20 galaxies within approximately 1 Mpc of our Milky Way Galaxy. Only a few are spirals; most of the rest are dwarf elliptical or irregular galaxies. Spirals and irregulars are shown in blue, ellipticals in red.
##
CONTENT:CHAP24:FIGURES:FG24_13A.PCT
Spiral Galaxy M33 - (1 of 2) - The spiral M33 and the dwarf elliptical M32 are two well-known neighbors of the Andromeda Galaxy (M31). M32 is also visible in Figure 23.1, a larger-scale view of the Andromeda system.
##
CONTENT:CHAP24:FIGURES:FG24_13B.PCT
Spiral Galaxy M32 - (2 of 2) - The spiral M33 and the dwarf elliptical M32 are two well-known neighbors of the Andromeda Galaxy (M31). M32 is also visible in Figure 23.1, a larger-scale view of the Andromeda system.
##
CONTENT:CHAP24:FIGURES:FG24_014.PCT
Doppler Effect From Galaxy's Rotation - A galaxy's rotation causes some of the radiation it emits to be blueshifted and some to be redshifted (relative to what the emission would be from an unmoving source). From a distance, when the radiation from the galaxy is combined into a single beam and analyzed spectroscopically, the redshifted and blueshifted components combine to produce a broadening of its spectral lines. The amount of broadening is a direct measure of the rotation speed of the galaxy.
##
CONTENT:CHAP24:FIGURES:FG24_015.PCT
Portion Of Virgo Cluster - Part of the Virgo Cluster of galaxies, about 20 Mpc from Earth. Several large spiral and elliptical galaxies can be seen. The galaxy near the center is a giant elliptical known as M86.
##
CONTENT:CHAP24:FIGURES:FG24_016.PCT
Several Clusters Of Galaxies In Local Universe - Schematic diagram of the locations of several galaxy clusters in our part of the universe. Our Milky Way is only one of these dots and our Local Group only one of the clusters of dots.
##
CONTENT:CHAP24:FIGURES:FG24_017.PCT
Plot Of The Local Supercluster - The Local Supercluster. Each of the 2200 points shown represents a galaxy, and the Sun is at the center of the diagram. The Virgo Cluster and the plane of our own Galaxy are marked. (Our Galaxy is seen edge-on. Its dust obscures our views to the top and the bottom, and two empty V-shaped regions on the map result.) The circle shown here is about 100 Mpc across.
##
CONTENT:CHAP24:FIGURES:FG24_018.PCT
Distant Galaxy Cluster CL0939+4713 - The galaxy cluster CL 0939+4713 contains huge numbers of galaxies and resides roughly a billion parsecs from the Earth. Every patch of light in this photograph is a separate galaxy. Thanks to the improved imaging capability of the Hubble Space Telescope, we can now discern, even at this great distance, spiral structure in some of the galaxies. In addition, we can see galaxies in collision-some tearing matter from one another, others merging into single systems.
##
CONTENT:CHAP24:FIGURES:FG24_019.PCT
Rotation Curves For Nearby Spiral Galaxies - Rotation curves for some nearby spiral galaxies indicate masses of a few hundred billion times the mass of the Sun. The corresponding curve for our own Galaxy (Figure 23.19) is marked in red for comparison.
##
CONTENT:CHAP24:FIGURES:FG24_20A.PCT
Estimating Mass In Binary Galaxy - In a binary galaxy, galaxy masses may be estimated by observing the orbit of one galaxy about another.
##
CONTENT:CHAP24:FIGURES:FG24_20B.PCT
Estimating Mass In Galaxy Cluster - In a galaxy cluster, cluster masses are measured by observing the motion of many galaxies in the cluster and then estimating how much mass is needed to prevent the cluster from flying apart.
##
CONTENT:CHAP24:FIGURES:FG24_021.PCT
X-Ray Images Of Distant Galaxy Cluster Abel 85 - (a) X-ray image of Abell 85, an old, distant cluster of galaxies, taken by the Einstein X-ray satellite observatory. The cluster's X-ray emission is shown in orange. The green graphs display a smooth, peaked intensity profile centered on the cluster but not associated with individual galaxies. (b) The contour map of X rays is superimposed on an optical photo, showing its X rays peaked on Abell 85's central supergiant galaxy. Images like these demonstrated for the first time that the space between the galaxies within galaxy clusters is filled with superheated gas. (c) A ROSAT X-ray image of hot gas within another cluster of galaxies (called Abell 2256). The cluster is nearly a billion pc from Earth and measures about 3 Mpc in diameter.
##
CONTENT:CHAP24:FIGURES:FG24_022.PCT
Relation Between Galaxy's Redshift And Distance - Optical spectra (on the left) of several different galaxies (on the right). The extent of the redshift (denoted by the horizontal yellow arrows) and the distance to each galaxy (in the center) increase from top to bottom.
##
CONTENT:CHAP24:FIGURES:FG24_023.PCT
Plot Of Recession Velocity Against Distance - Plots of recessional velocity against distance (a) for the galaxies shown in Figure 24.22, and (b) for numerous other galaxies within about 1 billion parsecs of the Earth.
##
CONTENT:CHAP24:FIGURES:FG24_024.PCT
Hubble Law On Cosmic Distance Ladder - Hubble's law tops the inverted pyramid of distance techniques. This last method is used to find the distances of astronomical objects all the way out to the limits of the observable universe.
##
CONTENT:CHAP24:FIGURES:FG24_025.PCT
Slice Survey Of Universe - The first slice of a survey of the universe, covering 1057 galaxies out to an approximate distance of 200 Mpc, clearly shows that galaxies and clusters are not randomly distributed on large scales. Instead, they appear to have a filamentary structure, surrounding vast, nearly empty voids. The distances shown assume H0 = 75 km/s/Mpc.
##
CONTENT:CHAP24:FIGURES:FG24_026.PCT
Plot Of 4500 Galaxies On Sky - Combination of data from several redshift surveys of the universe reveal the extent of large-scale structure within 200-300 Mpc of the Sun. The arc on the left is the Great Wall. The empty regions are mostly areas obscured by our Galaxy. Positions for more than 4500 galaxies are plotted here. We assume a Hubble constant of 75 km/s/Mpc.
##
CONTENT:CHAP24:FIGURES:FG24_027.PCT
Deepest View Of Heavens - (a) The present view of the formation of galaxies holds that large systems were built up from smaller ones through collisions and mergers, as shown schematically in this drawing. (b) This photograph, one of the deepest ever taken of the universe, shows objects down to 29th magnitude. It provides "fossil evidence" for hundreds of galaxy shards and fragments, most about 3000 Mpc distant.
##
CONTENT:CHAP24:FIGURES:FG24_028.PCT
Deep Optical Image Of Galaxy Field - Numerous small, irregularly shaped young galaxies can be seen in this very deep optical image (the 6-hour-long exposure captured objects as faint as the 26th magnitude). Redshift measurements indicate that the galaxies lie well over 1000 Mpc from the Earth. Their size, color, and irregular appearance support the theory that galaxies grew by merger and were smaller and less regular in the past. The entire field of view is only 3 by 5 arc minutes.
##
CONTENT:CHAP24:FIGURES:FG24_029.PCT
Peculiar Galaxy Ngc1275 - The peculiar galaxy NGC 1275 contains a system of long filaments that seem to be exploding outward into space. Its blue blobs, as revealed by the Hubble Space Telescope, are probably young globular clusters formed by the collision of two galaxies.
##
CONTENT:CHAP24:FIGURES:FG24_030.PCT
Galactic Cannibalism - This computer-enhanced, false-color composite optical photograph of the galaxy cluster known as Abell 2199 is thought to show an example of galactic cannibalism. The large central galaxy of the cluster (itself 120 kpc along its long axis) is displayed with a superimposed "window." (This results from a shorter time exposure, which shows only the brightest objects that fall within the frame.) Within the core of the large galaxy are several smaller galaxies (the three bright yellow images at center) apparently already "eaten" and now being "digested" (that is, being torn apart and becoming part of the larger system). Other small galaxies swarm on the outskirts of the swelling galaxy, almost certainly to be eaten too.
##
CONTENT:CHAP24:FIGURES:FG24_031.PCT
Computer Model Of Galactic Collision - Galaxies can change their shapes long after their formation. In this computer-generated sequence, two galaxies closely interact over several hundred million years. The smaller galaxy, in red, has gravitationally disrupted the larger galaxy, in blue, changing it into a spiral galaxy. Compare the result of this supercomputer simulation with a photograph (Figure 24.2b) of M51 and its small companion.
##
CONTENT:CHAP24:FIGURES:FG24_032.PCT
Interacting Galaxies, Starburst Activity - This interacting galaxy pair (IC 694, at left, and NGC 3690) shows starbursts now under way in both galaxies-hence the bluish tint. Such intense, short-lived bursts probably last for no more than a few tens of millions of years-a small fraction of a typical galaxy's lifetime.
##
CONTENT:CHAP24:FIGURES:FG24_033.PCT
Density Wave - Density waves are alternating regions of high and low density in a ring. When the density wave is wrapped into a tight spiral, as is the case in Saturn's rings, the high-density peaks form nearly concentric ringlets.
##
CONTENT:CHAP24:SLIDES:SL24_001.PCT
Core Of Starburst Galaxy - NGC235 is the nearest galaxy to us that is undergoing a rapid burst of star formation in its nucleus. Suchactivity can be triggered by an encounter with another  galaxy.
##
CONTENT:CHAP24:SLIDES:SL24_002.PCT
Coma Cluster Of Galaxies - The Coma cluster of galaxies contains a mixture of giant elliptical and spiral galaxies
##
CONTENT:CHAP24:SLIDES:SL24_003.PCT
Galaxy Evolution Mosaic - A selection of galaxies, some dating back to  half to age of the universe, are sorted according to the Hubble galaxy classification system
##
CONTENT:CHAP24:SLIDES:SL24_004.PCT
Ring Galaxy Collision - A rare, spectacular collision between two galaxies results in a spectacular burst of star formation along a 100,000 light year diameter ring.
##
CONTENT:CHAP24:SLIDES:SL24_005.PCT
Gravitational Lens In Cluster AC114 - This pair of L-shapes objects is actually an image of a single distant galaxy whose light has been split into two mirror images of the same object, by the optical  effect of the gravitational field of a foreground  galaxy cluster.  The gravity of the cluster acts a lens by bending, magnifying and brightening the light of  more distant object galaxies.  The unusual L-shape  suggests it is an active galaxy with a jet.
##
CONTENT:CHAP24:SLIDES:SL24_006.PCT
Hubble Deep Field - Several hundred never before seen galaxies are visible in this "deepest-ever" view of the universe,  called the Hubble Deep Field (HDF), made with Hubble Space Telescope.  Besides the classical spiral and elliptical shaped galaxies, there is a bewildering variety of other galaxy shapes and colors that are important clues to understanding the evolution of the universe. Some of the galaxies may have formed less that one billion years after the Big Bang.
##
CONTENT:CHAP24:SLIDES:SL24_007.PCT
Close-Up Of Deep Field - These three images represent select portions of the sky as seen in the Hubble Space Telescope "Deep Field" observation -- the "faintest" view of the universe.  These images were assembled from many separate exposures taken for ten consecutive days between December 18 and 28, 1995.  Besides the classical spiral and elliptical-shaped galaxies, there is a bewildering variety of other galaxy shapes and colors. The never before seen dimmest galaxies are nearly 30th magnitude.
##
CONTENT:CHAP24:SLIDES:SL24_008.PCT
Nucleus Of Galaxy M51 - Hubble Space Telescope has snapped a view of several stellar generations in the central region of the Whirlpool Galaxy (M51), 23 million light-years from Earth in the constellation Canes Venatici (the Hunting Dogs). The galaxy's massive center, the bright ball of light in the center of the photograph, is about 80 light-years across and has a brightness of about 100 million suns.  Astronomers estimate that it is about 400 million years old and has a mass 40 million times larger than our Sun.
##
CONTENT:CHAP24:SLIDES:SL24_009.PCT
Spiral Galaxy In Fornax Cluster - This color image from the Hubble Space Telescope shows a region in NGC 1365, a barred spiral galaxy located in a cluster of galaxies called Fornax. A barred spiral galaxy is characterized by a "bar" of stars, dust and gas across its center. The black and white photograph from a ground-based telescope shows the entire galaxy, which is visible from the Southern Hemisphere.
##
CONTENT:CHAP24:SLIDES:SL24_010.PCT
Core Of Colliding Galaxies - A collision between two galaxies results in a burst of new star formation.
##
CONTENT:CHAP24:VIDEO:VD24_001.MOV
Cepheid Star In Distant Galaxy - The pulsation period of a Cepheid variable star can be used to infer the absolute luminosity of that star -- and thus to determine the distance to the star.  This "yardstick" then provides an estimate of the distance from Earth to the distant galaxy housing the Cepheid star.
##
CONTENT:CHAP24:VIDEO:VD24_002.MOV
Big Dipper Zoom, HST Deep Field - This sequence of still images demonstrates that a small relatively empty-looking portion of sky is actually filled with hundreds of galaxies, when viewed at the faintest limits of current day telescopes, about 30th magnitude.  This faint threshold reveals objects which are less than a billionth the brightness of the faintest stars that can be seen by the human eye. The image was taken with the Hubble Space Telescope, by peering at this region of the sky for ten consecutive days.
##