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Chapter 14 Investigating Other Galaxies. “Island Universes”. Prior to the construction of large telescopes, it was not clear that galaxies really were other galaxies. Edwin Hubble was the first to demonstrate that M31 is actually a galaxy that lies far beyond the Milky Way.
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Chapter 14 Investigating Other Galaxies
“Island Universes” • Prior to the construction of large telescopes, it was not clear that galaxies really were other galaxies. • Edwin Hubble was the first to demonstrate that M31 is actually a galaxy that lies far beyond the Milky Way.
Measuring Galaxy Distances with Cepheid Variables • Hubble carefully examined photographic plates and discovered what he at first thought to be a nova. • Referring to previous plates of that region, he soon realized that the object was actually a Cepheid variable star. • Hubble realized that for these luminous stars to appear as dim as they were on his photographs of the Andromeda “Nebula,” they must be extremely far away.
Spiral Galaxies • Edwin Hubble classified spiral galaxies according to the texture of their spiral arms and the relative size of their central bulges. • Sa galaxies have smooth, broad spiral arms and the largest central bulges. • Sc galaxies have narrow, well-defined arms and the smallest central bulges.
Barred Spiral Galaxies • As with spiral galaxies, Hubble classified barred spirals according to the texture of their spiral arms. • SBa galaxies have the smoothest spiral arms and the largest central bulges. • SBc galaxies have narrow, well-defined arms and the smallest central bulges.
Elliptical Galaxies • Hubble classified elliptical galaxies according to how round or flattened they look. • A galaxy that appears round is labeled E0, and the flattest appearing elliptical galaxies are designated E7.
Giants and Dwarfs • Most galaxies are about the same size, except for the giant and dwarf ellipticals. • Giant ellipticals are rather rare. • Dwarf ellipticals are more common and transparent.
Hubble’s Tuning Fork Diagram • An elliptical galaxy is classified by how flattened it appears. • A spiral or barred spiral galaxy is classified by the texture of its spiral arms and the size of its central bulge. • A lenticular galaxy is intermediate between ellipticals and spirals. • Irregular galaxies do not fit into this simple classification scheme.
Variable Stars and Type I Supernovae • A standard candle—an object, such as a star, that lies within a galaxy and for which we know its luminosity. • For nearby galaxies, Cepheid variable stars make reliable standard candles. • For far galaxies we use Type I supernovae.
The Distance Ladder • Astronomers employ a variety of techniques for determining the distances to objects beyond the solar system. • One technique can be used to calibrate another. • Each division on the scale indicates a tenfold increase in distance, such as from 1 to 10 Mpc.
Clusters of Galaxies • Galaxies are not scattered randomly, but are found in clusters. • Members of a cluster are in constant motion around each other.
The Local Group • The Local Group is a poor, irregular cluster of which our Galaxy is part. • The largest and most massive galaxy in the Local Group is M31, the Andromeda Galaxy; in second place is the Milky Way, followed by the spiral galaxy M33. • Both the Milky Way and M31 are surrounded by a number of small satellite galaxies.
Cluster Types • Regular clusters contain few galaxies with spiral structures. • Irregular clusters have an even mixture of galaxy types.
Clusters of Clusters • Clusters of galaxies are themselves grouped together in huge associations called superclusters. • A typical supercluster contains dozens of individual clusters spread over a region of space up to 150 million ly across.
Structure in the Nearby Universe It seems that superclusters are not randomly distributed, but lie along filaments.
The Large Scale Distribution of Galaxies • This map encompasses two wedge-shaped slices of the universe. • Each dot represents a galaxy and extends out to a distance of nearly 3 billion light-years from Earth. • We see enormous voids where few galaxies are found. • These titanic sheets of galaxies are the largest structures known in the universe. • On scales much larger than 100 Mpc, the distribution of galaxies in the universe appears to be roughly uniform.
Colliding galaxies produce starbursts, spiral arms, and other spectacular phenomena. • When galaxies collide at high speed, the huge clouds of interstellar gas and dust in the galaxies slam into each other and can be stripped of their interstellar gas and dust. • The best evidence that such collisions take place is that many rich clusters of galaxies are strong sources of X rays. • The only way that such large amounts of gas could be heated to such extremely high temperatures is in violent collisions.
Starburst Galaxies • In less violent collisions, compressed interstellar gas may have more time to cool, allowing many protostars to form. • Such collisions may account for starburst galaxies, which blaze with the light of numerous newborn stars. • These galaxies have bright centers surrounded by clouds of warm interstellar dust, indicating recent, vigorous star birth. • Their warm dust is so abundant that starburst galaxies are among the most luminous objects in the universe at infrared wavelengths.
Collisions in Radio Light • This starburst galaxy is part of a cluster of about a dozen galaxies. • In radio light we can see streamers of hydrogen gas that connect the three bright galaxies as well as several dim ones.
Simulations of Galaxy Mergers • Tidal forces between colliding galaxies can deform the galaxies from their original shapes. • The galactic deformation is so great that thousands of stars can be hurled into intergalactic space along huge, arching streams. • Supercomputer simulations of such collisions show that while some of the stars are flung far and wide, other stars slow down and the galaxies may merge.
Dark matter can be inferred by observing the motions of galaxy clusters. • The rotation curves of four spiral galaxies provide evidence of dark matter. • 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
The Gravitational Deflection of Light • Further evidence about how dark matter is distributed comes from the gravitational bending of light rays. • A massive object like a galaxy can produce much greater deflections, and the amount of this deflection can be used to determine the galaxy’s mass. • Because of the warped space around the massive galaxy, light from the background source curves around the galaxy as it heads toward us. • Light rays can travel along two paths from the background source to us here on Earth. Thus, we should see two images of the background source.
Gravitational Lensing • This visible-light image of a galaxy cluster shows more than a thousand galaxies. • The superimposed image in red shows the distribution of the cluster’s hot, X-ray emitting gas, and the blue image shows the distribution of dark matter as determined by gravitational lensing. • We can understand the separation of dark matter and gas in this cluster if we assume that dark matter does not feel any force of fluid resistance. • This is what we would expect if dark matter responds to gravitational forces only.
Quasars • Quasars are ultraluminous objects located at the centers of remote galaxies. • Nearby radio-quiet quasars tend to be located in spiral galaxies, whereas radio-loud quasars as well as more distant radio-quiet quasars tend to be located in ellipticals. • Quasars are members of a diverse group of very distant, superluminous objects now collectively known as active galaxies.
Active Galactic Nuclei • One characteristic that is common to all types of active galactic nuclei is variability. • These fluctuations in brightness allow astronomers to place limits on the maximum size of a light source. (An object cannot vary in brightness faster than light can travel across that object.)
“Bottom Up” We now think that small objects merge to form full-sized galaxies.
