Key Ideas and Terms

14-1 When galaxies were first discovered, it was not clear that they lie far beyond the Milky Way until their variable stars were carefully observed

• Debates about the nature of and distance to so-called spiral nebulae were ongoing until about 100 years ago.
• Spiral nebulae were confirmed to be distant galaxies when Cepheid stars were identified in them, allowing distance to be measured.

14-2 Hubble devised a system for classifying galaxies according to their appearance

• Galaxies can be grouped into four major categories organized on a tuning fork diagram using the Hubble classification.
• The disks of spiral galaxies and barred spiral galaxies are sites of active star formation.

• Ranging greatly in size from giant elliptical galaxies to dwarf elliptical galaxies, elliptical galaxies are nearly devoid of interstellar gas and dust, resulting in inhibited star formation.
• Lenticular galaxies are intermediate between spiral and elliptical galaxies.
• Irregular galaxies have ill-defined, asymmetrical shapes and are often found associated with other galaxies.

14-3 Exploding stars release similar amounts of light and their distance can be inferred by measuring their apparent brightness

• Standard candles, such as Cepheid variables and the most luminous supergiants, globular clusters, H II regions, and supernovae in a galaxy are used in estimating intergalactic distances.
• The Tully-Fisher relation, which correlates the width of the 21-cm line of hydrogen in a spiral galaxy with its luminosity, can also be used for determining distance.
• The various techniques to determine distance to objects in the universe are collectively known as the distance ladder.

14-4 Galaxies are found in clusters and superclusters

• Galaxies are grouped into clusters and superclusters and are separated by voids rather than being scattered randomly throughout the universe.
• Along with about 50 other galaxies, the Milky Way Galaxy is a member of the Local Group cluster.
• A regular cluster has a nearly spherical shape with a central concentration of galaxies containing mostly elliptical and lenticular galaxies with large ellipticals near the center.
• Irregular clusters contain spiral, barred spiral, and irregular galaxies along with ellipticals.

14-5 Colliding galaxies produce starbursts, spiral arms, and other spectacular phenomena

• Intercluster gas at high temperatures is observed among galaxy clusters.
• A starburst galaxy occurs when a rapid and sudden onset of star formation causes it to shine more brightly.
• When two galaxies collide, their stars pass each other without colliding, but their interstellar media collide violently, either stripping the gas and dust from the galaxies or triggering prolific star formation.
• The gravitational effects during a galactic collision can throw stars out of their galaxies into intergalactic space.
• A large galaxy in a rich cluster may tend to grow steadily through galactic cannibalism, perhaps producing in the process a giant elliptical galaxy.

14-6 Dark matter can be inferred by observing the motions of galaxy clusters

• The luminous mass of a cluster of galaxies is not large enough to account for the observed motions of the galaxies; thus, a large amount of unobserved mass must also be present, resulting in a dark-matter problem.
• Detected by their X-ray emission, hot intergalactic gases in rich clusters only account for a small part of the unobserved mass.
• The remaining unobserved mass is probably in the form of dark-matter halos that surround the galaxies in these clusters.
• Gravitational lensing of remote galaxies by a foreground cluster enables astronomers to glean information about the distribution of dark matter in the foreground cluster.

14-7 Quasars are the ultraluminous centers of the most distant galaxies

• A quasar looks like a star but has a huge redshift showing they are quite distant from the Milky Way, according to the Hubble law.
• To be seen at such large distances, quasars must be very luminous, typically about 1000 times brighter than an ordinary galaxy.
• Quasars are a category of objects known as active galaxies with the energy source at the center. Hence, astronomers say that these galaxies possess active galactic nuclei, or AGNs.

14-8 Supermassive black holes may be the “central engines” that power active galaxies

• There is a natural limit to the luminosity that can be radiated by accretion onto a compact object like a black hole, called the Eddington limit.
• Rapid fluctuations in the brightness of active galaxies indicate that the region that emits radiation is quite small.
• The preponderance of evidence suggests that an active galactic nucleus consists of a supermassive black hole onto which matter accretes.
• As gases spiral in toward the supermassive black hole, some of the gas may be redirected to become two jets of high-speed particles that are aligned perpendicularly to the accretion disk.

14-9 Galaxies may have formed from the merger of smaller objects

• Observations indicate that galaxies arose from mergers of several smaller gas clouds.
• Whether a protogalaxy evolves into a spiral galaxy or an elliptical galaxy depends on its initial rate of star formation.