GalaxiesGuiding QuestionsWhen galaxies were first discovered, it was not clear that they lie far beyond the Milky WaySlide 4Slide 5Hubble proved that the spiral nebulae are far beyond the Milky WayGalaxies are classified according to their appearanceGalaxies can be grouped into four major categories: spirals, barred spirals, ellipticals, and irregularsThe disks of spiral and barred spiral galaxies are sites of active star formationElliptical galaxies are nearly devoid of interstellar gas and dust, and so star formation is severely inhibitedSlide 11Slide 12Irregular galaxies have ill-defined, asymmetrical shapesAstronomers use various techniques to determine the distances to remote galaxiesThe Distance LadderMasersThe Hubble law relates the redshifts of remote galaxies to their distances from the EarthThe Hubble law is v = H0dGalaxies are grouped into clusters and superclustersSlide 20Slide 21Slide 22Our Galaxy is a member of a poor, irregular cluster called the Local GroupSlide 24Slide 25Slide 26Slide 27Slide 28Colliding galaxies produce starbursts, spiral arms, and other spectacular phenomenaSlide 30The gravitational effects during a galactic collision can throw stars out of their galaxies into intergalactic spaceSlide 32Most of the matter in the universe has yet to be discoveredSlide 34Gravitational lensing of remote galaxies by a foreground cluster enables astronomers to glean information about the distribution of dark matter in the foreground clusterSlide 36Slide 37Galaxies formed from the merger of smaller objectsSlide 39Whether a protogalaxy evolves into a spiral galaxy or an elliptical galaxy depends on its initial rate of star formationSlide 41Slide 42Slide 43Key WordsGalaxies Chapter Twenty-SixGuiding Questions•How did astronomers first discover other galaxies?•How did astronomers first determine the distances to galaxies?•Do all galaxies have spiral arms, like the Milky Way?•How do modern astronomers tell how far away galaxies are?•How do the spectra of galaxies tell astronomers that the universe is expanding?•Are galaxies isolated in space, or are they found near other galaxies?•What happens when galaxies collide with each other?•Is dark matter found in galaxies beyond the Milky Way?•How do astronomers think galaxies formed?When galaxies were first discovered, it was notclear that they lie far beyond the Milky WayM51Hubble proved that the spiral nebulae are farbeyond the Milky Way•Edwin Hubble used Cepheid variables to show that the “nebula” were actually immense star systems far beyond our GalaxyGalaxies are classified according to theirappearanceGalaxies can be grouped into four major categories: spirals, barred spirals, ellipticals, and irregularsLenticular galaxies are intermediate between spiral and elliptical galaxiesThe disks of spiral and barred spiral galaxies are sites of active star formationElliptical galaxies are nearly devoid of interstellar gas and dust, and so star formation is severely inhibitedIrregular galaxies have ill-defined, asymmetrical shapesThey are often found associated with other galaxiesAstronomers use various techniques to determinethe distances to remote galaxiesStandard 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•A method that can be used for elliptical galaxies is the fundamental plane, which relates the galaxy’s size to its surface brightness distribution and to the motions of its starsThe Distance LadderMasers•One distance-measuring technique that has broken free of the distance ladder uses observations of molecular clouds called masers•“Maser” is an acronym for “microwave amplification by stimulated emission of radiation”The Hubble law relates the redshifts of remotegalaxies to their distances from the EarthThere is a simple linear relationship between the distance from the Earth to a remote galaxy and the redshift of that galaxy (which is a measure of the speed with which it is receding from us)The Hubble law is v = H0dThe value of the Hubble constant, H0, is not known with certainty but is close to 71 km/s/MpcGalaxies are grouped into clusters and superclusters Galaxies are grouped into clusters rather than being scattered randomly throughout the universe•A rich cluster contains hundreds or even thousands of galaxies•A poor cluster, often called a group, may contain only a few dozen•A regular cluster has a nearly spherical shape with a central concentration of galaxies•In an irregular cluster, galaxies are distributed asymmetricallyOur 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 spiral, barred spiral, and irregular galaxies along with ellipticals•Giant elliptical galaxies are often found near the centers of rich clustersColliding galaxies produce starbursts, spiral arms,and other spectacular phenomena When two galaxies collide, their stars pass each other, but their interstellar media collide violently, either stripping the gas and dust from the galaxies or triggering prolific star formationThe gravitational effects during a galactic collision can throw stars out of their galaxies into intergalactic spaceMost of the matter in the universe has yet to be discovered•The luminous mass of a cluster of galaxies is not large enough to account for the observed motions of the galaxies; a large amount of unobserved mass must also be present•This situation is called the dark-matter problem•Hot intergalactic gases in rich clusters account for a small part of the unobserved mass•These gases are detected by their X-ray emission•The remaining unobserved mass is probably in the form of dark-matter halos that surround the galaxies in these clustersGravitational lensing of remote galaxies by a foreground cluster enables astronomers to glean information about the distribution of dark matter in the foreground clusterGalaxies formed from the merger of smaller objects •Observations indicate that galaxies arose from mergers of several smaller gas clouds •A large galaxy in a rich cluster may tend to grow steadily through galactic cannibalism, perhaps producing in