UniverseUniverseEighth EditionEighth Edition

UniverseUniverseEighth EditionEighth Edition

Roger A. Freedman • William J. Kaufmann III

CHAPTER 24CHAPTER 24GalaxiesGalaxies

CHAPTER 24CHAPTER 24GalaxiesGalaxies

Chapter 24 and 26 online quizzes due Thursday Chapter 24 and 26 online quizzes due Thursday 12/912/9

Skip Chapter 27Skip Chapter 27

Final Exam – Monday 12/13, 5:30-7:30 PMFinal Exam – Monday 12/13, 5:30-7:30 PM

The final will cover chapters 23, 24 and 26 only, The final will cover chapters 23, 24 and 26 only, same format as Exams 1-3 same format as Exams 1-3

Last HW!

How did Edwin Hubble show that M31 in Andromeda is a distant galaxy and not part of the Milky Way?

A. By measuring the distance to M31 using Cepheid variables.

B. By measuring the distance to M31 using RR Lyrae variables.

C. By precisely measuring the parallax of M31.

D. By observing a nova in M31.

E. By observing a supernova in M31.

Q24.7

How did Edwin Hubble show that M31 in Andromeda is a distant galaxy and not part of the Milky Way?

A. By measuring the distance to M31 using Cepheid variables.

B. By measuring the distance to M31 using RR Lyrae variables.

C. By precisely measuring the parallax of M31.

D. By observing a nova in M31.

E. By observing a supernova in M31.

A24.7

Which of the following best describes the Hubble law in its simplest form?

A. The recessional velocity of a galaxy is proportional to its distance from us.

B. The recessional velocity of a galaxy is inversely proportional to its distance from us.

C. The brightness of a galaxy is proportional to its distance from us.

D. The brightness of a galaxy is inversely proportional to its distance from us.

E. The brightness of a galaxy is inversely proportional to its color.

Q24.10

Which of the following best describes the Hubble law in its simplest form?

A. The recessional velocity of a galaxy is proportional to its distance from us.

B. The recessional velocity of a galaxy is inversely proportional to its distance from us.

C. The brightness of a galaxy is proportional to its distance from us.

D. The brightness of a galaxy is inversely proportional to its distance from us.

E. The brightness of a galaxy is inversely proportional to its color.

A24.10

In which of the following types of galaxies would you be least likely to find a newly-formed star?

A. Elliptical

B. Spiral

C. Irregular

D. Misleading question — newly-formed stars can be found in ellipticals, spirals, and irregulars

Q24.8

In which of the following types of galaxies would you be least likely to find a newly-formed star?

A. Elliptical

B. Spiral

C. Irregular

D. Misleading question — newly-formed stars can be found in ellipticals, spirals, and irregulars

A24.8

Key Ideas The Hubble Classification:The Hubble Classification: Galaxies can be grouped Galaxies can be grouped

into four major categories: spirals, barred spirals, into four major categories: spirals, barred spirals, ellipticals, and irregulars.ellipticals, and irregulars.

The disks of spiral and barred spiral galaxies are sites of The disks of spiral and barred spiral galaxies are sites of active star formation.active star formation.

Elliptical galaxies are nearly devoid of interstellar gas Elliptical galaxies are nearly devoid of interstellar gas and dust, and so star formation is severely inhibited.and dust, and so star formation is severely inhibited.

Lenticular galaxies are intermediate between spiral and Lenticular galaxies are intermediate between spiral and elliptical galaxies.elliptical galaxies.

Irregular galaxies have ill-defined, asymmetrical shapes. Irregular galaxies have ill-defined, asymmetrical shapes. They are often found associated with other galaxies.They are often found associated with other galaxies.

Key Ideas Distance to Galaxies:Distance to Galaxies: Standard candles, such as Standard candles, such as

Cepheid variables and the most luminous supergiants, Cepheid variables and the most luminous supergiants, globular clusters, H II regions, and supernovae in a globular clusters, H II regions, and supernovae in a galaxy, are used in estimating intergalactic distances.galaxy, are used in estimating intergalactic distances.

The Tully-Fisher relation, which correlates the width of The Tully-Fisher relation, which correlates the width of the 21-cm line of hydrogen in a spiral galaxy with its the 21-cm line of hydrogen in a spiral galaxy with its luminosity, can also be used for determining distance. A luminosity, can also be used for determining distance. A method that can be used for elliptical galaxies is the method that can be used for elliptical galaxies is the fundamental plane, which relates the galaxy’s size to its fundamental plane, which relates the galaxy’s size to its surface brightness distribution and to the motions of its surface brightness distribution and to the motions of its stars.stars.

Key Ideas The Hubble Law:The Hubble Law: There is a simple linear relationship There is a simple linear relationship

between the distance from the Earth to a remote galaxy between the distance from the Earth to a remote galaxy and the redshift of that galaxy (which is a measure of the and the redshift of that galaxy (which is a measure of the speed with which it is receding from us). This speed with which it is receding from us). This relationship is the Hubble law, relationship is the Hubble law, vv = = HH00dd..

The value of the Hubble constant, The value of the Hubble constant, HH00, is not known with , is not known with

certainty but is close to 73 km/s/Mpc.certainty but is close to 73 km/s/Mpc.

Key Ideas Clusters and Superclusters:Clusters and Superclusters: Galaxies are grouped into Galaxies are grouped into

clusters rather than being scattered randomly throughout clusters rather than being scattered randomly throughout the universe.the universe.

A rich cluster contains hundreds or even thousands of A rich cluster contains hundreds or even thousands of galaxies; a poor cluster, often called a group, may galaxies; a poor cluster, often called a group, may contain only a few dozen.contain only a few dozen.

A regular cluster has a nearly spherical shape with a A regular cluster has a nearly spherical shape with a central concentration of galaxies; in an irregular cluster, central concentration of galaxies; in an irregular cluster, galaxies are distributed asymmetrically.galaxies are distributed asymmetrically.

Key Ideas Our Galaxy is a member of a poor, irregular cluster Our Galaxy is a member of a poor, irregular cluster

called the Local Group.called the Local Group. Rich, regular clusters contain mostly elliptical and Rich, regular clusters contain mostly elliptical and

lenticular galaxies; irregular clusters contain spiral, lenticular galaxies; irregular clusters contain spiral, barred spiral, and irregular galaxies along with ellipticals.barred spiral, and irregular galaxies along with ellipticals.

Giant elliptical galaxies are often found near the centers Giant elliptical galaxies are often found near the centers of rich clusters.of rich clusters.

Key Ideas Galactic Collisions and Mergers:Galactic Collisions and Mergers: When two galaxies When two galaxies

collide, their stars pass each other, but their interstellar collide, their stars pass each other, but their interstellar media collide violently, either stripping the gas and dust media collide violently, either stripping the gas and dust from the galaxies or triggering prolific star formation.from the galaxies or triggering prolific star formation.

The gravitational effects during a galactic collision can The gravitational effects during a galactic collision can throw stars out of their galaxies into intergalactic space.throw stars out of their galaxies into intergalactic space.

Galactic mergers may occur; a large galaxy in a rich Galactic mergers may occur; a large galaxy in a rich cluster may tend to grow steadily through galactic cluster may tend to grow steadily through galactic cannibalism, perhaps producing in the process a giant cannibalism, perhaps producing in the process a giant elliptical galaxy.elliptical galaxy.

Key Ideas The Dark-Matter Problem:The Dark-Matter Problem: The luminous mass of a The luminous mass of a

cluster of galaxies is not large enough to account for the cluster of galaxies is not large enough to account for the observed motions of the galaxies; a large amount of observed motions of the galaxies; a large amount of unobserved mass must also be present. This situation is unobserved mass must also be present. This situation is called the dark-matter problem.called the dark-matter problem.

Hot intergalactic gases in rich clusters account for a small Hot intergalactic gases in rich clusters account for a small part of the unobserved mass. These gases are detected part of the unobserved mass. These gases are detected by their X-ray emission. The remaining unobserved mass by their X-ray emission. The remaining unobserved mass is probably in the form of dark-matter halos that surround is probably in the form of dark-matter halos that surround the galaxies in these clusters.the galaxies in these clusters.

Gravitational lensing of remote galaxies by a foreground Gravitational lensing of remote galaxies by a foreground cluster enables astronomers to glean information about cluster enables astronomers to glean information about the distribution of dark matter in the foreground cluster.the distribution of dark matter in the foreground cluster.

Key Ideas Formation and Evolution of Galaxies:Formation and Evolution of Galaxies: Observations Observations

indicate that galaxies arose from mergers of several indicate that galaxies arose from mergers of several smaller gas clouds.smaller gas clouds.

Whether a protogalaxy evolves into a spiral galaxy or an Whether a protogalaxy evolves into a spiral galaxy or an elliptical galaxy depends on its initial rate of star elliptical galaxy depends on its initial rate of star formation.formation.