1. Objectives Describe how astronomers classify galaxies.
Section 30.2
Other Galaxies in the Universe
Objectives
Describe how astronomers classify galaxies.
Identify how galaxies are organized into clusters and superclusters.
Describe the expansion of the universe.

2. Section 30.2
Other Galaxies in the Universe
Finding galaxies with different shapes reveals the past, present, and future of the universe.
Review Vocabulary
elliptical: relating to or shaped like an ellipse or oval

3. New Vocabulary Other Galaxies in the Universe dark matter supercluster
Section 30.2
Other Galaxies in the Universe
New Vocabulary
dark matter
supercluster
Hubble constant
radio galaxy
active galactic nucleus
quasar

4. Discovering Other Galaxies
Section 30.2
Other Galaxies in the Universe
Discovering Other Galaxies
In 1924, Edwin Hubble discovered Cepheid variable stars in the Great Nebula in the Andromeda constellation. Using these stars to measure the distance to the nebula, Hubble showed that they were too far away to be located in our own galaxy. The Andromeda nebula then became known as the Andromeda galaxy.

5. Discovering Other Galaxies
Section 30.2
Other Galaxies in the Universe
Discovering Other Galaxies
Properties of galaxies
Masses of galaxies range from the dwarf ellipticals, which have masses of approximately 1 million times the mass of the Sun; to large spirals, with masses of around 100 billion times the mass of the Sun; to the largest galaxies, called giant ellipticals, which have masses as high as 1 trillion times that of the Sun.

6. Galaxies – classification.
In order of increasing mass and luminosity- dwarf ellipticals, large spirals, giant ellipticals.
According to shape : disk like, elliptical and irregular.
Disk like: 1)normal spiral, 2) barred spiral 3) no spiral arms.
Elliptical: not flattened into a disk shape, no spiral arms.
Irregular: no distinct shape.

7. Discovering Other Galaxies
Section 30.2
Other Galaxies in the Universe
Discovering Other Galaxies
Properties of galaxies
Luminosities of galaxies also vary over a wide range, from the dwarf spheroidals—not much larger or more brilliant than a globular cluster— to supergiant elliptical galaxies, more than times more luminous than the Milky Way.

8. Discovering Other Galaxies
Section 30.2
Other Galaxies in the Universe
Discovering Other Galaxies
Properties of galaxies
All galaxies show evidence that an unknown substance called dark matter dominates their masses. Dark matter is thought to be made up of a form of subatomic particle that interacts only weakly with other matter.

9. Discovering Other Galaxies
Section 30.2
Other Galaxies in the Universe
Discovering Other Galaxies
Classification of galaxies
Hubble classified the disklike galaxies with spiral arms as spiral galaxies. These were subdivided into normal spirals and barred spirals.

10. Discovering Other Galaxies
Section 30.2
Other Galaxies in the Universe
Discovering Other Galaxies
Classification of galaxies
Elliptical galaxies are not flattened into disks and do not have spiral arms. They are divided into subclasses based on the apparent ratio of their major and minor axes.

11. Discovering Other Galaxies
Section 30.2
Other Galaxies in the Universe
Discovering Other Galaxies
Classification of galaxies
The Hubble tuning-fork diagram summarizes Hubble classification for normal galaxies.

12. Discovering Other Galaxies
Section 30.2
Other Galaxies in the Universe
Discovering Other Galaxies
Classification of galaxies
Some galaxies do not have distinct shapes and are referred to as irregular galaxies.

13. Visualizing the Local Group
Section 30.2
Other Galaxies in the Universe
Visualizing the Local Group
All of the stars visible in the night sky belong to a single galaxy, the Milky Way. Just as stars compose galaxies, galaxies are gravitationally drawn into galactic groups, or clusters. The 40 galaxies closest to Earth are members of the Local Group of galaxies.

14. Groups and Clusters of Galaxies
Section 30.2
Other Galaxies in the Universe
Groups and Clusters of Galaxies
Local Group
The diameter of the Local Group is roughly 2 million ly. There are about 40 known member galaxies, of which the Milky Way and Andromeda galaxies are the largest. Most of the members are dwarf ellipticals that are companions to the larger galaxies.

15. Groups and Clusters of Galaxies
Section 30.2
Other Galaxies in the Universe
Groups and Clusters of Galaxies
Large clusters
Galaxy clusters larger than the Local Group might have hundreds or thousands of members and diameters in the range of about 5 to 30 million ly.

16. Groups and Clusters of Galaxies
Section 30.2
Other Galaxies in the Universe
Groups and Clusters of Galaxies
Large clusters
Most of the galaxies in the inner region of a large cluster are ellipticals. There is a more even mix of ellipticals and spirals in the outer portions.

17. Groups and Clusters of Galaxies
Section 30.2
Other Galaxies in the Universe
Groups and Clusters of Galaxies
Large clusters
In large clusters, gravitational interactions often cause galaxies to collide. They form strangely shaped galaxies, or they form galaxies with more than one nucleus, such as the Andromeda galaxy.

18. Groups and Clusters of Galaxies
Section 30.2
Other Galaxies in the Universe
Groups and Clusters of Galaxies
Masses of clusters
For clusters of galaxies, the mass determined by analyzing the motion of member galaxies is always much larger than the sum of the total masses of each of the galaxies, as determined by their total luminosity. This suggests that most of the mass in a cluster of galaxies is invisible, and supports the idea that the universe contains a great amount of dark matter.

19. Groups and Clusters of Galaxies
Section 30.2
Other Galaxies in the Universe
Groups and Clusters of Galaxies
Masses of clusters
Clusters of galaxies are organized into even larger groups called superclusters. These gigantic formations are hundreds of millions of light-years in size.

20. The Expanding Universe
Section 30.2
Other Galaxies in the Universe
The Expanding Universe
In 1929, Edwin Hubble measured the redshift and distances of many galaxies and found that the redshift of a galaxy depends on its distance from Earth. The farther away a galaxy is, the faster it is moving away. In other words, the universe is expanding.

21. The Expanding Universe
Section 30.2
Other Galaxies in the Universe
The Expanding Universe
Implications of redshift
An observer located in any galaxy, at any place in the universe, will observe the same thing in a medium that is uniformly expanding—all points are moving away from all other points, and no point is at the center. At greater distances, the expansion increases the rate of motion.

22. The Expanding Universe
Section 30.2
Other Galaxies in the Universe
The Expanding Universe
Implications of redshift
If the universe is expanding now, it must have been smaller and denser in the past. In fact, there must have been a time when all contents of the universe were compressed together. The Big Bang theory has been proposed to explain this expansion.
( Redshift – decreasing frequency, object moving away)

23. The Expanding Universe
Section 30.2
Other Galaxies in the Universe
The Expanding Universe
Hubble’s law
Hubble determined that the universe is expanding by making a graph comparing a galaxy’s distance to the speed at which it is moving.

24. The Expanding Universe
Section 30.2
Other Galaxies in the Universe
The Expanding Universe
Hubble’s law
Hubble’s graph resulted in a straight line with a slope called the Hubble constant. The Hubble constant is a value (H) used to calculate the rate at which the universe is expanding and is measured in kilometers per second per megaparsec.

25. The Expanding Universe
Section 30.2
Other Galaxies in the Universe
The Expanding Universe
Hubble’s law
Obtaining an accurate value for H was one of the key goals of astronomers who designed the Hubble Space Telescope (HST). It took nearly ten years after the launch of the HST to gather enough data to pinpoint the value of H, currently approximately 70 km/s/Mpc.

26. The Expanding Universe
Section 30.2
Other Galaxies in the Universe
The Expanding Universe
Hubble’s law
Once the value of H is known, it can be used to find distances to faraway galaxies.

27. Section 30.2
Other Galaxies in the Universe
Active Galaxies
Radio galaxies are very bright, often giant, elliptical galaxies that emit as much or more energy in the form of radio wavelengths as they do in wavelengths of visible light.

28. Section 30.2
Other Galaxies in the Universe
Active Galaxies
The cores of some unusual galaxies where highly energetic objects or activities are located are called active galactic nuclei (AGN).

29. Section 30.2
Other Galaxies in the Universe
Quasars
In the 1960s, astronomers discovered objects that looked like ordinary stars, but some emitted strong radio waves. Most stars do not. These starlike, very bright, extremely distant objects with emission lines in their spectra were called quasars.

30. Quasars Quasar redshift
Section 30.2
Other Galaxies in the Universe
Quasars
Quasar redshift
The redshift of quasars was much larger than any that had been observed in galaxies up to the 1960s, which would mean that the quasars were much farther away than any known galaxy.

31. Quasars Quasar redshift
Section 30.2
Other Galaxies in the Universe
Quasars
Quasar redshift
One piece of evidence indicates that those quasars associated with clusters of galaxies have the same redshift, verifying that they are the same distance away.
Another more important discovery is that most quasars are nuclei of very dim galaxies.

32. Quasars Looking back in time
Section 30.2
Other Galaxies in the Universe
Quasars
Looking back in time
Because quasars are distant, it takes their light a long time to reach Earth. Therefore, observing a quasar is seeing it as it was a long time ago.
If quasars are extra-bright galactic nuclei, then the many distant ones are nuclei of galaxies as they existed when the universe was young.

33. Quasars Looking back in time
Section 30.2
Other Galaxies in the Universe
Quasars
Looking back in time
Looking far back into time, the early universe had many quasars. Current theory suggests that they existed around supermassive black holes that pulled gas into the center, where in a violent swirl, friction heated the gas to extreme temperatures resulting in the bright light energy that was first detected.

34. Quasars Source of power
Section 30.2
Other Galaxies in the Universe
Quasars
Source of power
The AGN and quasars emit far more energy than ordinary galaxies, but they are as small as solar systems. This suggests that all of these objects are supermassive black holes.

35. Quasars Source of power
Section 30.2
Other Galaxies in the Universe
Quasars
Source of power
The beams of charged particles that stream out of the cores of radio galaxies and form jets are probably created by magnetic forces. As material falls into a black hole, the magnetic forces push the charged particles out into jets.
There is evidence that similar beams or jets occur in other types of AGN and in quasars.

36. Section 30.2 Other Galaxies in the Universe
Study Guide
Key Concepts
Section 30.2 Other Galaxies in the Universe
Finding galaxies with different shapes reveals the past, present, and future of the universe.
Galaxies can be elliptical, disk-shaped, or irregular.
Galaxies range in mass from 1 million Suns to more than a trillion Suns.

37. Section 30.2 Other Galaxies in the Universe
Study Guide
Key Concepts
Section 30.2 Other Galaxies in the Universe
Many galaxies seem to be organized in groups called clusters.
Quasars are the nuclei of faraway galaxies that are dim and seen as they were long ago, due to their great distances.
Hubble’s law helped astronomers discover that the universe is expanding.

38. 30 Galaxies and the Universe
30.2 Section Questions
All galaxies are moving apart from one another.
a. true
b. false

39. 30 Galaxies and the Universe
30.2 Section Questions
How were Edwin Hubble’s discoveries about redshifts used to conclude that the universe is expanding?
Answer: Hubble found that the redshift of a galaxy depended on its distance from Earth. The more distant a galaxy is, the greater its redshift; that is, the faster it is moving away. This implies that the universe is expanding.

40. 30 Galaxies and the Universe
30.2 Section Questions
Quasars are old and distant objects that emit several thousand times more energy than the entire Milky Way galaxy does.
a. true
b. false