Objectives

1. The Milky Way Galaxy Objectives • Determine the size and shape of the Milky Way, as well as Earth’s location within it. • Describe how the Milky Way formed. Vocabulary • variable star • RR Lyrae variable • Cepheid variable • halo • spiral density wave

2. The Milky Way Galaxy The Milky Way Galaxy • The Milky Way is a great disk made of stars orbiting a central point in the disk. • Our Sun is just one of perhaps 100 billion stars that make up the Milky Way.

3. The Milky Way Galaxy Discovering the Milky Way • It is difficult to see the Milky Way’s size and shape not only because are we too close, but we are also inside the galaxy. • We also cannot tell where its center is, or what Earth’s location is within it. • Astronomers are still refining their measurements.

4. The Milky Way Galaxy Discovering the Milky Way Variable Stars • Astronomers estimated the distances to globular clusters by identifying variable stars in them. • Variable stars are stars in the giant branch of the Hertzsprung-Russell diagram that pulsate in brightness because of the expansion and contraction of their outer layers. • RR Lyrae variables have periods of pulsation between 1.5 hours and 1 day, and on average, they have the same luminosity. • Cepheid variables have pulsation periods between 1 and 100 days, the longer the period of pulsation, the greater the luminosity.

5. The Milky Way Galaxy Discovering the Milky Way Variable Stars • By measuring a star’s period of pulsation, astronomers can determine the star’s luminosity. • The star’s luminosity, or absolute magnitude, can be compared to its apparent magnitude to calculate the distance to the star.

6. The Milky Way Galaxy Discovering the Milky Way The Galactic Center • Astronomers used RR Lyrae variables to determine that the globular clusters are distributed in space centered on a point, 28 000 ly away. • Astronomers reasoned that the globular clusters were orbiting the center of the Milky Way which is a region of very high star density. • The direction of the galactic center is toward the constellation Sagittarius.

7. The Milky Way Galaxy The Shape of the Milky Way • Astronomers have been able to determine the galaxy’s shape by mapping it with radio waves. • The galactic center, also called the nucleus, is surrounded by a nuclear bulge, which sticks out of the galactic disk. • The halo is a spherical region, around the nuclear bulge and disk, where globular clusters are located.

8. The Milky Way Galaxy The Shape of the Milky Way The Milky Way consists of a nuclear bulge in the center of a disk. The disk and bulge are surrounded by a spherical region called the halo. (not to scale)

9. The Milky Way Galaxy The Shape of the Milky Way Spiral Arms • Hydrogen atoms in a very low-density gas can emit radiation at a wavelength of 21 cm. • Using this emission as a guide, astronomers have identified four major spiral arms and numerous minor arms in the Milky Way. • The Sun is located in the minor arm Orion at a distance of about 28 000 ly from the galactic center. • The Sun’s orbital speed is about 220 km/s, and thus its orbital period is about 240 million years.

10. The Milky Way Galaxy The Shape of the Milky Way Spiral Arms

11. The Milky Way Galaxy Mass of the Milky Way • The mass located within the circle of the Sun’s orbit through the galaxy is about 100 billion times the mass of the Sun. • Because the Sun is about average in mass, astronomers have concluded that the galaxy contains about 100 billion stars within its disk. • Evidence indicates that as much as 90 percent of the galaxy’s mass is contained in the halo. • Some of this unseen matter, or dark matter, is probably in the form of dim stellar remnants such as white dwarfs, neutron stars, or black holes.

12. The Milky Way Galaxy Mass of the Milky Way A Galactic Black Hole • Sagittarius A*, the center of the galaxy, has about 2.6 million times the mass of the Sun but is smaller than our solar system. • Data gathered by the Chandra X-Ray Observatory reveal intense X-ray emissions as well. • Astronomers believe that Sagittarius A* is a supermassive black hole that probably formed at the time when the galaxy’s disk was forming.

13. The Milky Way Galaxy Stars in the Milky Way • Globular clusters in the halo of the Milky Way, which are estimated to be as old as 12 to 14 billion years, are the oldest-known objects in the galaxy. • Stars in the globular clusters have extremely small amounts of elements that are heavier than hydrogen and helium. • The nuclear bulge of the galaxy also contains stars with compositions like those of globular cluster stars.

14. The Milky Way Galaxy Stars in the Milky Way Stellar Populations • Most of the young stars in the galaxy are located in the spiral arms of the disk, where the interstellar gas and dust are concentrated. • The galaxy could be divided into two components: • The round part made up of the halo and bulge, where the stars are old and contain only traces of heavy elements • The disk, especially the spiral arms, where stars are still forming

15. The Milky Way Galaxy Stars in the Milky Way Stellar Populations • Astronomers divide stars in these two regions into two classes. • Population I stars, like the Sun, are those in the disk and arms and have small amounts of heavy elements. • Population II stars are those in the halo and bulge and contain only traces of heavy elements.

16. The Milky Way Galaxy Stars in the Milky Way Stellar Populations Globular clusters and the nuclear bulge contain old stars poor in heavy elements. The disk contains young stars that have higher heavy element content. (not to scale)

17. The Milky Way Galaxy Formation and Evolution of the Milky Way • The fact that the halo and bulge are made exclusively of old stars suggests that these parts of the galaxy formed first. • The galaxy began as a spherical cloud in space with the first stars forming while this cloud was round in what is now the halo. • The nuclear bulge represents the inner portion of the original cloud. • The cloud eventually collapsed under the force of its own gravity, and rotation forced it into a disklike shape. • Stars that formed after this time have orbits lying in the plane of the disk.

18. The Milky Way Galaxy Maintaining Spiral Arms • The way in which the spiral arms are maintained is not clearly understood. • There are two different theories about how the arms are maintained: 1. Spiral density waves, which can be thought of alternating dense and less-dense regions that are frozen in place and rotate as a rigid pattern, cause a buildup of material as they move through space. 2. The spiral arms are not permanent structures but instead, are continually forming as a result of disturbances such as supernovae explosions.

19. The Milky Way Galaxy Maintaining Spiral Arms A slow truck on a highway causing a buildup of cars around it is similar to how spiral density waves create spiral arms in a galaxy.

20. The Milky Way Galaxy Maintaining Spiral Arms • The Milky Way has a broken spiral-arm pattern, which fits the disturbances model best. • Some galaxies that have a prominent 2-armed pattern are almost certainly created by density waves.

21. The Milky Way Galaxy Section Assessment 1. Match the following terms with their definitions. ___ RR Lyrae variable ___ Cepheid variable ___ halo ___ spiral density wave B D C A A. alternating dense and less-dense regions that are frozen in place and rotate as a rigid pattern B.star with pulsation periods between 1.5 hours and 1 day C.a spherical region in the Milky Way where globular clusters are located D. star with a pulsation period between 1 to 100 days

22. The Milky Way Galaxy Section Assessment 2. How did astronomers discover the arms of the Milky Way? Astronomers discovered the arms of the Milky Way through observing the radiation emitted by very-low density hydrogen gas.

23. The Milky Way Galaxy Section Assessment 3. Identify whether the following statements are true or false. ______ The Sun has orbited the galaxy approximately 20 times in its history. ______ Most of the Milky Way’s Mass is located in the nuclear bulge. ______ Population I stars are younger than Population II stars. ______ The Sun is located on the Perseus arm of the Milky Way. true false true false

24. End of Section 1

25. 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. Vocabulary • supercluster • Hubble constant • radio galaxy • active galactic nucleus • quasar

26. Other Galaxies in the Universe Other Galaxies in the Universe • Our galaxy is just one of billions of galaxies in the universe. • By observing those galaxies that are farthest away, astronomers get an idea of how the universe looks as a whole. • Because it takes so long for light to reach us from remote galaxies, these far away galaxies also provide an idea of what the universe was like long ago.

27. Other Galaxies in the Universe Discovering Other Galaxies • Astronomers were aware of galaxies outside the Milky Way long before they knew what they were or if they were part of our own galaxy. • In 1924, Edwin Hubble discovered Cepheid variable stars in the Great Nebula in the Andromeda constellation. • Measuring the distance to the nebula, Hubble showed that they were much too far away to be located in our own galaxy and it became known as the Andromeda Galaxy.

28. Other Galaxies in the Universe Discovering Other Galaxies Classification of Galaxies • Hubble went on to study galaxies and sort them into categories according to their shapes. • The disklike galaxies with spiral arms were called spiral galaxies and were divided into two subclasses: normal spirals (S) and barred spirals (SB). • These are further subdivided based on: (a) tightly wound arms and a large, bright nucleus; (c) loosely wound arms and a small, dim nucleus; (b) having characteristics between a and c.

29. Other Galaxies in the Universe Discovering Other Galaxies Classification of Galaxies

30. Other Galaxies in the Universe Discovering Other Galaxies Classification of Galaxies • Galaxies with flat disks that do not have spiral arms are denoted S0. • Elliptical galaxies are not flattened into disks, do not have spiral arms, and are denoted E. • Ellipticals are divided into subclasses (E0–7) based on the apparent ratio of their major and minor axes: round ellipticals are classified as E0, while very elongated ellipticals are classified as E7.

31. Other Galaxies in the Universe Discovering Other Galaxies Classification of Galaxies

32. Other Galaxies in the Universe Discovering Other Galaxies Classification of Galaxies • Some galaxies do not have distinct shapes, and thus do not fit into either the spiral or elliptical classification. • They are called irregular galaxies and are denoted by Irr.

33. Other Galaxies in the Universe Discovering Other Galaxies Masses of Galaxies • Dwarf elliptical galaxies have masses of perhaps one million Suns. • Large spirals, such as the Milky Way, have masses of around 100 billion Suns. • The largest galaxies, called giant ellipticals, have masses as high as 100 trillion times the Sun’s mass.

34. Other Galaxies in the Universe Groups and Clusters of Galaxies • Most galaxies are located in groups, rather than being spread uniformly throughout the universe. • The Milky Way belongs to a small cluster of galaxies called the Local Group which is roughly 2 million ly in diameter. • There are about 35 known members including the Milky Way and Andromeda Galaxies, but most of are dwarf ellipticals. • Clusters larger than the Local Group may have hundreds of members and diameters in the range of about 5 to 30 million ly.

35. 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 visible masses of the galaxies. • This provides the strongest evidence that the universe contains a great amount of dark matter, the nature of which is still unknown.

36. Other Galaxies in the Universe Groups and Clusters of Galaxies Superclusters • Superclusters are clusters of galaxies organized into even larger groups that are hundreds of millions of light-years in size. • These superclusters appear in sheetlike and threadlike shapes.

37. Other Galaxies in the Universe The Expanding Universe • In 1929, Edwin Hubble, by measuring the redshifts and distances of many galaxies, found that the farther away from Earth a galaxy is, the faster it is moving away. • The universe is expanding. • In a medium that is uniformly expanding, all points are moving away from all other points, and no point has to be at the center.

38. Other Galaxies in the Universe The Expanding Universe Hubble’s Law • Hubble determined that the universe is expanding by making a graph of the speed at which a galaxy is moving versus its distance. • The result is a straight line, which can be expressed as a simple equation, v = Hd. • v is the speed at which a galaxy is moving away measured in kilometers per second • d is the distance to the galaxy measured in megaparsecs (Mpc), where 1 Mpc = 1 × 106 pc • H is a number called the Hubble constant

39. Other Galaxies in the Universe The Expanding Universe Hubble’s Law • The Hubble constant, which is the value (H) that is used to calculate the rate that the universe is expanding, is measured in kilometers per second per megaparsec. • To measure H requires finding distances and speeds for many galaxies, out to the largest possible distance, and constructing a graph to find the slope. • Currently, the best measurements indicate a value of approximately 70 kilometers per second per megaparsec. • Once the value of H is known, it can be used to find distances to far away galaxies.

40. Other Galaxies in the Universe Active Galaxies • Radio galaxies, often giant elliptical galaxies, emit as much as or more energy in radio wavelengths than they do in wavelengths of visible light. • The radio emission usually comes from two huge lobes of very hot gas located on opposite sides of the visible galaxy, linked by jets of gas. • Active galactic nuclei, or AGNs, in some unusual galaxies, contain a highly energetic object or activity that emits as much or more energy than the rest of the galaxy.

41. Other Galaxies in the Universe Quasars • In the 1960s, astronomers discovered another new type of object that looked like ordinary stars, but were strong radio emitters. • Quasars are starlike objects with emission lines in their spectra. • The emission lines in the spectra of quasars are shifted very far toward longer wavelengths. • The redshifts of quasars indicate that they are very far away. • Most quasars appear to be extra-bright active galactic nuclei of very dim galaxies.

42. Other Galaxies in the Universe Quasars Looking Back in Time • Because many quasars are far away, it takes their light a long time to reach Earth. • The most remote quasars are several billion light-years away, which indicates that they existed billions of years ago. • This suggests that many galaxies went through a quasar stage when they were young. • Today’s active galactic nuclei might be former quasars that are not quite as energetic as they were long ago.

43. Other Galaxies in the Universe Quasars Source of Power • The AGNs and quasars emit far more energy than ordinary galaxies, but they are as small as solar systems which suggests that all of these objects are supermassive black holes. • The beams of charged particles that stream out of the cores of radio galaxies and form jets are probably created by the magnetic forces of black holes. • Radio-lobed quasars have jets that are essentially related to radio galaxies.

44. Other Galaxies in the Universe Section Assessment 1. Match the following terms with their definitions. ___ supercluster ___ radio galaxy ___ active galactic nucleus ___ quasar B A D C A. galaxies that emit as much or more energy in radio wavelengths than they do in visible light B.organizations of clusters of galaxies that are hundreds of millions of light years in size C.starlike objects with emission lines in their spectra D. galaxy cores that emit as much or more energy than the rest of the galaxy

45. Other Galaxies in the Universe Section Assessment 2. What two important discoveries did Edwin Hubble make? Edwin Hubble discovered that there were other galaxies outside of the Milky Way and that the universe is expanding.

46. Other Galaxies in the Universe Section Assessment 3. Identify whether the following statements are true or false. ______ We see the Sun as it was about eight minutes ago. ______ There are about 15 known members of the Local Group. ______ The universe will appear to be expanding from any location within it. ______ Radio galaxies are often elliptical. true false true true

47. End of Section 2

48. Cosmology Objectives • Explain the different theories about the formation of the universe. • Describe the possible outcomes of universal expansion. Vocabulary • cosmology • Big Bang theory • steady-state theory • cosmic background radiation • inflationary universe

49. Cosmology Cosmology • Cosmology is the study of the universe, its current nature, and its origin and evolution. • Astronomers use a combination of observations and theoretical models in cosmology. • In cosmology, objects that have a range of properties cannot be compared, as there is only one universe to consider and nothing to compare it with.

50. Cosmology Models of the Universe • The fact that the universe is expanding implies that it had a beginning. • The Big Bang theory is the theory that the universe began as a point and has been expanding ever since. • The steady-state theory proposes that the universe is the same as it has always been and new matter is being created as the universe expands maintaining its density. • The evidence from observational tests weighs in favor of the Big Bang.