Our Place in the Galaxy

Transcription

1 Our Place in the Galaxy The Milky Way has been known for thousands of years. Galileo showed that it consisted of a myriad of stars. Kapteyn counted stars and believed he had shown that the galaxy was about 10,000 light years across, with the Sun in the center. AST 303: Chapter 20 1

2 Our Place in the Galaxy (2) In 1917, Harlow Shapley had a different idea. He studied the distribution of globular clusters. Globular clusters contained RR Lyrae variable stars, a standard candle. From the brightness of these stars, he was able to calculate the distance to the cluster. He also knew the direction to the cluster (its position in the sky). Some globular clusters were too far away for the individual RR Lyrae stars to be seen. Shapley inferred their distances from their diameters, using nearby globular clusters whose distances were known as a guide. AST 303: Chapter 20 2

3 Globular Cluster 47 Tuc AST 303: Chapter 20 3

4 Globular Cluster M3 AST 303: Chapter 20 4

5 Our Place in the Galaxy (4) Shapley showed that the globular clusters formed a spherical system. What was even more surprising was that the Sun was nowhere near the center of this spherical system of globular clusters. Instead, the center of the system of globular clusters is 10,000 parsecs away, in the direction of Sagittarius. Shapley inferred that the center of the spherical system of globular clusters must be related to the center of our galaxy. Kapteyn was wrong! The galaxy was much bigger than he thought. Kapteyn s error: Dust in the galaxy was dimming the distant stars, systematically removing them from the sample AST 303: Chapter 20 5

6 Our Place in the Galaxy (3) The surprising results are shown in this diagram: Sun Kapteyn Universe 30,000 L.Y. AST 303: Chapter 20 6

7 Schematic Structure of the Galaxy AST 303: Chapter 20 7

8 Our Place in the Galaxy (5) There are a number of RR Lyrae stars in the general direction of the galactic center that are not associated with globular clusters. RR Lyrae stars are pretty rare, so there must be many other stars there as well. There may also be other matter (gas or dust) that we cannot see as stars. This generally spherical distribution of globular clusters, stars, and other matter is known as the galactic halo. There is reason to believe that the galactic halo is quite extensive, even if rather rarefied. It may be 100,000 parsecs or more in diameter. One reason why we believe that it may be quite large is that it is difficult to explain the motions of stars around the center of the galaxy if we assume that most of the mass of the galaxy is confined to the disc. AST 303: Chapter 20 8

9 Our Place in the Galaxy (6) When we look at other galaxies, we see that there is a flattened region, the disc; and a central bulge, the nucleus. When we look at the Milky Way in the sky, the brightest and widest regions seem to be in the direction of Sagittarius. Infrared and radio observations show a hot spot in the direction of Sagittarius AST 303: Chapter 20 9

10 Wide Angle View of Milky Way (Optical) AST 303: Chapter 20 10

11 First Radio Map of Milky Way (1939) AST 303: Chapter 20 11

12 Milky Way at Radio Wavelengths AST 303: Chapter 20 12

13 Galactic Center in Radio Wavelengths AST 303: Chapter 20 13

14 Our Place in the Galaxy (7) In the 1930s and 1940s, methods for correcting for interstellar dust using the interstellar reddening began to be developed. It became possible to locate the positions of hot, bright stars in our galaxy: O, B and A stars, Cepheid variables, and others. Astronomers began to notice a pattern in the distribution of these stars. They noticed a similar pattern with regard to where the gas and dust was located in the galaxy. AST 303: Chapter 20 14

15 Local Spiral Arms AST 303: Chapter 20 15

16 Orion Arm AST 303: Chapter 20 16

17 Our Place in the Galaxy (8) The similarity of this pattern of distribution with the distribution of bright stars, gas, and dust in external galaxies is very suggestive. Star formation probably takes place in the arms? AST 303: Chapter 20 17

18 Our Place in the Galaxy (9) Sun AST 303: Chapter 20 18

19 The Motion of Stars in the Galaxy (3) The speed of the Sun about the center of the galaxy is about 250 km/sec. It would take the Sun 200,000,000 years to complete a single orbit around the center of the galaxy. Some objects have very high velocities relative to the Sun (typically km/sec compared to km/sec for most stars). These are known as high velocity stars. Other objects with high velocity are the globular clusters. We these objects actually move about the galactic center at rather low velocities. Their high velocities with respect to the Sun are due to the Sun s high velocity. AST 303: Chapter 20 19

20 The Motion of Stars in the Galaxy (4) We can estimate the mass of the galaxy from Kepler s third law: P = years D = 10,000 pc 200,000 AU/pc = AU So, M = D3 P 2 = (2! 109 ) 3 (2! 10 8 ) 2 = 2!1011 Solar Masses We now know that there is even more mass than this. We assumed that all of the mass of the galaxy is concentrated at the center, which is not true. The calculation does not count matter that is well out of the galactic plane. The galaxy appears to have halo containing a significant amount of mass, not visible in telescopes. AST 303: Chapter 20 20

21 The Motion of Stars in the Galaxy (5) We do not know what form this mass takes. Black holes? Planetary-sized bodies? Very faint stars (brown dwarfs)? Clouds of gas whose temperature is such that they do not radiate significant amounts of detectable energy? When all of the missing mass is accounted for, the mass of the galaxy may exceed Solar masses. AST 303: Chapter 20 21

22 21 Cm Radio Map of Spiral Structure AST 303: Chapter 20 22

23 Nucleus of M100 AST 303: Chapter 20 23

24 The Sun's Position in the Galaxy AST 303: Chapter 20 24

25 The Cause of Spiral Structure (2) Spiral structure cannot be due to material arms. Modern theories (Lin, Shu) suggest that they are due to density waves. Stars and gas move through the arm region; the gravitational field causes them to bunch up in the arms. It s like driving in traffic. When the speed is high, the cars are widely spaced; when traffic is obstructed, everyone slows down and cars are closely spaced. At different times, different stars are in the arms. Star moves fast Star moves slowly AST 303: Chapter 20 25

26 The Cause of Spiral Structure (2) The theory predicts that there will be shock waves along the leading edge of the arms. This may lead to star formation. Thus we may expect the arms to be marked by young, hot stars This is not the whole story. Although density waves do not wind up quickly, they must be rejuvenated over times of the order of a billion years. AST 303: Chapter 20 26

27 Stellar Populations Astronomers distinguish two main populations of stars. Population I Hot, luminous, main sequence stars, clusters, H II regions. Associated with spiral arms. Circular motion in the plane of the galaxy. Young in age. Contain 1-2% of elements other than hydrogen and helium. Population II RR Lyrae stars, globulars, high velocity stars, planetary nebulae. Nuclear region, halo, and extended galactic disc. Orbits more eccentric and with less angular momentum. Old in age. Depleted in elements other than hydrogen and helium (less than 1% sometimes much less). AST 303: Chapter 20 27

28 The Formation and Evolution of Our Galaxy As stars evolve, they convert hydrogen into heavy elements. When they explode as supernovae, these elements, as well as new ones created during the explosion, are spewed out into the interstellar medium. Over time, the interstellar medium becomes enriched in heavy elements This explains why the Population II stars, which are old, are depleted in heavy elements, compared to Population I stars. What about Population III stars: Stars with no heavy elements at all? Might such stars exist? AST 303: Chapter 20 28

29 The Formation and Evolution of Our Galaxy (2) Also, the fact that Population II stars are dispersed over a spherical rather than a disklike system is presumably a consequence of their having been formed while the galaxy was in formation AST 303: Chapter 20 29

30 Age vs. Position in the Galaxy AST 303: Chapter 20 30

31 Statistical Issues There is plenty of room for improvement in statistical methods in statistical and kinematic astronomy, which describes the orbits and motions of stars of various kinds in the galaxy. The information we have is the proper motion and radial velocites of the stars, together with their apparent magnitudes. If we can estimate the absolute magnitude, we also get the distance modulus and distance. All are measured with (sometimes significant) errors. Combining this information to accurately estimate the interesting parameters is a challenge AST 303: Chapter 20 31

32 Statistical Issues We have also seen that the kinematical properties of different classes of stars (e.g., Population I, Population II) are different. This further complicates the problem of accurately modeling the kinematics and mass distribution of the stars in the galaxy. The gas component throws a further complication into the mix. Unlike stars, gas motions are affected by the gas pressure, shock waves, and possible magnetic fields. Gas is a significant fraction of the mass of the galaxy, so it is an important component in any galactic model Then there is the dark matter. We don t know what it is, but we know it is there. How should it be modeled? AST 303: Chapter 20 32