Lecture 25 The Milky Way Galaxy November 29, 2017

Transcription

1 Lecture 25 The Milky Way Galaxy November 29,

2 2 Size of the Universe The Milky Way galaxy is very much larger than the solar system Powers of Ten interactive applet

3 3 Galaxies Large collections of stars, dust and gas Held together by their gravity. Contain millions to billions of stars. Stars rotate around the center of the galaxy. Our galaxy is the Milky Way

4 4 Structure of the Milky Way Hard to observe our Galaxy -- we are inside. William Herschel -- tried to find where Sun was in the galaxy by counting stars Found same density of stars on all sides Concluded we are in the center.

5 5 Dust Blocks Our View of the Center of the Milky Way

6 6 Shapley Uses Globular Clusters Globular Clusters often orbit outside disk of MW. Shapley observed them mainly on one side of the sky. We are not at the center of the galaxy. to Find the Center Figure 23.9, Chaisson and McMillan, 6 th ed. Astronomy Today, 2008 Pearson Prentice Hall

7 7 Structure of the Milky Way Figure 23.10, Chaisson and McMillan, 6 th ed. Astronomy Today, 2008 Pearson Prentice Hall Animation

8 8 Harlow Shapley first located the center of our Galaxy in 1917 by A. observing supernova explosions throughout the Galaxy. B. observing the distribution of globular clusters. C. making redshift measurements of stars in the galactic disk. D. observing the distribution of hydrogen gas in the Galaxy Response Counter 0% 0% 0% 0% A. B. C. D. 90

9 9 Components of the Milky Way Disk Young and older stars. Much gas and dust. Extends ~30 kpc in diameter Sun is about halfway out from center of the disk.

10 10 Motions of gas and stars Disk Takes 230 million yrs for Sun to go about the center. Stars rotate differentially Takes different amounts of time for stars to get around the center

11 11 Bulge Young and older stars Gas and dust. At center of disk.

12 12 Halo Mostly old stars. Many stars in globular clusters orbiting center. Little gas so no new star formation.

13 13 Motions of Stars Bulge Some stars move in elongated orbits coplanar with disk, others have random orientations. Halo Stars move in all directions around the center Figure 23.13, Chaisson and McMillan, 6 th ed. Astronomy Today, 2008 Pearson Prentice Hall

14 14 Edge-on Spiral Galaxy Galaxy NGC 4565 Figure 23.3b, Chaisson and McMillan, 6 th ed. Astronomy Today, 2008 Pearson Prentice Hall

15 15 Milky Way Milky Way galaxy from our perspective and at visible wavelength of light

16 16 Astronomers deduce that the Milky Way is a disk because they A. see stars arranged in a circular pattern around the north celestial pole. B. see far more stars along the band of the Milky Way than in other directions. C. see a large, dark circle silhouetted against the Milky Way in the Southern Hemisphere. D. see the same number of stars in all directions in the sky. Response Counter 0% 0% 0% 0% A. B. C. D. 90

17 17 Mapping the Structure of the Milky Way Dust obscures our view of the galaxy Need to use radio and IR wavelengths of light 21-cm emission line Comes from neutral atomic hydrogen (not ionized) The radio wavelength is not obscured by dust H very common in the universe

18 18 Hydrogen emits 21-cm-wavelength radio waves because of a spin-flip transition.

19 19 Milky Way at different wavelengths (Interactive Figure)

20 20 Distribution of Gas Much of the gas is concentrated in arms in the disk spiraling around the center Figure 15.18, Arny and Schneider, 5 th ed. Explorations, 2008 The McGraw-Hill Companies

21 21 Spiral Structure in the Milky Way A "God's view" map of Milky Way as seen from far Galactic North (in Coma Berenices). The star-like lines center in a yellow dot representing the position of Sun. The spokes of that "star" are marked with constellation abbreviations, "Cas" for "Cassiopeia", etc. The spiral arms are colored differently in order to highlight what structure belongs to which arm. HII regions are marked as dots colored in the same color as their spiral arm. From Krisciunas and Yenne, The Pictorial Atlas of the Universe, p.145 (ISBN X)

22 22 Spiral Structure in the Milky Way Recent (2008) data from the Spitzer Space Telescope indicates our galaxy has a large bar and perhaps only two major arms with several minor arms. Note in this diagram our Sun is at the bottom. (NASA) Our galaxy may therefore resemble NGC 1365 (below), click here for image info.

23 23 Radio waves of 21-cm wavelength originate from which component of the interstellar medium? A. Cool, neutral atomic hydrogen B. Cool, carbon monoxide, CO C. Cold, molecular hydrogen, H 2 D. Hot, ionized atomic hydrogen Response Counter 0% 0% 0% 0% A. B. C. D. 90

24 24 Destruction of Spiral Structure If the spiral arms consisted of matter moving together with the stars, they would wind up and be destroyed within a few rotations. See animation applet. (Applet from M. Seeds, Foundations of Astronomy 12 th ed. ITP Nelson; Dr. Brian Martin, The King s University College) Figure 23.17, Chaisson and McMillan, 6 th ed. Astronomy Today, 2008 Pearson Prentice Hall

25 25 Density Waves Form Spiral Arms Spiral structure may be caused by waves of higher density. Same material is not always in the arm, just temporarily compacted. (See Wikipedia animations and Interactive Figure) Slinky Traffic on the highway Lower density Area Density wave Slow moving car Lower density Area

26 26 Density Wave Theory holds that the spiral arms are waves of gas compression that form stars as they go. In the painting at right, gas enters an arm from behind, is compressed, and forms stars. The inset shows spiral galaxy NGC Spiral Arm Formation Figure 23.18, Chaisson and McMillan, 6 th ed. Astronomy Today, 2008 Pearson Prentice Hall

27 27 Spiral Arm Formation Stars form in arms since they have higher density. Recent star formation indicated by some O and B stars. Arms are bright, blue regions

28 28 Spiral Arm Formation Between arms, star formation less recent, less bright. Arms have only 5% more stars than the other areas, but they are mostly very luminous (O and B spectral types)

29 29 Self-Propagating Star Formation Stars form in higher density area. Supernovae later on compress nearby areas, start new star formation Figure 23.19, Chaisson and McMillan, 6 th ed. Astronomy Today, 2008 Pearson Prentice Hall

30 30 How can you tell the difference between a young collection and an old collection of stars? A. Collections of young stars are redder. B. Collections of young stars are darker. C. Collections of young stars move faster. D. Collections of young stars are bluer. Response Counter 0% 0% 0% 0% A. B. C. D. 90

31 31 Center of the Galaxy Center suspected to contain billions of stars Cannot observe center in visible light due to dust, so we use IR and radio observations Bright source near center in radio - Sagittarius A* Probably a black hole at the center

32 32 Galactic Center Radio image of Galactic center. ~60 parsecs across Filaments may be associated with magnetic field. Radio image of Galactic center ~7 parsecs across Sagittarius A* at center.

33 33 Black Hole at the Center Stars are orbiting center (Sagittarius A*) very quickly. Estimated 4.3 million solar masses contained in an area 0.30 AU across Likely black hole. IR image of Galactic Center movie

34 34 The best measurements of the mass of the black hole at the galactic center come from A. the orbits of gas clouds near the center. B. analysis of the X-ray emission from the accretion disk. C. the orbits of stars near center. D. the rate at which the black hole orbits its companion black hole. the orbits of gas clouds n... 0% 0% 0% 0% Response Counter 90 analysis of the X-ray emis... the orbits of stars near c... the rate at which the bla..