Astronomy 114. Lecture 27: The Galaxy. Martin D. Weinberg. UMass/Astronomy Department

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1 Astronomy 114 Lecture 27: The Galaxy Martin D. Weinberg UMass/Astronomy Department A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 1/23

2 Announcements Quiz #2: we re aiming for this coming Friday... A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 2/23

3 Announcements Quiz #2: we re aiming for this coming Friday... Today: a bit more on telescopes Galaxies! Our Galaxy, Chap. 25 Galaxies, Chap. 26 A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 2/23

4 X-ray and gamma-ray telescopes High-energies photons interact with most materials X-ray telescopes use ring-shaped "glancing" mirrors Made of heavy metals Reflect the rays just a few degrees Mirrors are rotated parabolas and hyperbolas A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 3/23

5 X-ray and gamma-ray telescopes High-energies photons interact with most materials X-ray telescopes use ring-shaped "glancing" mirrors A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 3/23

6 X-ray and gamma-ray telescopes High-energies photons interact with most materials X-ray telescopes use ring-shaped "glancing" mirrors A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 3/23

7 X-ray and gamma-ray telescopes High-energies photons interact with most materials X-ray telescopes use ring-shaped "glancing" mirrors A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 3/23

8 X-ray and gamma-ray telescopes High-energies photons interact with most materials X-ray telescopes use ring-shaped "glancing" mirrors Gamma-ray telescopes give up on focusing entirely Use coded aperture masks The pattern of shadows the mask creates can be reconstructed to form an image A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 3/23

9 X-ray and gamma-ray telescopes High-energies photons interact with most materials X-ray telescopes use ring-shaped "glancing" mirrors Gamma-ray telescopes give up on focusing entirely Use coded aperture masks The pattern of shadows the mask creates can be reconstructed to form an image Satellites and balloons... why? A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 3/23

10 Transmittance of the Earth s atmosphere A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 4/23

11 Detectors (1/3) First detector: human eye Use secondary lens ("eyepiece") to make converging rays parallel again Limitations: Short exposure time (1/30 second) Have to rely on observer s description or drawing A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 5/23

12 Detectors (2/3) Better: film or photographic plates Put in focal plane, so extended image forms on plate Incoming photons cause permanent chemical change Long exposure increased sensitivity Permanent record Inefficient: best emulsions miss 99% of incoming photons A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 6/23

13 Detectors (3/3) State of the art: electronic detectors (CCDs) Incoming photons knock electrons out of silicon Electrons counted electronically Excellent efficiency: up to 80% of incoming photons counted Digital data, easily analyzed by computers CCDs have revolutionized optical astronomy (since c. 1980) In digital cameras and camcorders A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 7/23

14 A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 8/23

15 Structure of the Milky Way The halo: a roughly spherical distribution which contains the oldest stars in the Galaxy The bulge and Galactic Center The disk: contains the majority of the stars, including the Sun, and most of the gas and dust A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 9/23

16 Structure of the Milky Way Stellar disk: radius 16 kpc Sun s position: radius 8 kpc Period of Sun s rotation: 250 million years Speed of Sun s motion: 200 km/s A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy 114 9/23

17 The Galactic Disk Rotating, flattened due to angular momentum Contains the Sun and young stars, atomic and molecular gas, dust A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

18 The Galactic Disk Rotating, flattened due to angular momentum Contains the Sun and young stars, atomic and molecular gas, dust A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

19 The Galactic Disk Rotating, flattened due to angular momentum Contains the Sun and young stars, atomic and molecular gas, dust A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

20 Discovery of the Milky Way Herschel ( 1800) and Kapteyn ( 1920) counted stars to infer the shape of the Galaxy Spectral types to estimate luminosity L Brightness-distance relation, b = L/(4πR 2 ), tomap the locations of the stars His answer: we are near the center of an elliptical distribution of stars! A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

21 What happened? Interstellar extinction! Dust particles in interstellar space (approx. 0.1 micron in size) Absorb and scatter light strongly in the UV and visible Stars appear dimmer We infer larger distances In the disk, we can only see stars to about 1 to 2 kpc away (in visible light) Dust Dust A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

22 How did astronomers figure this out? 1920s: Harlow Shapley measured distance to globular clusters using RR Lyrae variables Deduced the correct size and shape of the Milky Way A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

23 How did astronomers figure this out? 1920s: Harlow Shapley measured distance to globular clusters using RR Lyrae variables Deduced the correct size and shape of the Milky Way A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

24 How did astronomers figure this out? 1920s: Harlow Shapley measured distance to globular clusters using RR Lyrae variables Deduced the correct size and shape of the Milky Way A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

25 How did astronomers figure this out? 1920s: Harlow Shapley measured distance to globular clusters using RR Lyrae variables Deduced the correct size and shape of the Milky Way A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

26 General problem with studying Milky Way Like being inside a forest, and seeing only the trees Because our view is obscured, it looks as if there are about as many stars in all directions Need to use radio waves or other forms of radiation to see through interstellar matter What we now know: The Milky Way is a spiral galaxy containing about 100 billion stars A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

27 External galaxies as a model Visible The galaxy M51 is a face-on spiral galaxy It shows spiral arms much like the Milky Way There is lots of gas and dust in the spiral arms Young O and B stars are present in the arms A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

28 External galaxies as a model Near infrared The galaxy M51 is a face-on spiral galaxy It shows spiral arms much like the Milky Way There is lots of gas and dust in the spiral arms Young O and B stars are present in the arms A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

29 External galaxies as a model Ultraviolet The galaxy M51 is a face-on spiral galaxy It shows spiral arms much like the Milky Way There is lots of gas and dust in the spiral arms Young O and B stars are present in the arms A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

30 External galaxies as a model X-ray The galaxy M51 is a face-on spiral galaxy It shows spiral arms much like the Milky Way There is lots of gas and dust in the spiral arms Young O and B stars are present in the arms A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

31 Radio emission from neutral hydrogen (1/2) The n = 1 level (ground state) of H is split into 2 levels separated by a very small energy This splitting is due intrinsicspin of electron and proton Behave like small magnets When the North poles are aligned, the energy is higher and vice versa A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

32 Radio emission from neutral hydrogen (1/2) The n = 1 level (ground state) of H is split into 2 levels separated by a very small energy This splitting is due intrinsicspin of electron and proton Behave like small magnets When the North poles are aligned, the energy is higher and vice versa A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

33 Radio emission from neutral hydrogen (2/2) HI emission is rare (per atom) but hydrogen is plentiful in the Galaxy A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

34 Rotation of the Disk Measure using the Doppler Effect A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

35 Rotation of the Disk Measure using the Doppler Effect Stars: Doppler shifts of stellar absorption lines Ionized Gas: emission lines from HII regions Atomic Hydrogen (HI) Gas: Cold H clouds emit a radio emission line at a wavelength of 21-cm Can trace nearly the entire disk beyond where the stars have begun to thin out A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

36 Mapping the Milky Way (1/4) Distribution of gas in velocity along the line of sight distance map A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

37 Mapping the Milky Way (2/4) A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

38 Mapping the Milky Way (3/4) NGC 1232 (similar to the Milky Way, face on... ) A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

39 Mapping the Milky Way (4/4) NGC 4565 (similar to the Milky Way, edge on... ) A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

40 Artist s view of the Milky Way from above Milky Way: four-armed barred spiral A114: Lecture Apr 2007 Read: Ch. 25,26 Astronomy /23

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