1	Astro 1050 Mon., Nov. 4, 2002 Today: Chapter 12, The Milky Way
2	Chapter 12 The Milky Way Galaxy Band of light running around sky in a “great circle” Name from Greeks and Romans: Milky Circle, Milky Road Galileo saw it was made of thousands of faint stars Great Circle suggests a plane of material with us in plane (like ecliptic)
3	The Milky Way (during the Leonid Meteor Shower) Milky Way made of many faint stars Bands of dark dust visible too Many types of objects (eg. O, B stars, Hydrogen clouds) concentrated along plane of Milky Way
4	Where are we within the plane? Great Circle suggests a plane of material with us in plane (like ecliptic) Similar brightness in all directions in plane. Does that really mean we are located in the center?
5	To sort out location will need: Bright objects visible at large distances Objects above or below the plane – so not as obscured Ways to measure distances to those objects Ways to see material other than stars Gas, dust, ???, mass distribution Ways to map motion of objects in our Galaxy Examples of other Galaxies The Shapley-Curtis debate: Are spiral nebulae external galaxies or a type of object within our own galaxy
6	Distances to the farther stars Parallax only works for nearby stars Spectroscopic “Parallax” works somewhat further out Measure spectra and get Spectral Type and Luminosity Class From those get Luminosity and then use m-M to find distance Limited because need relatively bright stars to get high resolution spectra Need another way to find M, then use m-M to get distances Variable stars and the Period-Luminosity relationship: Some stars vary in intrinsic brightness with time The larger, more massive, and brighter stars vary more slowly Like the relative pitch of a large vs. small organ pipe
7	Example of a Variable Cepheid Variables named after prototype Delta Cephei RR Lyrae Variables names after prototype RR Lyrae Related to presence of partially ionized He at right level of star Partially ionized material can act as a local energy source or sink
8	The Instability Strip If T too low partially ionized He too deep to cause instability If T too high partially ionized He too high to cause instability Larger stars oscillate with longer periods
9	The Period-Luminosity Relationship Relationship discovered by Henrietta Leavitt in 1912 stars in Small Magellanic Cloud all at roughly same distance but didn’t have absolute M, just apparent M need absolute M to get distances Calibrated by Harlow Shapley If you can find distance (so M-m) for just one nearby Cepheid, you can convert Leavitt’s “m” scale to the “M” you want.
10	Calibrating the Period-Luminosity Relationship using Proper Motion Suppose all planes fly at 500 MPH = 733 ft/sec Observe the angle that a plane shifts in 1 second of time A plane that moves 1⁰ in 1 sec (90⁰ in 90 sec) is at 42,000 ft A plane that moves 2⁰ in 1 sec (90⁰ in 45 sec) is at 21,000 ft Works for stars too: closer stars have faster “proper motion” Can only get average distances using average proper motion, since any given star might be moving faster or slower than average Harlow Shapley found 11 Cepheids with proper motion Used average proper motion, and average distance, to find average (M-m) Let him replace Leavitt’s relative “m” axis with absolute “M” Now period Þ M then M-m Þ d
11	Globular vs. open clusters Open Clusters Typically a few thousand stars Not gravitationally bound Often contain young stars Concentrated in plane of Milky Way Distributed “randomly” around the circle of the Milky Way Globular Clusters Typically >hundred thousand stars Only contain older stars Are gravitationally bound Not strongly concentrated in plane of Milky Way Not randomly distributed around the circle of the Milky Way – more towards Sagittarius
12	The Distribution of Globular Clusters Assume Globular Clusters orbit around center of galaxy Center of Globular Cluster distribution is 8.5 kpc in direction of Sagittarius We are about 2/3 of the way out to one side – so “diameter” is approx. 25 kpc or 75,000 ly. Dust within the galactic plane fools us with respect to distribution of ordinary stars
13	The Shapley-Curtis Debate: 1920 Are spiral nebulae really other galaxies, or just swirling clouds of gas and dust within our own galaxy? Many spiral galaxies had much larger radial velocities than other objects within our own galaxy
14	The Andromeda Galaxy
15	M51: The Whirlpool Galaxy
16	Sensing Hydrogen Gas Radio emission at 21 cm wavelength Penetrates gas and dust Requires little energy to excite
17	The Structure of our Galaxy The Disk Component Stars, gas, and dust The spiral arms Size: Luminous Diameter ~ 25 kpc Thickness 300 pc – 1 kpc O stars and dust 30 pc Sunlike stars greater The Spherical Component Old Stars, but little gas or dust The Halo Globular clusters Isolated old stars red dwarfs, giants, white dwarfs The Nuclear Bulge
18	Disk vs. Halo Orbits The Disk Component Stars, gas, and dust The spiral arms Size: Luminous Diameter ~ 25 kpc Thickness 300 pc – 1 kpc O stars and dust smaller Sun-like stars greater The Spherical Component Old Stars, but little gas or dust The Halo Globular clusters Isolated old stars red dwarfs, giants, white dwarfs The Nuclear Bulge
19	Differential Galactic Rotation Stars far from the center take longer to orbit galaxy If all mass is at the center get Keplerian Rotation: If M is distributed, M_effective grows with distance, so velocity does not drop in same way
20	The Galactic Rotation Curve Keplerian fall-off near center indicates compact mass at center Flat curve throughout disk indicates much distributed mass Lack of fall-off beyond visible “edge” indicates “dark matter”
21	Stellar Population and Galaxy Evolution “Metal” abundance during time “Metals” are elements heavier than He A given star’s atmospheric abundance is approx. fixed at birth Interstellar metal abundance grows with each new generation of stars Red giants and supernova eject new heavy elements into interstellar gas Orbits during time A given star’s orbit is approx. fixed at birth – just plows through gas Orbits of gas clouds evolve with time since they can collide Orbits get more circular and disk-like with time
22	Traditional Model of Galaxy Evolution Stars are stuck with their original orbits They plow through gas like bullets Orbits of gas can evolve Gas clouds collide and only average motion (rotation) survives Metal abundance grows with time System starts out with little organized motion,few metals Halo stars form at this time It contracts, velocities average out leaving only rotation Gas collapses to form the disk Disk stars form after this has happened Some problems with traditional model Globular clusters not all same age Gap in ages between halo and disk objects Presence of some metals in even in oldest stars System may have formed by merger of smaller galaxies Galactic Cannibalism
23	M51: The Whirlpool Galaxy
24	Possible Origin of Spiral Arms Differential rotation smears features out into spiral patterns But can’t be whole story: Number of times Sun has orbited the galaxy: 10 billion yr/200 million yr = 50 times Spiral arms would have been wound up very tightly Something must continuously rebuild them
25	Degree of Organization of the Spiral Arms Different degrees of organization Grand Design Spirals: M51 Flocculent (“wooly”) Spirals
26	Degree of Organization of the Spiral Arms Different degrees of organization Grand Design Spirals: M51 Flocculent (“wooly”) Spirals
27	Tracing the Spiral Arms Arms NOT obvious if you look at: Old objects like the sun Arms ARE obvious if you look at: Maps of gas clouds 21 cm Hydrogen Radio maps of CO Far infrared observations of dust Young stars O, B stars “HII” ionized hydrogen regions surrounding O,B stars Clouds somehow form in arms , then dissipate between them Short lived objects only get a short distance from their places of birth O stars, Lifetime = few million years, at 250 km/s Þ500 pc
28	M51: The Whirlpool Galaxy
29	Density Wave Theory SPIRAL WAVE rotates with galaxy, but slower than individual stars Like moving traffic jam after an accident has been cleared Gas (and stars) catch up with wave, move through it, eventually reach front Just like cars catching up with moving traffic jam, eventually get through it Gas is more crowded in wave – clouds collapse to form new stars More collisions in the traffic jam There are slightly more old stars in the arm too, because they speed up slightly coming into it and slow down slightly moving out of it. But the best tracers are the things that mark recent cloud collapses: O,B stars, etc.
30	M51: The Whirlpool Galaxy
31	Self Sustaining Star Formation Cloud collapse Þ New stars New stars Þ Supernova after few million years Supernova Þ Shock Waves Shock Waves Þ Nearby clouds collapse Differential Rotation twists pattern into spiral
32	Two limiting cases of spirals Grand Design: Density Wave Flocculent: Self Sust. Star Form. + Diff. Rot. In most Galaxies you have some combination of the two
33	The Nucleus of the Galaxy Likely Black hole High velocities Large energy generation At a=275 AU P=2.8 yr Þ 2.7 million solar masses Radio image of Sgr A about 3 pc across, with model of surrounding disk
34	A movie of stars at the core www.mpe.mpg.de/www_ir/GC Very cool, brand new, and worth a look! This is the best evidence to date for a massive black hole at the Galactic core. Now essentially “proven.”