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Lecture 21: The Milky Way

Lund Observatory composite of the Milky Way in galactic coordinates - a diffuse band of light across the sky (requires a dark , clear site !). Objectives: to describe our view of the Milky Way identify and understand its major components. Lecture 21: The Milky Way.

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Lecture 21: The Milky Way

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  1. Lund Observatory composite of the Milky Way in galactic coordinates - a diffuse band of light across the sky (requires a dark, clear site !) • Objectives: • to describe our view of the Milky Way • identify and understand its major components Lecture 21: The Milky Way Additional reading: Kaufmann (chap. 25), Zeilik (chap. 14, 20) PHYS1005 – 2003/4

  2. IRAS (IR) - reduces effect of interstellar absorption • Radio (21cm) • shows distribution of H PHYS1005 – 2003/4

  3. Schematic overview and properties: • diameter: 30 kpc • thickness:~300 pc • no. of stars: ~2x1011 • mass: ~1012 MO • (from rotation curve) • Sun’s rotation period around Galaxy: 250 million yrs PHYS1005 – 2003/4

  4. How our view of the Milky Way has changed with time ! • 1610: • Galileo (Italy) observed the Milky Way with the first telescope and noted For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters. • 18th century: • Thomas Wright (England) and Immanuel Kant (Germany) suggested we were located in a disk of stars, shown here: • Kant even suggested that some fuzzy patches were other large groups of stars, or Island Universes! • William Herschel (England) constructed a model for the Milky Way by counting star densities in different directions with his 48” telescope, producing this sketch: PHYS1005 – 2003/4

  5. Application of 20th century technology: • 1900: Johannes Kapteyn (Holland) • began project to photographically measure stellar motions and distances (nearby stars only) • by 1922 had produced the Kapteyn model with: • Sun 650pc from centre of • oblate spheroid, ~5x wider than it is thick • and with radius ~3000pc • not bad, compared to modern values! • but he assumed the Sun was near the centre of the Galaxy • main problem was getting accurate distances • 1920: Harlow Shapley (US) • (using Cepheids) noted that globular clusters were distributed about a point 15kpc away from the Sun! • correctly assumed they were spherically distributed about Galactic Centre • got direction (Sagittarius) right, but distance wrong PHYS1005 – 2003/4

  6. current value: Sun is 8.5kpc from Galactic Centre • why so different? • Extinction makes • us appear to be at centre of Milky Way “band of stars” • and GCs to be further away than they are • Kapteyn realised this was possible, but could not find evidence Adding the final details: • 1927: Jan Oort (Holland) • measures rotation rate of Milky Way  estimate of total mass • 1940 – 1960s: Oort and Hendrik van der Hulst (Holland) • reveal spiral structure of Milky Way using radio (21cm) maps of H distribution • nearby spiral arms (and stellar associations): PHYS1005 – 2003/4

  7. 21cm radio map of Milky Way spiral structure: PHYS1005 – 2003/4

  8. PHYS1005 – 2003/4

  9. The Modern Picture: • total mass within 100kpc - few x 1011 MO, consisting of: • Disk • R ~ 15kpc; Sun at 8.5kpc • few x 100pc thick • young clusters, star-forming regions trace spiral arms • disk material moves in circular orbits at ~200 km/s • ~250 million yrs for one orbit • Halo • fuzzy-edged • R ~ 25kpc, but • ~108 stars; most M in form of dark matter • Halo does not rotate  appears to move backwards wrt us • low metal abundances (up to 1000x less than Solar) – why? • Halo stars are old, like globular clusters • may be remnant of early star systems that merged to form Milky Way PHYS1005 – 2003/4

  10. Nucleus • centre of Milky Way in Sagittarius • impossible to study in optical due to I/S extinction, but can be done in IR (see lecture 6) • e.g. CIRSI images of Galactic Centre region with INT 2.5m on La Palma PHYS1005 – 2003/4

  11. most detailed studies of the Galactic Centre region have been performed at radio wavelengths (as radio easily penetrates the gas and dust) • bright, variable radio source (Sgr A*) found at centre in 1974, the nature of which has been a major astronomical puzzle • higher resolution NRAO image of Sgr A* (box) : PHYS1005 – 2003/4

  12. VLT IR images with resolution 0.04” A “Monster” in the Nucleus ? • inner 30pc – the nucleus • v high stellar densities • > 106 stars pc-3 • stars only ~1000AU apart  collide every 106 yrs ! • cf ~0.1 – 1 pc-3 around Sun • rapid stellar motions discovered in 1998 with AO techniques • and a flare in May 2003 ! PHYS1005 – 2003/4

  13. Calculating the mass at the Nucleus: • essentially it’s a “super” binary system: • orbital P = 15.2 yrs • separation, a ~ 5 light-days • highly eccentric orbit (at closest, S2 only ~17 lt-hrs from SgrA* and moving at 5000 km/s) • 1 AU = 8 light-mins  a = 900 AU • recall Kepler 3 from lecture 10: • a(AU)3 / P(yr)2 = M1 + M2 • M2 is tiny (~15MO red giant)  M1 = 9003 / 15.22 = 3 million MO • i.e. supermassive black hole ! • but it’s quiescent! Why? • N.B. X-ray (2001), IR (2003) flares have now been seen PHYS1005 – 2003/4

  14. Effect of employing Adaptive Optics on large ground-based telescopes: Gemini-N IR images (taken in 2000) PHYS1005 – 2003/4

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