1. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw 2. Constituents of the Galaxy 3. Structure of the Galaxy 4. The system of galactic coordinates 5. Stellar populations

2. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Constituents of the Galaxy The Galaxy is composed of the following constituents: Stars Interstellar gas Dust particles Cosmic rays Dark matter (of unknown type)

3. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Stars Stars occur in the Galaxy as field stars (i.e. isolated stars like the Sun) stars in open (galactic) clusters stars in globular clusters l: Messier 67, an open cluster r: ω Centauri, a globular cluster

4. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Interstellar gas Interstellar gas is found mainly in the galactic disk, in a thin layer. Much of the gas is in discrete clouds, especially neutral atomic hydrogen (HI) and dense molecular clouds (H 2 ). Also there are diffuse gaseous nebulae (HII) and a low density intercloud medium, either neutral hydrogen, or highly ionized coronal gas.

5. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw l: Lagoon nebula, M8, in Sagittarius r: Rosette nebula, NGC2237 in Monoceros

6. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw l: Tarantula nebula, 30 Dor, in the LMC r: The Trifid nebula, M20, in Sagittarius

7. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw The η Carinae nebula in the southern Milky Way

8. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Dust Interstellar dust consists of small solid particles, possibly mainly silicates and some water ice, in interstellar space, size typically ~100 nm. Dust grains occur in a general thin layer throughout the galactic disk, in dark clouds (e.g. the Coalsack dark nebula) and in small very dense dark globules. Dust is also seen as “reflection” nebulae near some hot stars (e.g. in Pleiades).

9. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw The Coalsack dark nebula in Crux, the Southern Cross

10. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw The Horsehead nebula is a dark nebula in Orion silhouetted against a bright HII background

11. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Dust clouds: above: Barnard 86 right: dark clouds in Sagittarius

12. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Distribution of dust clouds around Sun within 3 kpc, from data on star colours and reddening

13. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Cosmic rays Galactic cosmic rays are high energy particles E > 10 9 eV and occasionally as high as 10 19 eV. They are mainly protons, with some helium nuclei (α particles), and nuclei of other light elements, especially C, N and O. They are trapped in the galactic magnetic field. Their origin may be exploding massive stars known as supernovae.

14. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Dark matter Dark matter emits no light and is a form of mass of unknown type, and probably not even baryonic (i.e. composed of protons and neutrons). But it has a gravitational field and causes the Galaxy to rotate faster than can otherwise be explained. As much as 90 per cent of the mass of the Galaxy may be “dark matter”. Its nature is a major problem of modern astrophysics.

15. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Structure of the Galaxy The Galaxy has the following components: The halo Large roughly spherical region of low density. Possibly as much as 50 or 100 kpc in diameter The disk A thin disk, diameter up to 50 kpc, thickness ~1 kpc or less. Spiral arms are embedded within the disk The bulge Central roughly spherical region, ~1 kpc diameter

16. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Schematic model of the Milky Way

17. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Schematic diagram showing the structure of the Galaxy

18. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Other spiral galaxies give a good impression of the true structure of our own Milky Way system above: M51 in Canes Venatici right: M31 in Andromeda

19. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Edge-on spiral galaxy, M104. Note the thin but dark dust lane and the unusually large bulge relative to the disk

20. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Spiral galaxy NGC1232. The redder stars in the central bulge and bluer stars in the spiral arms are typical of spiral galaxies, including our Milky Way

21. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Galactic coordinates The galactic equator is defined by a great circle which follows the HI distribution in the galactic disk Galactic longitude and latitude are (l,b) in degrees (l,b) = (0º,0º) is the galactic centre (l,b) = (90º,0º) is the direction of galactic rotation (l,b) = (180º,0º) is direction of galactic anticentre (l,b) = (-,±90º) are the directions of N and S galactic poles

22. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw The system of galactic coordinates Note that the galactic equator is inclined at 63.5º to the celestial equator

23. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Stellar populations The concept of stellar populations was introduced by Walter Baade (German-American astronomer in California) in 1944 from observations of the spiral galaxy M31 in Andromeda. Spiral arms: bluer stars – population I Nuclear bulge: redder stars – population II The halo stars are also assigned to population II

24. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Stellar populations l: Andromeda galaxy, M31 above: Walter Baade

25. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Baade’s 1944 colour- magnitude diagram of stars in M31 showing two populations. The brightest pop II stars are redder than the brightest pop I stars.

26. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw Stellar populations

27. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw

28. ASTR112 The Galaxy Lecture 2 Prof. John Hearnshaw End of lecture 2