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2. Constituents of the Galaxy3. Structure of the Galaxy4. The system of galactic coordinates5. Stellar populations

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Constituents of the GalaxyThe Galaxy is composed of the following constituents:• Stars• Interstellar gas• Dust particles• Cosmic rays• Dark matter (of unknown type)

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StarsStars 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 clusterr: ω Centauri, a globular cluster

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Interstellar gas

Interstellar gas is found mainly in the galacticdisk, in a thin layer. Much of the gas is in discrete clouds, especially neutral atomic hydrogen (HI)and dense molecular clouds (H2 ). Also there arediffuse gaseous nebulae (HII) and a low density intercloud medium, either neutral hydrogen, orhighly ionized coronal gas.

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awl: Lagoon nebula, M8, in Sagittariusr: Rosette nebula, NGC2237 in Monoceros

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l: Tarantula nebula, 30 Dor, in the LMCr: The Trifid nebula, M20, in Sagittarius

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The η Carinae nebula in the southern Milky Way

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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 throughoutthe galactic disk, in dark clouds (e.g. the Coalsackdark nebula) and in small very dense dark globules.Dust is also seen as “reflection” nebulae near some hot stars (e.g. in Pleiades).

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The Coalsack dark nebula in Crux, the Southern Cross

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The Horsehead nebulais a dark nebula in Orionsilhouetted against a bright HII background

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Dust clouds:above: Barnard 86right: dark cloudsin Sagittarius

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Distribution of dust clouds around Sun within 3 kpc, from data on star colours and reddening

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Cosmic rays

Galactic cosmic rays are high energy particlesE > 109 eV and occasionally as high as 1019 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.

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Dark matter

Dark matter emits no light and is a form of massof 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 Galaxymay be “dark matter”. Its nature is a major problemof modern astrophysics.

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Structure of the GalaxyThe 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

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Schematic model of the Milky Way

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Schematic diagram showing the structure of the Galaxy

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Other spiral galaxies give a good impression ofthe true structure of our own Milky Way system

above: M51 in Canes Venaticiright: M31 in Andromeda

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Edge-on spiral galaxy, M104. Note the thin but darkdust lane and the unusually large bulge relative to the disk

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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

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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

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The system of galactic coordinatesNote that the galactic equator is inclined at 63.5º to thecelestial equator

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Stellar populations

The concept of stellar populations was introducedby Walter Baade (German-American astronomer in California) in 1944 from observations of thespiral galaxy M31 in Andromeda.

Spiral arms: bluer stars – population INuclear bulge: redder stars – population IIThe halo stars are also assigned to population II

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Stellar populations

l: Andromeda galaxy, M31above: Walter Baade

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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.

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Stellar populations

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End of lecture 2End of lecture 2