the milky way galaxy · the milky way galaxy a.k.a. the galaxy (galaxias kyklos) nb this is a...
TRANSCRIPT
The Milky Way Galaxya.k.a. the Galaxy (galaxias kyklos)
NB this is a painting, not a photograph
Milky Way Galaxy
• Appearance, size, morphology– Disk, bulge, halo
• Constituents:– Stars of various populations– Gas (cold, warm, hot) and dust = ISM– Dark matter
• Dynamics• Evolution and recycling • Galactic centre
The local group, a photo montage
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Artist’s impresseion
The Milky Way Galaxy
Galaxy = MWgalaxy = others
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Milky way look alike
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NGC 891
MW (COBE)
Equatorialvs
Galacticcoordinates
Extent of optical emission (starlight) vs 21cm (neutral gas)In Milky Way and other spiral galaxies
21 cm spin fliptransition of neutral Hydrogen (HI) (see box 1.3, page 30)
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Nearest stars Brightest stars
Nearest stars dominated by low luminosity brighest stars dominated by highbecause these are most common lumionsity as we can see them far away
Pleiades youngstar cluster
Horsehead nebulain Orion
Pop I = youngStars in the disk
Globular cluster (population II) on the order of a million stars, gravitationally bound
X-ray binaries in globular cluster ”47 Tuc”
Colour distribution of halo and disk stars- Age vs Metallicity
Metal-richGlobularClusters(near the plane)
Metal-poorGlobularClusters(in the halo)
Vertical distribution of stars
I = I0 e−r / hDisks have exponential light distribution: I = surface brightness, h = scale length (height)
Thin (hz= 300 pc)
vs
Thick (hz= 1 kpc)
Disk
Thick disk older, hence no A-stars
Disks have exponential light distribution: I = surface brightness, intensity (flux per angular area) h = scale length (height)
I = I0 e−r / h
Surface photometry
NB surface brightness independent of distance- Flux per angular area - Luminosity per metric area
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Radial distribution of neutral atomic (HI) and molecular (H2) gas in the Milky Way Galaxy
H2 has no ’dipole moment’ and therefore no suitably observabletransitions at typical ISM cloud temperatures, but the CO molecule can be used as tracer (see Box 1.4 on page 33 in JL)
Property Population I Intermediate Population IIOrbits Circular Elongated Very ellipticalShape spiral arms disk spherical/haloThickness(pc) 120 400 2000Metals (%) 3-4 0.4-2 0.4 or lessMass (Msun) 2x109 5x1010 2x1010
Age (yr) 108 109 1010
Typical objects Open clusters, Sun Globular clustersHII regions, RR Lyrae stars
Stellar populations in MW
Population III ? Zero metallicity
Chemical compositionof starclusters in MW
High latitude neutral hydrogen