2. Constituents of the Galaxy

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2. Constituents of the Galaxy 3. Structure of the
Galaxy 4. The system of galactic coordinates 5.
Stellar populations
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• Constituents of the Galaxy
• The Galaxy is composed of the following
• constituents
• Stars
• Interstellar gas
• Dust particles
• Cosmic rays
• Dark matter (of unknown type)

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• 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 ? Centauri, a
globular cluster
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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 (H2 ). Also there are diffuse
gaseous nebulae (HII) and a low density
intercloud medium, either neutral hydrogen,
or highly ionized coronal gas.
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l Lagoon nebula, M8, in Sagittarius r
Rosette nebula, NGC2237 in Monoceros
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l Tarantula nebula, 30 Dor, in the LMC r
The Trifid nebula, M20, in Sagittarius
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The ? 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 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).
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The Coalsack dark nebula in Crux, the Southern
Cross
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The Horsehead nebula is a dark nebula in
Orion silhouetted against a bright HII background
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Dust clouds above Barnard 86 right dark
clouds in 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
particles E gt 109 eV and occasionally as high
as 1019 eV. They are mainly protons, with some
helium nuclei (a 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 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.
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• 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

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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
of the true structure of our own Milky Way system
above M51 in Canes Venatici right M31 in
Andromeda
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Edge-on spiral galaxy, M104. Note the thin but
dark dust 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 coordinates Note that the
galactic equator is inclined at 63.5º to
the celestial equator
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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
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Stellar populations
l Andromeda galaxy, M31 above Walter Baade
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Baades 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 2