THE GALAXY
Composite infrared
colour image of
Galactic Centre
region taken at
1.25, 2.2 and 3.5
microns with
COBE/DIRBE
instrument
(NASA/GSFC).
• GALAXY: A conglomeration of stars, gas + dust
• Topics:
– Binary stars
– Star clusters
– Stellar evolution: an overview
– Variable stars and their use as distance indicators
– Mass loss from stars
– Binary stars with compact components
– The interstellar medium
– Structure and rotation of the Galaxy
AS 1001 The Galaxy
Two main themes:
• An overview of the structure of the Milky Way
Galaxy (MWG)
– how the various components are inter-related and mutually
interacting
– leading to hints about the formation and evolution of galaxies.
• An understanding of the distance scale in the
Universe
– starting from stars with accurately known parallaxes
– using properties of star clusters, binary stars, variable stars to
build a picture of the MWG + distances to other galaxies.
AS 1001 The Galaxy
Analysing radiation from stars
Spectra
Apparent brightnesses
Colours
Positions
Astrophysical theory
Radial velocities Calibration of types via:
Proper motions nearby stars, stars in
Classification of stars clusters, binary stars
into different types for : masses, sizes,
intrinsic brightnesses,
temperatures, chemical
composition.
Distances
Space motions
Chemical composition
or ‘population’ group
3-D distribution of stars
of different types
AS 1001 The Galaxy
Star clusters and
colour-magnitude diagrams
• CLUSTERS: Congregations of
stars with stronger mutual
gravitational attraction than in
general stellar field.
• Range from loose associations
(~ 100 stars) to open clusters (n
x 103 stars) and globular
clusters (n x 105 stars).
Globular cluster: 47 Tuc
Open cluster: The Pleiades (D. Malin/UKSTU)
AS 1001 The Galaxy
The importance of star clusters
• Stars in a cluster are all at ~ same distance
– e.g. Pleiades: diameter 10 pc, distance 126 pc
– h Persei: diameter 10 pc, distance 2200 pc
– Hence, accurate distances to these objects.
• Stars in a cluster had a common origin
– formed about the same time
– from same pre-stellar gas, i.e. same chemical composition.
• Hence direct tests of theoretical models for
luminosity (L) and temperature (T) as function of
stellar mass (M) and stellar ages (t).
AS 1001 The Galaxy
Distances to clusters - 1
• Parallaxes and moving clusters
Convergent point
• Need space motions of individual stars
• Proper motions + radial velocities + direction to
convergent point give distance to cluster.
– e.g. the Hyades star cluster at d = 40 pc.
• Measure apparent magnitudes (V) and colour
indices (e.g. B-V) of moving cluster stars
– Hence obtain absolute magnitudes MV (using V-MV=5 log d - 5 )
• Hence get calibration of MV vs. (B-V, spectral type),
etc.
AS 1001 The Galaxy
Moving clusters
• group of stars with same space velocity
Vr
θ Vt
V
θ
convergent
point
• measure radial velocity
– Vr = V cos θ
• measure angle to convergent point
– V and then Vt
– Vt = V sin θ
• measure proper motions µ in arcsec/year
– Vt = 4.74 µ d
Vr
• gives distance d= tanθ
– V in km/s, d in pc
4.74µ
AS 1001 The Galaxy
Distances to clusters - 2
• Colour-magnitude diagrams
• Now better, more accurate than moving-cluster
method.
• Many ordinary F-type stars in solar neighbourhood
(d < 20 to 25 pc) with accurate parallaxes (P = 1/d).
• V, (B-V), etc values easily measured to accuracies
< 1%.
• Also known that interstellar space in solar
neighbourhood is free of dust
– hence no scattering, or extinction of starlight between these
local F stars and us.
AS 1001 The Galaxy
Main-sequence fitting
• Calibration: 2.8
– via V - AV - MV = 5 log (d/10)
with AV = 0, and MV
– d = 1/P
– can determine accurate MV vs.
(B-V)0 etc relationships for 4.7
main-sequence F stars -- 0.3 0.6
empirical! (B-V)0
• Application: MV Move cluster C-M diagram
– Observe a cluster of stars to until the two main
determine V, (B-V) etc. sequences
(V-AV) overlap.
– Correct for interstellar
extinction.
– Compare colour-magnitude Hence get
diagram for cluster [(B-V)0 vs. distance modulus
(V-AV)] with (B-V)0 vs. MV (V - AV - MV)
diagram for solar-
neighbourhood stars. (B-V)0
AS 1001 The Galaxy
Interstellar reddening and extinction
• Effect of dust in interstellar space is to scatter
starlight
– blue light is scattered more easily then red light:
−1
I scat ∝ λ
– Thus a star seen through dust will appear fainter than it would if
there were no dust along the line of sight.
– A star’s colour will also be affected, because of the λ–1
dependence, so that a star appears to be redder (or less blue)
than its intrinsic colour.
AS 1001 The Galaxy