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ASTR112 The Galaxy
Lecture 3
Prof. John Hearnshaw
The Pleiades open cluster
5. Stellar populations
6. Galactic (open) clusters
ASTR112 The Galaxy
Lecture 3
Stellar populations
Spiral arms: bluer stars – population I
Nuclear bulge: redder stars – population II
The halo stars are also assigned to population II
Prof. John Hearnshaw
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.
ASTR112 The Galaxy
Lecture 3
l: Andromeda galaxy, M31
above: Walter Baade
Prof. John Hearnshaw
Stellar populations
ASTR112 The Galaxy
Lecture 3
Prof. John Hearnshaw
Baade’s 1944 colourmagnitude diagram of
stars in M31 showing
two populations. The
brightest pop II stars
are redder than the
brightest pop I stars.
ASTR112 The Galaxy
Lecture 3
Prof. John Hearnshaw
Stellar populations
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 3
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 3
Galactic star clusters
Main points
• Also known as open star clusters
• About 400 known; ~18000 may exist in Galaxy
• Gal. latitude b ≤ 5º in most cases, very few >10º
• Open clusters are mainly young Popn I objects
• Typically they contain a few hundred stars
• The stars are coeval (of same age), at essentially
same distance and of same metallicity
• There is a spread in star masses (given by IMF)
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 3
The Pleiades star cluster
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 3
Some open clusters
top l: double cluster h and χ Per
top centre: the Hyades
top r: Messier 67
l: κ Crucis, the Jewel Box cluster
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 3
ASTR112 The Galaxy
Lecture 3
Prof. John Hearnshaw
Young open
clusters and OB
associations in
the galactic plane
are found mainly
in the spiral arms
The galactic distributions of both early B-type field
stars and of open clusters closely follow the Milky
Way and are only found close to the galactic equator
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 3
Most famous young Popn I clusters include:
• the Pleiades
• the Hyades
• h and χ Persei (the double cluster)
• Praesepe
• κ Crucis (the Jewel Box)
Old Popn I clusters are much rarer and include:
• M67
• NGC188
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 3
ASTR112 The Galaxy
Lecture 3
Galactic cluster Hertzsprung-Russell diagrams
Prof. John Hearnshaw
• This is a plot of magnitude as a function of
colour index (often (B–V)) or of spectral type.
• If C.I. is used, then also known as a colourmagnitude diagram (CMD)
Colour-magnitude diagrams
for the Pleiades and Praesepe
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 3
Colour-magnitude diagrams
for the Hyades and NGC188
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 3
Messier 67 colour-magnitude diagram
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 3
Composite HR diagram for open star clusters
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 3
Features of the HR diagram for a galactic cluster:
• Zero-age main sequence (ZAMS)
The locus of stars which have just started to shine
• Subgiant branch
Stars that have just exhausted H in their cores,
and are now moving off the main sequence
• Red giants
Evolved stars in upper right-hand part of diagram
with either He cores, or they are burning He to
C and O in their cores. They have a H-burning
shell. These were once the more massive MS stars.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 3
ASTR112 The Galaxy
Lecture 3
Moving cluster distances
Stars in cluster have common space motion. But
because of the perspective effect, the proper motions
appear to converge on a given point in sky – the
convergent point.
Prof. John Hearnshaw
Fundamental distance method applicable to
Hyades (the nearest cluster) and Sco-Cen association
Proper motions of
stars in the Hyades
cluster, showing
the convergent point
located in the sky
but several degrees
away from the
cluster itself.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 3
ASTR112 The Galaxy
Lecture 3
Measure
• θ, the angle between each star and convergent point
• Radial velocity of stars VR from the Doppler shift of
spectral lines
• Proper motion of each star, μ, in arc seconds/yr
θ
V
to convergent point
star
θ
Earth
Prof. John Hearnshaw
μ
VR
ASTR112 The Galaxy
Lecture 3
V  V cos 
R
T
V  V tan
T
R
But V  4.74d
T
V tan
d 
4.74
4.74
or p 
V tan
R
R
Prof. John Hearnshaw
V  V sin 
The basic equations for
the moving cluster method.
VR radial velocity (km/s)
μ proper motion (arc s/yr)
d distance (pc)
p parallax (=1/d) (arc s)
ASTR112 The Galaxy
Lecture 3
This is a fundamental distance determination in
astronomy, relative to which distances to other
more distant objects are measured.
Prof. John Hearnshaw
For the Hyades the moving cluster method gives
mV – MV (distance modulus) = 3.25
Hence d = 44.3 pc.
ASTR112 The Galaxy
Lecture 3
1. Plot colour-magnitude diagram mV vs (B-V)
2. Compare with MV vs (B-V) for Hyades
(MV is known from moving cluster method)
3.Hence find mV – MV (distance modulus)
= 5 log d – 5 where d is disatnce in parsecs.
Prof. John Hearnshaw
Photometric distances for galactic clusters
ASTR112 The Galaxy
Lecture 3
Cluster
Hyades
Pleiades
Praesepe
Sco-Cen
M67
h Persei
χ Persei
distance
44 pc
127 pc
159 pc
170 pc
830 pc
2250 pc
2400 pc
Prof. John Hearnshaw
Distances of some well-known clusters
ASTR112 The Galaxy
Lecture 3
Ages of clusters
Mass (M⊙)
15
5
3
1
M-S lifetime (yr)
10 × 106
66 × 106
22 × 107
10 × 109
Prof. John Hearnshaw
Lifetimes of main-sequence stars depend strongly
on mass.
ASTR112 The Galaxy
Lecture 3
The cluster turn-off point
• This is the (B-V) colour index of bluest mainsequence stars, and corresponds to most massive
stars still on M-S (core hydrogen-burning stage).
Prof. John Hearnshaw
• Turn-off goes to redder M-S stars as cluster ages.
ASTR112 The Galaxy
Lecture 3
Cluster
age (yr)
h and χ Persei
Pleiades
Praesepe
Hyades
M67
NGC188
3 × 106
5 × 106
4 × 108
5 × 108
5 × 109
8 × 109
Prof. John Hearnshaw
Ages of some well-known galactic clusters
ASTR112 The Galaxy
Lecture 3
Prof. John Hearnshaw
End of lecture 3