5. Stellar populations

1
5. Stellar populations 6. Galactic (open) clusters
The Pleiades open cluster
2
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
3
Stellar populations
l Andromeda galaxy, M31 above Walter Baade
4
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.
5
Stellar populations
6
(No Transcript)
7
(No Transcript)
8

• 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
  gt10º
• 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)

9
The Pleiades star cluster
10
Some open clusters top l double cluster h and ?
Per top centre the Hyades top r Messier 67 l ?
Crucis, the Jewel Box cluster
11
Young open clusters and OB associations in the
galactic plane are found mainly in the spiral
arms
12
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
13

• Most famous young Popn I clusters include
• the Pleiades
• the Hyades
• h and ? Persei (the double cluster)
• Praesepe
• ? Crucis (the Jewel Box)
• Old Popn I clusters are much rarer and include
• M67
• NGC188

14

• Galactic cluster Hertzsprung-Russell diagrams
• This is a plot of magnitude as a function of
• colour index (often (BV)) or of spectral
  type.
• If C.I. is used, then also known as a colour-
• magnitude diagram (CMD)

15
Colour-magnitude diagrams for the Pleiades and
Praesepe
16
Colour-magnitude diagrams for the Hyades and
NGC188
17
Messier 67 colour-magnitude diagram
18
Composite HR diagram for open star clusters
19

• 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.

20
Moving cluster distances Fundamental distance
method applicable to Hyades (the nearest
cluster) and Sco-Cen association 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.
21
Proper motions of stars in the Hyades cluster,
showing the convergent point located in the
sky but several degrees away from the cluster
itself.
22

• Measure
• ?, 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

VR
µ
?
star
V
?
Earth
to convergent point
23
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)
24
For the Hyades the moving cluster method gives mV
MV (distance modulus) 3.25 Hence d 44.3
pc. This is a fundamental distance determination
in astronomy, relative to which distances to
other more distant objects are measured.
25

• Photometric distances for galactic clusters
• Plot colour-magnitude diagram mV vs (B-V)
• Compare with MV vs (B-V) for Hyades
• (MV is known from moving cluster method)
• Hence find mV MV (distance modulus)
• 5 log d 5 where d is disatnce in parsecs.

26
Distances of some well-known clusters Cluster
distance Hyades 44 pc Pleiades 127
pc Praesepe 159 pc Sco-Cen 170 pc M67 830
pc h Persei 2250 pc ? Persei
2400 pc
27
Ages of clusters Lifetimes of main-sequence
stars depend strongly on mass.
Mass (M?) M-S lifetime (yr) 15
10 106 5 66
106 3 22 107 1
10 109
28

• The cluster turn-off point
• This is the (B-V) colour index of bluest main-
• sequence stars, and corresponds to most
  massive
• stars still on M-S (core hydrogen-burning
  stage).
• Turn-off goes to redder M-S stars as cluster
  ages.

29
Ages of some well-known galactic
clusters Cluster
age (yr) h and ? Persei 3
106 Pleiades 5
106 Praesepe 4
108 Hyades 5
108 M67 5
109 NGC188 8 109
30
End of lecture 3