astr112 the galaxy lecture 3 prof. john hearnshaw 5. stellar populations 6. galactic (open) clusters...
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ASTR112 The GalaxyLecture 3
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5. Stellar populations6. Galactic (open) clusters
The
Ple
iade
s op
en c
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ASTR112 The GalaxyLecture 3
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Stellar populations
The concept of stellar populations was introducedby Walter Baade (German-American astronomer in California) in 1944 from observations of thespiral galaxy M31 in Andromeda.
Spiral arms: bluer stars – population INuclear bulge: redder stars – population IIThe halo stars are also assigned to population II
ASTR112 The GalaxyLecture 3
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Stellar populations
l: Andromeda galaxy, M31above: Walter Baade
ASTR112 The GalaxyLecture 3
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Baade’s 1944 colour-magnitude diagram of stars in M31 showing two populations. The brightest pop II stars are redder than the brightest pop I stars.
ASTR112 The GalaxyLecture 3
Pro
f. J
ohn
Hea
rnsh
aw
Stellar populations
ASTR112 The GalaxyLecture 3
Pro
f. J
ohn
Hea
rnsh
aw
ASTR112 The GalaxyLecture 3
Pro
f. J
ohn
Hea
rnsh
aw
ASTR112 The GalaxyLecture 3
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f. J
ohn
Hea
rnsh
aw
Galactic star clustersMain 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)
ASTR112 The GalaxyLecture 3
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The Pleiades star cluster
ASTR112 The GalaxyLecture 3
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Some open clusterstop l: double cluster h and χ Pertop centre: the Hyadestop r: Messier 67l: κ Crucis, the Jewel Box cluster
ASTR112 The GalaxyLecture 3
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Young open clusters and OB associations in the galactic planeare found mainlyin the spiral arms
ASTR112 The GalaxyLecture 3
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The galactic distributions of both early B-type fieldstars and of open clusters closely follow the Milky
Way and are only found close to the galactic equator
ASTR112 The GalaxyLecture 3
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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
ASTR112 The GalaxyLecture 3
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Galactic cluster Hertzsprung-Russell diagrams
• 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 colour- magnitude diagram (CMD)
ASTR112 The GalaxyLecture 3
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Colour-magnitude diagrams for the Pleiades and Praesepe
ASTR112 The GalaxyLecture 3
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Colour-magnitude diagrams for the Hyades and NGC188
ASTR112 The GalaxyLecture 3
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Messier 67 colour-magnitude diagram
ASTR112 The GalaxyLecture 3
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Composite HR diagram for open star clusters
ASTR112 The GalaxyLecture 3
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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.
ASTR112 The GalaxyLecture 3
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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 – theconvergent point.
ASTR112 The GalaxyLecture 3
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Proper motions ofstars in the Hyadescluster, showingthe convergent pointlocated in the skybut several degreesaway from the cluster itself.
ASTR112 The GalaxyLecture 3
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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
VRμ θ
θto convergent point Earth
star
V
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tan74.4
or
74.4tan
74.4But
tan
sin
cos
R
R
T
RT
T
R
Vp
Vd
dV
VV
VV
VV
The basic equations forthe 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 GalaxyLecture 3
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For the Hyades the moving cluster method givesmV – MV (distance modulus) = 3.25Hence d = 44.3 pc.
This is a fundamental distance determination in astronomy, relative to which distances to other more distant objects are measured.
ASTR112 The GalaxyLecture 3
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Photometric distances for galactic clusters
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.
ASTR112 The GalaxyLecture 3
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Distances of some well-known clusters
Cluster distanceHyades 44 pcPleiades 127 pcPraesepe 159 pcSco-Cen 170 pcM67 830 pch Persei 2250 pcχ Persei 2400 pc
ASTR112 The GalaxyLecture 3
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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
ASTR112 The GalaxyLecture 3
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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.
ASTR112 The GalaxyLecture 3
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Ages of some well-known galactic clusters
Cluster age (yr)
h and χ Persei 3 × 106
Pleiades 5 × 106
Praesepe 4 × 108
Hyades 5 × 108
M67 5 × 109
NGC188 8 × 109
ASTR112 The GalaxyLecture 3
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End of lecture 3End of lecture 3