Other stellar types
• Some clusters have hotter stars than we find in the solar neighbourhood -- O, B, A stars -- as well as F stars, and cooler stars (G, K, M)
• Hence we can establish intrinsic values (MV, (B-V)0 etc) for stars of different types (O to M) and luminosity classes (I, II, III, IV, V)
• This is done empirically, i.e direct from observation and independently of theoretical models.
AS 1001 The Galaxy
Open and globular clusters: chemical compositions • Basic assumption of MS fitting technique is that L-Teff relationship for MS stars is same for stars in clusters as in solar neighbourhood (s-n). • Stellar structure theory says this is correct provided the stars have the same chemical composition. • For s-n stars and most open clusters, this is found to be true from detailed spectroscopic studies, but globular-cluster stars are different. • For many studies in astronomy, sufficient to consider 3 main chemical constituents in stars: Constituent: Hydrogen Helium “Metals” (everything else) % by mass: X Y Z s-n and open clusters: 0.73 0.25 0.02 Globular clusters: 0.75 0.25 0.001 to 0.0001 AS 1001 The Galaxy
Page ‹#› Open and globular clusters: spatial distribution • With distances to open and globular clusters from MS fitting, via empirical relationships and theoretical models, and observed directions in sky, we determine their 3-D distribution. • Open clusters: – Together with solar neighbourhood, all in highly flattened disc system, the Milky Way – more than 1000 clusters known.
• Globular clusters: – In Galactic halo : spheroid with centre at centre of Galaxy. – about 130 clusters known – most numerous within 3 kpc of Galactic Centre, but – many found ~ 10 kpc from centre.
AS 1001 The Galaxy
Stellar populations
Halo Globular Open clusters clusters Milky Way plane
• Hence we find two broad populations of stars in the Galaxy: – Population I -- disc stars, Z ~ 0.02: “normal” – Population II -- halo stars, Z < 0.001: “metal-poor”
AS 1001 The Galaxy
Page ‹#› The composite colour-magnitude diagram
• CM diagrams: used for MS fitting and hence distances to clusters.
• Can now make a composite CM diagram [(B-V)0 vs. MV] for a number of clusters of known distance. • The left-hand envelope of this sequence of open- cluster main sequences defines the zero-age main sequence (ZAMS) for population I stars. – NB progressive sequence of cluster “turn-off points” as one moves down the main sequence -- direct result of the age of each cluster. – Note also that CM diagram for old open cluster M67 is similar to that of the globular M3 -- turnoff point, subgiants/giants. • Hence need to consider stellar evolution -- the ageing of stars -- how they change their characteristics with time.
AS 1001 The Galaxy
O -4 ZAMS: zero-age MV B -2 main sequence
0 A
2 F 4
6 G
K
M
-0.4 0 0.4 0.8 1.2 1.6 (B-V)0
AS 1001 The Galaxy
Page ‹#› H-R diagrams for clusters
Hertzsprung gap young
L
old
Age:
temperature
AS 1001 The Galaxy
Main-Sequence Turnoffs: Ages
• Very luminous, early type stars (O & B) live shorter lives than late type stars (G & K) • Turnoff used to determine age of cluster • If turnoff occurs around B stars, then cluster is around 10 million years old • For Pleiades, turnoff occurs around A stars, so age is around 100 million years • Globular clusters, turnoff is at much later spectral types, giving ages of around 1010 years
AS 1001 The Galaxy
Page ‹#› 3 Pleiades Turnoff ~ A stars V Age ~ 108 yr 5
7
9
11
13
-0.4 0 0.4 0.8 1.2 1.6 B-V
AS 1001 The Galaxy
2 V Turnoff ~ Late A 4 Age ~ 109 yr 6 Praesepe 8
10
12
-0.4 0 0.4 0.8 1.2 1.6 B-V
AS 1001 The Galaxy
Page ‹#› V 6 Turnoff ~ F stars 8 Age ~ 1010 yr 10 M 67 12
14
16
-0.4 0 0.4 0.8 1.2 1.6 B-V
AS 1001 The Galaxy
O 11 V B Turnoff ~ F stars 13 Age ~ 1010 yr 15 A Glob. cluster 17 M 5 F 19
21 G
K
M
-0.4 0 0.4 0.8 1.2 1.6 B-V
AS 1001 The Galaxy
Page ‹#› The evolution of stars: an outline • Star formation: – Occurs in molecular clouds in interstellar medium -- regions of cold gas with T ~ 10 K to 100 K – Random motions of molecules are slow [v = (3kT/m)1/2] – Denser parts of cloud can contract gravitationally into clumps that form protostars – Collapse from inside-out. Initial protostellar core contains a fraction of the total mass of the collapsing clump – Protostar grows by accreting the rest of the infalling gas.
– Protostars are large (many R) which contract, converting gravitational PE into thermal energy.
– 1 M protostar has radius ~ 10R, T~3000 K by this stage
AS 1001 The Galaxy
Star and Planet formation
• Protostellar discs – Collapsing gas cloud has some angular momentum – Conservation of angular momentum makes collapsing gas spin up j = v x r – Bulk of matter forms a disc around protostar – Accretes over 105 to 106 years
Residual disc not accreted by star Forms reservoir for planet formation Our solar system is a disc Typical disc sizes ~ few hundred AU Dark absorption lane Starlight scattered off of disc, starlight upper and lower scattered in envelope surfaces of disc
AS 1001 Images from Hubble Space Telescope The Galaxy
Page ‹#› Later stages of star formation
• Young star is big (10R) and fairly hot (T ~ 3000 K) – high luminosity – located above main sequence • Thermal energy radiated into space, allowing contraction to continue – young star descends (in Lumonisity) towards main-sequence. • Contraction continues: 4 7 – 10 years for 15 M, 10 years for 1 M – until central core temperature reached ~ 106 K at pressures ~ 108 bar, when the first thermonuclear fusion reactions can take place. • Star settles to equilibrium state, on the zero-age main sequence, with fusion of H to He providing the energy source and hence the gas pressure to counteract the gravitational contraction.
AS 1001 The Galaxy
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