astr112 the galaxy lecture 3 prof. john hearnshaw 5. stellar populations 6. galactic (open) clusters...

ASTR112 The GalaxyLecture 3

Pro

f. J

ohn

Hea

rnsh

aw

5. Stellar populations6. Galactic (open) clusters

The

Ple

iade

s op

en c

lust

er


Pro

f. J

ohn

Hea

rnsh

aw

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


Pro

f. J

ohn

Hea

rnsh

aw

Stellar populations

l: Andromeda galaxy, M31above: Walter Baade


Pro

f. J

ohn

Hea

rnsh

aw

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.


Pro

f. J

ohn

Hea

rnsh

aw

Stellar populations


Pro

f. J

ohn

Hea

rnsh

aw


Pro

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)


Pro

f. J

ohn

Hea

rnsh

aw

The Pleiades star cluster


Pro

f. J

ohn

Hea

rnsh

aw

Some open clusterstop l: double cluster h and χ Pertop centre: the Hyadestop r: Messier 67l: κ Crucis, the Jewel Box cluster


Pro

f. J

ohn

Hea

rnsh

aw

Young open clusters and OB associations in the galactic planeare found mainlyin the spiral arms


Pro

f. J

ohn

Hea

rnsh

aw

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


Pro

f. J

ohn

Hea

rnsh

aw

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


Pro

f. J

ohn

Hea

rnsh

aw

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)


Pro

f. J

ohn

Hea

rnsh

aw

Colour-magnitude diagrams for the Pleiades and Praesepe


Pro

f. J

ohn

Hea

rnsh

aw

Colour-magnitude diagrams for the Hyades and NGC188


Pro

f. J

ohn

Hea

rnsh

aw

Messier 67 colour-magnitude diagram


Pro

f. J

ohn

Hea

rnsh

aw

Composite HR diagram for open star clusters


Pro

f. J

ohn

Hea

rnsh

aw

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.


Pro

f. J

ohn

Hea

rnsh

aw

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.


Pro

f. J

ohn

Hea

rnsh

aw

Proper motions ofstars in the Hyadescluster, showingthe convergent pointlocated in the skybut several degreesaway from the cluster itself.


Pro

f. J

ohn

Hea

rnsh

aw

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


Pro

f. J

ohn

Hea

rnsh

aw

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)


Pro

f. J

ohn

Hea

rnsh

aw

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.


Pro

f. J

ohn

Hea

rnsh

aw

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.


Pro

f. J

ohn

Hea

rnsh

aw

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


Pro

f. J

ohn

Hea

rnsh

aw

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


Pro

f. J

ohn

Hea

rnsh

aw

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.


Pro

f. J

ohn

Hea

rnsh

aw

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


Pro

f. J

ohn

Hea

rnsh

aw

End of lecture 3End of lecture 3

astr112 the galaxy lecture 3 prof. john hearnshaw 5. stellar populations 6. galactic (open) clusters...

Documents

galaxy lecture

john hearnshaw slide

john hearnshaw messier

john hearnshaw baades

john hearnshaw features

andromeda galaxy

praesepe slide

imf slide