Stellar Populations in the Galaxy
Stars are fish in the sea of the galaxy, and like fish they often travel in schools. Star
clusters are relatively small groupings, the true “schools” are stellar populations.
Populations often correlate with specific components or structures of the galaxy.
Open Cluster NGC 6520 from CFHT Credit & Copyright: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT
Explanation: Did you ever have a day when it felt like a dark cloud was following you around? For the open cluster of
stars NGC 6520, every day is like this. On the left of the above picture are many of NGC 6520's bright blue stars. They
formed only millions of years ago - much more recently than our ancient Sun which formed billions of years ago. On
the right is an absorption nebula, molecular cloud Barnard 86, from which the stars of NGC 6520 surely formed. This
nebula contains much opaque dust that blocks light from the many stars that would have been visible in the
background. Surrounding NGC 6520 is part of the tremendously dense starscape in the bulge of our Milky Way
Galaxy, the extended halo of stars that surrounds the center of our Galaxy. NGC 6520 spans about 10 light years and
lies about 5500 light years away toward the direction of Sagittarius.
The most basic populations
Characteristic
Population I
II
Age:
Young-intermediate
Old
(≤ 3 x 109 yr.)
(≥ 5 x 109)
Color (Sp type):
Blue (type O-M)
Red (G-M)
Heavy Elements:
≥ 2%
≤ 1%
Preferred Habitat:
Galactic disks,
Galactic bulges and halos
especially spiral arms
Social Groupings:
Young, open clusters,
Globular clusters or alone.
young stellar associations,
travel with gas & dust clouds.
Reproductive Rate:
~ 3 Msun/yr typically
0
(but can be much more
(but not endangered.)
prolific in short bursts!)
Refined Stellar Populations in the Milky Way
Population
Characteristics
Thin disk:
= Baade’s Pop. I (young to intermediate). Near circular,
high angular momentum orbits. High metallicity,
well-defined age-metallicity relationship in solar
neighborhood. Product of extended infall?
Thick disk:
Old, ≈ 12 Gyr. Intermediate metallicity ([Fe/H] ≈ -0.6,
with significant spread. Distinct chemical enrichment
history. Product of minor merger?
Central bulge:
Most stars in the universe are probably in bulges. Old and
metal-rich, with broad spread. Probably enriched rapidly
by SNII, though need more data. High “phase space”
density.
Stellar halo:
Baade’s Pop. II. Old and metal-poor. Element ratios like
those produced in SNII, indicating short formation
timescale. Small scatter in abundance ratios indicates
efficient mixing. Low angular momentum like bulge.
Formation probably connected to bulge pop., though only
1/10 the star mass.
Outer halo:
Very little mass, much of it in ‘streams’ from accreting
dwarf galaxies.
Pop. III?
The first generation(s)? A few very low metallicity stars
have been found.
Basic Milky Way Properties
Property
Spatial dimension
Disk
2
Spheroid
3
Size
20 kpc ≈ 6 x 1020 m
≤ 100 kpc (bulge smaller)
Kinematics
differential rotation
“random” orbits
≈ 250 km/s
≈ 250 km/s
Pop. I, including:
Pop. II, including:
Young associations
Field halo stars,
& clusters, spiral arm
nuclear bulge, globular
pops., intermediate age
clusters (130), dwarf
pops. (≤ 1010 yrs)
spheroidals (~ 10)
Stellar Populations
Milky Way Properties continued
Property
Disk
Spheroid
Metallicity
Z ≈ Zsun
ISM
Gas & dust clouds,
Few clouds,
star-forming complexes,
hot corona.
Z ≈ (0.1 - 0.01) Zsun
warm edges, bubbles.
% Dark matter
≤ 50%
≈ 90%
The Milky Way's Center Credit: DIRBE, COBE, NASA
Explanation: Although the Earth is round, our Galaxy appears truly flat. This was shown in dramatic fashion by the
COsmic Background Explorer (COBE) satellite which produced this premier view of the central region of our own
Milky Way Galaxy in infrared light in1990. The Milky Way is a typical spiral galaxy with a central bulge and
extended disk of stars. However, gas and dust within the disk obscure visible wavelengths of light effectively
preventing clear observations of the center. Since infrared wavelengths are less affected by the obscuring material, the
Diffuse InfraRed Background Experiment (DIRBE) on board COBE was able to detected infrared light from stars
surrounding the Galactic center and produce this image. Of course, the edge on perspective represents the view from
the vicinity of our Sun, a star located in the disk about 30,000 light years out from the center.