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Chapter 14: Contʼd
14.3 How did the Milky Way Galaxy Form?
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Halo: No ionization nebulae, no blue stars ⇒ no star formation
(and hence no recycling)
Disk: Ionization nebulae, blue stars ⇒ star formation
Milky Wayʼs star formation rate is about 1 MSun/yr.
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Halo Stars:
Halo stars
0.02-0.2% heavy elements (O, Fe…), formed first,
only old stars
then stopped.
Disk stars formed
Disk Stars:
later, & keep on
2% heavy elements,
forming.
stars
of
all
ages
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Our galaxy probably formed from a giant
gas cloud
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Halo stars formed first as gravity caused cloud to
contract
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Remaining gas settled into spinning disk due to conservation of angular momentum
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Stars continuously form in disk as galaxy
grows older
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The collapsing cloud model explains the age, chemical, and
orbital differences between halo and disk stars.
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More detailed studies  Halo stars formed in clumps that later merged (“galactic cannibalism”).
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What lies at the Center of our
Galaxy
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Galactic center in IR light
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Galactic center in IR light
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Galactic center in radio
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Galactic center in radio
Strange radio sources in
galactic center
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Strange radio sources in
galactic center
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Stars at galactic center
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Stellar Orbits
Use Newtonʼs version
of Keplerʼs 3rd law:
 mass = 3-4 106 MS
 In size of solar
system
black hole in center
of Milky Way
Recent evidence from
bright X-ray “flare”
emission
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End of Chapter 14
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Chapter 15
A Universe of Galaxies
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What is a galaxy?
•  An enormous collection of stars held together by their
common gravity.
•  Have a wide range of masses: from 100 million stars
(dwarf galaxies) to >1 trillion stars (giant galaxies).
•  Lower mass galaxies are more common.
•  Galaxies have a wide range of ages, stellar populations
(the mix of stars in a galaxy), and gas content.
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How far away are other galaxies? •  Light travels at a finite speed = 300,000 km/s.
Destination
Distance
Sun
8 light-minutes
Pluto
8 light-hours
Nearest star (α Centauri)
4 light-years
Center of our galaxy
30,000 light-years
Nearest large galaxy
2.5 million light-years
Nearest cluster of galaxies
50 million light-years
Most distant known galaxy
~12 billion light-years
•  The farther away we look in distance, the further back we
look in time.
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Hubble Ultra
Deep Field (10 days of
exposures)
Elliptical Galaxy
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Irregular Galaxies
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Spiral Galaxy
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Spiral Galaxy
halo
disk
bulge
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NGC 4414
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Barred Spiral Galaxy
NGC 1300 – 110, 000 light years in diameter
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Disk Component:
Stars of all ages,
& many gas clouds
Spheroidal Component
(halo & bulge): Old stars,
few gas clouds
NGC 4594 (The Sombrero Galaxy) – 82,000
light years across
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Disk
Component:
stars of all ages,
many gas clouds
Blue-white color
indicates ongoing
star formation
(massive stars)
Spheroidal
Component:
bulge & halo, old stars,
few gas clouds
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Red-yellow color
indicates older
star population
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Elliptical Galaxies
All spheroidal
component, virtually no
disk component.
Red-yellow color
indicates older star
population.
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Irregular Galaxies
The Large Magellanic Cloud – 30,000 lys across
Blue-white color indicates ongoing star formation
Irregulars are small
and much
lessLecture
massive
than spirals
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Role of Angular Momentum in
Galaxy Formation
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Recap: The 3 main types of galaxies
•  Spiral galaxies have
prominent disks and
spiral arms.
•  Elliptical galaxies are
rounder and redder than
spiral galaxies and
contain less cool gas
and dust. •  Irregular galaxies are
neither disk-like nor
rounded in appearance. 4/20/2009
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Hubbleʼs galaxy classification scheme
Spheroid
Dominates
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Disk
Dominates31
Spirals
Groups of galaxies
Spiral
galaxies
are often
found in
groups of
galaxies
(up to a few
dozen
galaxies)
Elliptical
(Edge-on view)
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Hickson Compact Group 87
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Elliptical
galaxies are
much more
common in
huge
clusters of
galaxies
(hundreds to
thousands of
galaxies)
Central part ofHabbal
galaxy
cluster Abell 1689
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