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The Universe of Galaxies
A Brief History
• 1610 - Galileo
• 1610 - Galileo
resolves the Milky
Way into stars
• 1784 – Hershel maps
the Milky Way
• 1784 – Hershel maps
the Milky Way
• 1912 – Leavitt
discovers the periodluminosity relation for
Cepheid variable
stars.
• This eventually
became the key to
measuring distances
to galaxies
• 1917 – Shapley
measures the Galaxy
using globular star
clusters..
Using star clusters
• Instead of 1000 ly
across, it is 100,000
ly and the sun is not
at the center.
The Galactic system
The First Extragalactic Nebula
The Distance to NGC 6822
• 1925 Hubble
measures the
distance to NGC 6822
and finds that it is a
separate, distant
galaxy, 500,000 ly
away (the correct
distance is 2 million
ly)
M31
The Hubble Deep Field
Types of galaxies
Ellipticals
• Elliptical in shape
• Smooth light
distribution – no
structure
• Mostly old stars (red)
• Predominate in large
clusters
Ellipticals
• Elliptical in shape
• Smooth light
distribution – no
structure
• Mostly old stars (red)
• Predominate in large
clusters
A cluster of elliptical galaxies
Spiral galaxies
• Sa galaxies
– Large central bulge
– Tightly wound arms
– Arms smooth, dusty
Spiral galaxies
• Sb galaxies
– smaller central bulge
– Less tightly wound
arms
– Arms blue, patchy
Spiral galaxies
• Sc galaxies
– small central bulge
– Loosely wound arms
– Arms blue, very patchy
Barred spiral galaxies
• SB galaxies are
divided into SBa,
SBb, SBc, with similar
characteristics to
regular spirals, except
for a centrallyoriented bar
Barred spiral galaxies
• SB galaxies are
divided into SBa,
SBb, SBc, with similar
characteristics to
regular spirals, except
for a centrallyoriented bar
Barred spiral galaxies
• SB galaxies are
divided into SBa,
SBb, SBc, with similar
characteristics to
regular spirals, except
for a centrallyoriented bar
Irregular galaxies
• Irr galaxies
– No spiral structure
– Blue, many young
stars
– Patchy light
distribution
Irregular galaxies
Dwarf galaxies
• Dwarf galaxies
– Low surface
brightness
– Less than 5000 ly
across
– Some are irregular,
some are elliptical
– Various star formation
histories
NGC 4414
Interacting galaxies
• Merging galaxies are
found in the nearby
universe and may
have been very
common in the early
universe.
• This is NGC 520
• “The Mice”
Centaurus A
Cen A in radio
Cen A in x-rays
Milky Way mergers
• The MW galaxy is
tidally interacting with
two small galaxies
(the MCs) and
recently has
swallowed two dwarf
galaxies.
• This is a map of the
Sagittarius dwarf,
which is falling into
the MW.
Milky Way mergers
Galaxies with active galactic nuclei
AGNs
An AGN and a normal galaxy
Seyfert galaxies
The extreme case: quasars
3C273, the first quasar
Identified by Schmidt in 1961
Quasars are at cosmological
distances
Studied in detail by Burbidge and
others
HST showed that quasars are the
nuclei of galaxies
The Local Group
MCs, Sculptor, Fornax, Ursa Minor, Draco,
Sagittarius are too close to be identified here
Spiral Members
• The Milky Way
MWG
•
•
•
•
Type: Sbc
Absolute magnitude: -21.0
Diameter: 40 kpc
Disk Thickness: ~200 pc, depends on
spectral type of stars; thin for massive
stars, thick for old, low mass stars
• Mass: 2 x 1011 solar masses
M31, the Andromeda Galaxy
M31 = NGC 224
•
•
•
•
•
Type: Sb
Asolute magnitude: -21.8
Diameter: 50 kpc +
Active star formation, especially at ~10 kpc
2 close but separate companions: M32&NGC
205
• Evidence for past mergers
• Mass: 4 x 1011 solar masses
M31, M32 and NGC 205
M33
M33 = NGC 598
•
•
•
•
•
•
Type: Sc
Absolute magnitude: -18.87
Tilted by 57 degrees
High rate of star formation
Mass: 1.1 x1010 solar masses
Mass of gas: 2.6 x 109 solar masses
NGC 604 in M33
The Magellanic Clouds
LMC
The Magellanic Clouds
SMC
The Magellanic Clouds
•
•
•
•
Type: Irregular
Absolute magnitudes: -18.7, -17.8
Orbiting MWG
Bursts of star formation
NGC 6822
WLM
Phoenix
Leo II
Sculptor
Sextans I