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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 period-luminosity 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 centrally-oriented bar
Barred spiral galaxies
• SB galaxies are divided into SBa, SBb, SBc, with similar characteristics to regular spirals, except for a centrally-oriented bar
Barred spiral galaxies
• SB galaxies are divided into SBa, SBb, SBc, with similar characteristics to regular spirals, except for a centrally-oriented 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 nucleiAGNs
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