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The Nature of Galaxies
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Chapter 17
Elliptical
Other Galaxies
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• External to Milky Way
– established by Edwin Hubble
– used Cepheid variables to measure distance
• M31 (Andromeda Galaxy) far outside Milky Way
Spiral
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• Three basic types:
– elliptical
– spiral
– irregular
Irregular
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Spiral Galaxies
• Similar to Milky Way:
– thin disk + nuclear bulge + halo
– Disk contains:
• dust and gas
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– H II regions, H I regions,
molecular clouds
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spiral arms
active star formation
open clusters
mixture of young & old stars
– Halo contains:
• old stars
• Globular Clusters
– Bulge contains:
• old stars
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Spiral Galaxies
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Barred Spiral Galaxies
• Some spirals have bar in center
– “barred spirals”
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Elliptical Galaxies
• Shape ranges from:
– spherical to ellipsoidal
• Characteristics:
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– no disk or spiral arms
– old reddish stars
• similar to halo or bulge in spirals
– little gas or dust
– little star formation
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Irregular Galaxies
• No specific shape
– often appear chaotic
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• Often have intense star formation
– gravitational interaction with other
galaxies?
• Mixture of old and new stars
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Interacting
Galaxies
Cartwheel Galaxy
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NASA/HST
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Simulation by C. Mihos et al., CWRU
Galaxy Masses
• For spirals:
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– use Doppler shift; measure galaxy rotation
– make rotation curve
– calculate mass using Kepler’s Law
• For ellipticals:
– use Doppler shift; measure stellar orbital
velocities
– calculate mass using Kepler’s Law
• Results
– Giant ellipticals and spirals are most
massive;
– irregulars & dwarf ellipticals least
massive
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Mass-to-Light Ratios
• ratio of mass to luminosity
– for Sun,
• M/L = 1
– average star
• M/L = 2 to 3
– for entire Galaxy
• M/L ~ 100
• 90% of galaxy mass is unseen
Extragalactic Distances
• Compare apparent and absolute brightness
• Variable Stars:
– Cepheids, RR Lyrae
• Standard Candles:
– brightest stars, supernovae, planetary nebulae
• Galaxy techniques:
– For spirals:
• rotation rate gives mass
• mass depends on number of stars, hence luminosity
– For ellipticals:
• range of stellar velocities depends on mass (hence luminosity)
Fifteen years ago, a quasar was observed that
was found to be located 8 billion light years
away. If our universe is approximately
15 billion years old, when did the quasar emit
the light that we observe?
A. 15 years ago
B. 7 billion years ago
C. 8 billion years ago
D. 15 billion years ago
Galaxy Motion
• Galaxy spectra:
– absorption lines redshifted
– more distant galaxies have
larger redshift
• ALL galaxies moving away
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The Hubble Law
• Hubble Law:
– The more distant the galaxy, the faster it is moving away.
v=Hd
(H is the Hubble constant, d is distance)
Implications of Hubble Law
• Every galaxy moving away
• Farther away = faster
• Conclusion:
– Universe is expanding
• Predicted by Einstein’s Theory of Relativity
• Are we at center? NO
– universe same in all directions
– there is no center!
The Expanding Universe
• A uniformly expanding universe
– explains Hubble law
• example: expanding loaf of raisin bread
• Galaxies (like raisins) not moving, not expanding
• Space is expanding