The Nature of Galaxies Chapter 17. Other Galaxies External to Milky Way –established by Edwin...
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Transcript of The Nature of Galaxies Chapter 17. Other Galaxies External to Milky Way –established by Edwin...
The Nature of Galaxies
Chapter 17
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Other Galaxies
• External to Milky Way– established by Edwin Hubble– used Cepheid variables to measure distance
• M31 (Andromeda Galaxy) far outside Milky Way
• Three basic types:– elliptical– spiral– irregular
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Elliptical
Spiral
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Irregular
Spiral Galaxies• Similar to Milky Way:
– thin disk + nuclear bulge + halo
– Disk contains:• dust and gas
– H II regions, H I regions, molecular clouds
• 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”
Elliptical Galaxies
• Shape ranges from:– spherical to ellipsoidal
• Characteristics:– 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
• Often have intense star formation– gravitational interaction with other
galaxies?
• Mixture of old and new stars
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Interacting Galaxies
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Cartwheel Galaxy
Simulation by C. Mihos et al., CWRU
NASA/HST
Galaxy Masses
• For spirals:– 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 agoB. 7 billion years agoC. 8 billion years agoD. 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 = H d (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