Download - The Nature of Galaxies

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


Elliptical

Spiral


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






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



Irregular Galaxies• No specific shape

– often appear chaotic

• Often have intense star formation– gravitational interaction with other

galaxies?

• Mixture of old and new stars




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



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


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