Lecture 34 Galaxies and the Universe

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Lecture 34 Galaxies and the Universe

• Repeatedly said there are other galaxies. Now
  details
• Large Magellanic Cloud (satellite to Milky Way) .
  Distance50 kpc. Pichttp//antwrp.gsfc.nasa.gov/
  apod/ap010804.html
• M31 Distance730 kpc, Pichttp//antwrp.gsfc.nasa
  .gov/apod/ap021021.html
• M33 Distance 670 kpc, Pic http//antwrp.gsfc.n
  asa.gov/apod/ap030924.html

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These are the closest galaxies

• Question How do we know their distances?

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Our Galactic Neighborhood- The Local Group

• Galaxies come in clusters, too
• A new unit, the Megaparsec

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Beyond the Local Group

• Other small groups of galaxies
• Typical spacing of a few Megaparsecs
• Our neighbors include some of the brightest
  binocular galaxies
• See Appendix 15 for list

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Types of Galaxies

• Spirals, Ellipticals, Irregulars

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The Larger Neighborhood
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The Virgo Cluster

• Distance 20 Megaparsecs
• gt 1000 galaxies
• Largest are giant ellipticals like M87

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Not the end of the story the Abell clusters

• Published in 1958
• 2718 Rich Clusters like the Virgo cluster
• Most distant 2 billion light years

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The Abell Clusters
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Now the question how can you know the distances
to these objects?

• You cant see Cepheids that far out

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Turn to one of Hubbles greatest discoveries

• He discovered galaxies as island universes
• Measured distances to galaxies
• Measured velocities (toward us or away)
• How?

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Illustration of a galactic redshift
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The Hubble Relation for Galaxies

• Galaxies in all directions receding from us
• The more distant they are, the faster they are
  receding
• vH0d
• v speed of recession (km/sec), d distance
  (Mpc), H0 Hubble Constant, (km/sec/Mpc), H0
  70 km/sec/Mpc

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The Hubble Relation for Galaxies

• If you measure the redshift, you know the distance

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With measurements of distances, you can map out
the distribution of galaxies