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) . Distance=50 kpc. Pic: http://antwrp.gsfc.nasa.gov/apod/ap010804.html M31: Distance=730 kpc, Pic: http://antwrp.gsfc.nasa.gov/apod/ap021021.html

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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) . Distance=50 kpc. Pic:http://antwrp.gsfc.nasa.gov/apod/ap010804.html

  • M31: Distance=730 kpc, Pic:http://antwrp.gsfc.nasa.gov/apod/ap021021.html

  • M33: Distance = 670 kpc, Pic: http://antwrp.gsfc.nasa.gov/apod/ap030924.html


These are the closest galaxies

Question: How do we know their distances?


Our Galactic Neighborhood- The Local Group

Galaxies come in clusters, too

A new unit, the Megaparsec


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


Types of Galaxies

Spirals, Ellipticals, Irregulars


The Larger Neighborhood


The Virgo Cluster

  • Distance ~ 20 Megaparsecs

  • > 1000 galaxies

  • Largest are giant ellipticals like M87


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


The Abell Clusters


Now the question: how can you know the distances to these objects?

You can’t see Cepheids that far out


Turn to one of Hubble’s greatest discoveries

  • He discovered galaxies as “island universes”

  • Measured distances to galaxies

  • Measured velocities (toward us or away)

  • How?


Illustration of a galactic redshift


The Hubble Relation for Galaxies

  • Galaxies in all directions receding from us

  • The more distant they are, the faster they are receding

  • v=H0d

  • v = speed of recession (km/sec), d= distance (Mpc), H0 “Hubble Constant”, (km/sec/Mpc), H0 = 70 km/sec/Mpc


The Hubble Relation for Galaxies

If you measure the redshift, you know the distance


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


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