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

Spirals, Ellipticals, Irregulars 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