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