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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 28: Groups & Clusters of Galaxies

Readings: Ch 26, section 26-6

Key Ideas

Galaxies tend to gather into Groups and Clusters

The Milky Way is part of the Local Group

Hierarchy of Structure:
Groups: 3 to 30 bright galaxies
Clusters: 30 to 300+ bright galaxies
Superclusters: Clusters of Clusters
Voids, Filaments, & Walls

Groups & Clusters of Galaxies

Most galaxies are found in groups and clusters

Basic Properties:

About 3000 clusters have been cataloged to date.

The Local Group

Group of 39 galaxies including the Milky Way and Andromeda:

Virgo Cluster

Nearest sizable cluster to the Local Group

Relatively loose cluster, centered on two bright Ellipticals: M87 & M84


Image of the Virgo Cluster (83k) Credit: NOAO/AURA/NSF

Rich Clusters

Contain 1000's of galaxies: Image of the rich Coma Cluster (189k) Credit: NOAO/AURA/NSF


Clusters of Clusters


Superclusters are the largest coherent structures seen in the Universe.

Local Supercluster

Roughly centered on the Virgo Cluster


The Local Group is on the outskirts of the Local Supercluster, and falling into the Virgo Cluster at about 250 km/sec.

Voids, Filaments & Walls

The Universe looks foamy on large scales


Voids: Empty bubbles

Map of the Local Universe

The "Great Wall"

Found in a large-scale galaxy survey.

Sheet of superclusters:

Mass is ~2x1016 Msun

One of the largest structures known.


The existence of "Large Scale Structure" tells us something about how galaxies formed.

(Representative) Unanswered Questions:

Supplement: The Sloan Flyout Movie

In class, I showed a wonderful "fly-out" movie that used the Sloan Digital Sky Survey data. The movie was prepared by the Cosmus Project at the University of Chicago. The specific animation is available on this webpage:
Downloads from the SDSS/WMAP Universe
And look for the link under "Movies" labeled "Sloan Galaxies only". This is the 30Mb "straight flyout" movie I showed in class.

There are lots of other cool movies on the Cosmus site, but beware: the files are *big*.

Return to [ Unit 4 Index | Astronomy 162 Main Page ]
Updated: 2006 February 19
Copyright Richard W. Pogge, All Rights Reserved.