The University Record, May 21, 1996

Galaxy cluster can help to determine size, age of universe

By Sally Pobojewski
News and Information Services

If you look high up in the southern sky on May evenings, you are actually looking out from our Milky Way Galaxy into the remove universe, according to U-M astronomer Richard Teske. Far beyond the constellation of Virgo, the Reclining Maiden, lies a great cluster of galaxies that only can be seen with medium- or large-sized amateur telescopes. This cluster of 3,000 galaxies is a focal point of astronomers' efforts to learn the true size and age of the universe.

Galaxy clusters are made up of large numbers of individual galaxies, which are themselves huge aggregations of tens of billions of stars. Some galaxies take the shape of huge spirals, others are football-shaped, while some are ragged and without any recognizable structure.

"Galaxies like to stay near other galaxies, and are seldom found alone," Teske says. "This tendency to form groups or clusters is an important clue to their history."

Before galaxies formed, the universe was filled with immense clouds of gas. These clouds split or fragmented into smaller galaxy-sized clouds. Stars condensed out of gases within the fragments, generating the galaxies we see today. The process gave rise to groups of galaxies, each cluster a descendant of one of the original, huge clouds. Galaxies in a cluster have remained with their original companions since they formed---their mutual gravitational pull upon one another keeping them from parting company as the universe expands. "Galaxy clusters are often regarded as the true 'units' of the universe's structure---the real bricks of the building," Teske explains.

The Hubble Space Telescope is now being used in a project designed to improve methods of distance measurement. The Virgo Cluster supplies a key reference point in the project, according to Teske.

"The central difficulty is knowing the real distance of one of the objects," Teske says. "This is the problem the Hubble Space Telescope will address through observations of the Virgo Cluster."

Astronomers are using the Hubble to pin down the distance to the Virgo Cluster with measurements of brightnesses of variable stars and other objects in the galaxies there. When the different methods reliably give the same distance to Virgo, they' ll be used to determine distances to even more remote galaxy clusters.