The University Record, September 18, 1995

Ancient star cluster visible in September skies

One of the most ancient objects visible with the naked eye—the great globular star cluster in the constellation of Hercules, the Hero—can be seen in Michigan skies this September, according to astronomer Richard Teske.

This star cluster formed shortly after the birth of the universe and was already eight to 10 billion years old by the time our sun and its planets were born. Astronomers study the Hercules star cluster and others like it to determine just how old the universe really is.

“Michigan skywatchers can find Hercules almost directly overhead just after dark,” Teske says. “The cluster lies about a third of the way between Hercules’ western shoulder and waist, where it appears as a fuzzy, faint ‘star.’ Choose a haze-free night and look for it from a dark location away from city lights and shopping malls.”

Seen with a small telescope, the Hercules star cluster appears about a third as large as the full moon—its hazy glow broken up into many tiny star-points of light. Astronomers estimate the cluster contains about one million stars crammed together into a globe-shaped area 100 light-years in diameter, Teske says. Despite the immense number of stars, the cluster looks faint and small, because it is about 23,000 light-years from Earth.

“Stars in clusters swarm together like a spherical horde of gnats on tranquil late summer evenings,” Teske says. “Around 130 globular star clusters have been discovered in our Milky Way galaxy. All are very distant from us.

“Imagine the flat, spiral Milky Way galaxy spread out on the floor of a two-car garage,” Teske explained. “It is about 80,000 light-years across. Each globular cluster would be the size of a pencil eraser and each star a microscopic dot.”

Globular star clusters have attracted astronomers’ attention because of their great age, which is uncertainly estimated to be around 13 to 15 million years. They are the oldest datable astronomical objects known, and astronomers cited their ages as evidence that the Universe must be more than 13 to 15 billion years old.

“This hypothesis received a nasty blow recently when the Hubble Space Telescope uncovered evidence that the Universe may only be 10 to 12 billion years old—making it younger than these star clusters,” Teske says.

Faced with a glaring discrepancy between the ages of star clusters and the Universe, astronomers are now studying globular clusters intensively. One group of scientists is trying to find double stars in the clusters—stars so close together they orbit around one another in much the same way as Earth orbits the sun. The ages of such paired stars can often be independently appraised by careful observation.

“Still waiting offstage in the scientific wings are attempts to make independent estimates of the age of the Universe. This is usually done by comparing the distances of far-off galaxies with the speed at which the Universe is expanding,” Teske explains. “Some scientists think the comparison has not been done correctly and that an incorrect age has been proposed. It could take years to find the solution to the problem.”