Open Clusters

Week of October 11, 1999

Two weeks ago we covered constellations, groups of usually unrelated stars in the sky, and last week we did associations, loosely bound groups of stars that, if nearby, can be as large as constellations. The next step up are true clusters, called open clusters.

Open clusters (sometimes misleadingly called "galactic clusters") are groups of stars ranging in size from a few dozen stars up to tens of thousands. Almost all open clusters we see in the sky are relatively young. The youngest is about 1 or 2 million years old, while the oldest is about 6 or 7 billion years, making it just older than our own solar system. Most are younger than 100 million years old. We can tell the ages of the clusters by studying the stars in them. Stars evolve, and astronomers have a pretty good handle on just how fast stars age. So by looking at the stars in the cluster we can tell pretty well how old they are.

Clusters of stars probably all form from the same collapsing gas cloud, much like how we believe the solar system formed. Imagine how big the cloud must be to form hundreds or thousands of stars! Once they form from the gas, the hot, bright stars burn and blow away the remaining gas in a process that, to be honest, is still not very well understood.

What happens to an old cluster? It dissipates! Over time, the stars in the cluster pass near each other, and affect each other gravitationally. Some stars can slingshot out of the cluster that way. The biggest and hottest stars tend to blow up before they get too old, and we no longer can detect them easily. Also, the Galaxy itself has a gravitational field, and complex interactions with the cluster can strip stars away that way. Many stars in the Galaxy formed in clusters and were ejected in one way or another.

You've probably seen a cluster without even knowing it. The constellation Taurus is usually recognizable because of a "V" shaped group of stars that is supposed to represent the bull's head and horns. Those stars (with the exception of the brightest one) are actually a cluster, called the Hyades (pronounced hi aye deez). It's the nearest cluster to the Earth at a distance of roughly 40 parsecs (130 light years). Near the Hyades in the sky but actually three times farther away is the cluster called the Pleaides (plee uh deez). Many people mistake them for the Little Dipper, but the cluster is much smaller. It's easily seen in November and December after sunset. Time exposures of it reveal that many of the stars are still embedded in the gas of dust of their birth nebula-- so the Pleaides must be very young indeed! Calculations show it to be about 50 million years old-- only 1 percent of our own Sun's age.