Hubble Spots Young Globular Star Clusters

The globular clusters are only a few hundred million years old.

By Sam Flamsteed|Monday, June 01, 1992
Who knows what violence lurks in the hearts of galaxies? The Hubble Space Telescope, apparently. Consider the case of NGC 1275, about 200 million light-years away in the constellation Perseus and an astronomical deviant if ever there was one. Its shape is peculiar, neither spiral nor elliptical; it is unusually bright for its size; hot gas is streaming into its core; and energetic particles, in turn, are jetting out of the core into intergalactic space. Now, thanks to the Hubble and a group led by Jon Holtzman of the Lowell Observatory in Flagstaff, Arizona, astronomers can add another striking feature to NGC 1275, one that helps explain how the galaxy acquired its disparate pieces--and one that also hints at unimaginable violence in the past of our own placid galaxy, the Milky Way.

What we found, says Holtzman, was a group of bright objects around the galaxy’s core that appear to be young globular star clusters. It was totally unexpected. The operative word here is young. Globular clusters are remarkable enough--each one consists of 100,000 stars or more stuffed in a space just 100 light-years across--but many galaxies, including the Milky Way, are peppered with the things. Invariably, though, the stars in the clusters shine with the low-energy, reddish light of extreme age. In fact, they are the oldest stars in the universe, as much as 16 billion years old, which is more than three times the age of our sun. Since in the Milky Way globular clusters also tend to be found outside the main disk of stars, many astronomers think they are fossils of the galaxy’s prehistory--the first stars to turn on when the vast spinning cloud of hydrogen began to collapse into a spiral disk.

The star clusters in NGC 1275, on the other hand, are in its core, and they are not red. They are uniformly bright blue. That means they are just a few hundred million years old, and they all formed at about the same time.

How could that be? Holtzman and his colleagues have an idea that fits in with earlier speculations about NGC 1275. They say it may be the tangled wreck of a collision between two other galaxies. Astronomers believe such violent encounters are not all that unusual. Indeed, computer simulations support the hypothesis that many elliptical galaxies were actually born from the merger of two or more spirals. In the simulations, the intermediate step before two individual galaxies settle into a single elliptical is an irregular blob of stars--like NGC 1275. Judging from its shape, one of the galaxies that collided to form it was already a large elliptical; the other may have been a small spiral.

This scenario would explain NGC 1275’s other peculiarities. In such a collision some of the gas in the small spiral would be ripped away, heating up and plunging toward the center of the large elliptical. If it found a massive black hole when it got there--a reasonable possibility, since astronomers believe black holes squat like voracious spiders in the cores of most galaxies--it could spew out a jet of particles as it spiraled in. Finally, the violent encounters of gas clouds in both galaxies might push many of the clouds simultaneously toward star-forming collapse, producing a fleet of new globular clusters in one fantastic burst.

That would neatly iron out a wrinkle in the merging-galaxies hypothesis. One of the major objections to the hypothesis, says Holtzman, is that large ellipticals tend to have many times the number of globular clusters that spirals do. So if the ellipticals were formed from spirals, where did the extra clusters come from? Now we may have an answer: they could have come from the collision itself.

What’s more, if colliding galaxies really do throw off globular clusters like sparks--and Holtzman is quick to stress that the idea is still speculative--that would reflect strangely on our own galaxy. No one really knows exactly how the Milky Way got its globular clusters, but it seems reasonable to suppose, given what extraordinary objects they are, that the universe has only one method of producing them. If the clusters in NGC 1275 did emerge from a collision, it would suggest that the Milky Way, for all its present majesty, suffered a wild accident in its youth.
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