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May 13, 2015

A globular cluster is an impressive sight — a tightly packed ball of hundreds of thousands of stars, held together by their mutual gravitational pull.

The finest globular in northern skies is M13, in the constellation Hercules. It’s a favorite target of amateur astronomers. But professional astronomers are just as likely to point their telescopes at another cluster in Hercules. That’s because M92 may well be the oldest star cluster in the entire galaxy.

One way astronomers know that is because the stars in a globular have much less iron relative to hydrogen than the Sun does. Hydrogen was formed in the Big Bang, so it’s the most abundant element in the universe. But iron and other heavy elements were forged inside stars, then expelled into space when the stars died, enriching the clouds that gave birth to new stars. So any stars with low abundances of iron must have formed quite early, before there was much iron for them to incorporate.

Even for a globular cluster, though, M92 is extreme — the amount of iron in its stars is tiny. From that, astronomers estimate that the cluster is 12-and-a-half billion years old, which means it formed less than a billion-and-a-half years after the Big Bang. M92 therefore preserves a record of the conditions that prevailed in the early universe. That makes it an excellent target for those who want to study the birth of our home galaxy.


Script by Ken Croswell, Copyright 2015

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