October 21, 2008

In Today's One Minute Astronomer...

Globular Clusters: Ancient Stellar Relics

Most globular clusters are spherical collections of ancient stars born not long after the universe began. At first, you might think globular clusters all look the same: just fuzzy balls in the eyepiece of your telescope. But look closer. Each differs in shape and structure, as distinct as a human face. And these clusters contain some of the oldest stars in the universe.

The Basics

• Globular clusters like M13 and Omega Centauri are ancient relics of star formation in the early universe. Distinct from open clusters, which hold hundreds of brand-new stars, globulars have tens to hundreds of thousands of stars more than 12 billion years old. You can see dozens of these spherical, tightly-bound clusters with binoculars or a small telescope.

• Nearly every galaxy has a collection of globulars. Our Milky Way holds some 180 globular clusters. Giant elliptical galaxies like M87 have thousands of globs, many of which were ripped away from smaller galaxies.

• Stars like our Sun contain heavier elements like calcium, silicon, and carbon. But the stars in globulars formed long before such elements were formed: they are called “metal poor” stars and consist almost entirely of hydrogen and helium. Such stars belong to what astronomers call Population II. Newer stars, like our Sun, are from Population I.

M13, The Hercules Cluster

A Deeper Look

• How did globular clusters form? That's a good question for which there is no clear answer. But they likely collapsed from clouds of gas too small to form a galaxy but too large to form an open star cluster. In fact, globular clusters aren't that different in size and shape from dwarf spherical galaxies. In a way, globulars are “micro-galaxies”.

• In the early 1900's, famed astronomer Harlow Shapley determined most globular clusters in the Milky Way are found around the nucleus of our galaxy. He used globular clusters to determine the size of the Milky Way and the Sun's position on its outskirts.

• Unlike open star clusters, globular clusters are strongly bound by gravity and stable over time. Eventually, most of the stars in globular clusters will form white dwarfs and neutron stars and fade from view. But as far as we know, globular clusters… even as they darken… will remain bound forever.

Good To Know

For many years, detailed calculations showed that stars in globular clusters were older than the universe itself. Oops! But in 2000, the models of stellar evolution were reconciled with models of how the universe formed. It turns out the universe is about 13.7 billion years old, while globular clusters are roughly 12 billion years old, give or take half a billion years.

Personal View

For years, I was never keen on globular clusters. I always preferred open star clusters and nebulae because they are so much prettier. But I'm coming around. I think about it this way: I can go out into my backyard, look up, and see at a glance a hundred thousand stars that are 12 billion years old… more than twice the age of our Sun. It's hard not to be impressed!

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