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If you have dark skies and a pair of binoculars, you can scan for some of the oldest stars in the entire galaxy this evening. The stars may have formed as early as 13 billion years ago — when the universe was less than a billion years old.
The stars belong to globular clusters, which reside in the Milky Way’s halo, outside the galaxy’s bright disk.
Each cluster is a tightly packed ball of a hundred thousand stars or more. And all of those stars are among the most ancient in the galaxy.
Astronomers infer a star’s age by measuring its composition. The first stars in the universe consisted almost entirely of hydrogen and helium — elements forged in the Big Bang. Nuclear reactions in those stars made heavier elements. When the stars died, they expelled some of the heavy elements into space.
Over time, those elements were incorporated into new stars, which in turn made and distributed even more heavy elements. So stars like the Sun, which formed billions of years after the Big Bang, contain fairly high concentrations of these elements. But the stars in globular clusters contain almost none, suggesting that they’re extremely old.
Three clusters are in the southeast at nightfall. M15 is highest in the sky, to the upper right of the Great Square of Pegasus. M2 is not far to its lower right. And M30 is down in Capricornus. Through binoculars, each cluster looks like a fuzzy star — the glow of some of the oldest stars in the universe.