Incredibly Ancient, Stunningly Beautiful Cluster of Stars Is Just 7,800 Light-Years Away

Some of the oldest stars in our galaxy belong to a cluster located 7,800 light-years away, according to a new paper published in The Astrophysical Journal Letters. The stars themselves are well-known, but the distance measurement is more precise than scientists have ever managed before for stars located beyond the main disk of the Milky Way.

That measurement could become a sort of stepping-stone to accurately measure even more distant stars, including the residents of about 150 other clusters that hover on the outskirts of our galaxy.

The cluster, called NGC 6397, is incredibly old—more than 13 billion years, which means it's practically as old as the universe itself. "The globular clusters are so old that if their ages and distances deduced from models are off by a little bit, they seem to be older than the age of the universe," co-author Tom Brown, an astronomer at the Space Telescope Science Institute in Maryland, said in a press release.

Read more: NASA's Hubble Space Telescope photographs its most distant star ever

Astronomers like clusters because there are many stars to study, and those stars are identical twins. Of particular importance to this project, they're all about the same distance from Earth. So astronomers set about using the Hubble Space Telescope and a technique called parallax to try and pin down that distance as precisely as possible.

Here's the idea behind parallax: Stare at the tip of your nose and close one eye and then the other. The background behind your nose will appear to jump back and forth. If you were really dedicated to the task, you could use the distance between your eyes and the apparent scale of the jump to calculate how far the tip of your nose is from the point directly between your eyes.

In astronomy, it's the same basic idea. To substitute for opening and closing each eye, astronomers harness Earth's movement around the sun, taking observations six months apart, when Earth is on opposite extremes of its orbit. That cosmically tiny difference is enough to cause targets to jump a tiny bit across the celestial background.

The globular cluster, called NGC 6397, whose distance to Earth scientists measured. NASA, ESA, and T. Brown and S. Casertano (STScI) ; Acknowledgement: NASA, ESA, and J. Anderson (STScI)

Parallax itself isn't new, but the authors of the new paper then made their measurements even more precise by applying it to 40 different stars in the cluster in a process called spatial scanning. Putting all that information together, they determined the cluster's distance with an error range of just 3 percent. In comparison, previous error rates on the same measurement have fallen at 10 or 20 percent.

The team thinks it could hone the distance estimate even more precisely, thanks to Gaia, a European instrument that is working on pinning down the locations of a billion stars in and around the Milky Way. The scientists say that a data dump due to be published later this month should let them shave their margin of error down to just 1 percent.