Astronomers used the Hubble Space Telescope to produce the first precise measurement of the distance to NGC 6397, a collection of stars that formed shortly after the Big Bang. The object is located 7,800 light-years from Earth and the uncertainty on this estimate has an error margin of about 3 percent.
NGC 6397 is a 13.4 billion-years-old globular cluster. These grouping of stars are very important to our understanding of stars. The stars in these clusters while all looking different, are at roughly the same distance, have the same chemical composition, and are the same age. They are the ideal test for stellar evolution models.
The results, published in The Astrophysical Journal Letters, are a significant improvement on previous globular cluster distance estimate, which had uncertainties between 10 and 20 percent. The new estimate was produced using a new technique combined with a very old one.
The team used the so-called spatial scanning approach. The Hubble telescope can provide hundreds of position measurement for dozens of stars in the cluster. By repeating the measurement every six months, the astronomers were able to measure tiny angle differences as the Earth's position changed in space. Based on the angle, by using trigonometric formulae known since classic times, they were able to work out the distances. This is the same approach as parallax measurements. By combining measurements from 40 stars they achieved the impressive estimate.
“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,” lead author Tom Brown of the Space Telescope Science Institute (STScI) in Baltimore, said in a statement.
This approach had been already tested on nearby star cluster but not on something like NGC 6397.
“Any model that agrees with the measurements gives you more faith in applying that model to more distant stars,” Brown said. “The nearby star clusters serve as anchors for the stellar models. Until now, we only had accurate distances to the much younger open clusters inside our galaxy because they are closer to Earth.”
There are about 150 globular clusters that surround the much younger galaxy disk of the Milky Way, where the spiral arms (and us) reside. By increasing precision, the researchers will produce more reliable models. And while this measurement is a significant step forward, the researchers are already planning to push it further by combining this data with measurements from the European Space Agency’s telescope, Gaia. “Getting to 1 percent accuracy will nail this distance measurement forever,” Brown said.
Gaia’s new data release comes out at the end of the month so this record might only last a short while. Watch this space.