Free Astronomy Magazine July-August 2018

16 JULY-AUGUST 2018 First precise distance measurement to a globular star cluster by NASA/ESA A stronomers using NASA’s Hubble Space Telescope have for the first time precisely measured the distance to one of the oldest objects in the universe, a collection of stars born shortly after the big bang. This new, refined dis- tance yardstick provides an inde- pendent estimate for the age of the universe. The new measurement also will help astronomers improve models of stellar evolution. Star clusters are the key ingredient in stellar models because the stars in each grouping are at the same dis- tance, have the same age, and have the same chemical composition. They therefore constitute a single stellar population to study. This stellar assembly, a globular star cluster called NGC 6397, is one of the closest such clusters to Earth. The new measurement sets the clus- ter’s distance at 7,800 light-years away, with just a 3 percent margin of error. Until now, astronomers have estimated the distances to our galaxy’s globular clusters by com- paring the luminosities and colors of stars to theoretical models, and to the luminosities and colors of similar stars in the solar neighbor- hood. But the accuracy of these es- timates varies, with uncertainties hovering between 10 percent and 20 percent. However, the new measurement uses straightforward trigonometry, the same method used by surveyors, and as old as an- cient Greek science. Using a novel observational technique to measure extraordinarily tiny angles on the sky, astronomers managed to stretch Hubble’s yardstick outside of the disk of our Milky Way galaxy. The research team calculated NGC 6397’s age at 13.4 billion years old. “The globular clusters are so old that if their ages and distances de- duced from models are off by a lit- tle bit, they seem to be older than the age of the universe,” said Tom Brown of the Space Telescope Sci- ence Institute (STScI) in Baltimore, Maryland, leader of the Hubble study. Accurate distances to globu-