Free Astronomy Magazine July-August 2018

18 JULY-AUGUST 2018 SPACE CHRONICLES searchers then combined the results to obtain the precise distance meas- urement. “Because we are looking at a bunch of stars, we can get a better measurement than simply looking at individual Cepheid vari- able stars,” team member Caser- tano said. The tiny wobbles of these cluster stars were only 1/100 th of a pixel on the telescope’s camera, measured to a precision of 1/3000 th of a pixel. This is the equivalent to measuring the size of an automo- bile tire on the moon to a precision of one inch. The researchers say they could reach an accuracy of 1 percent if they combine the Hubble distance measurement of NGC 6397 with the upcoming results obtained from the European Space Agency’s Gaia space observatory, which is measuring the positions and dis- tances of stars with unprecedented precision. “Getting to 1 percent ac- curacy will nail this distance meas- urement forever,” Brown said. trigonometric parallax to nail down the cluster’s distance. This tech- nique measures the tiny, apparent shift of an object’s position due to a change in an observer’s point of view. Hubble measured the appar- ent tiny wobble of the cluster stars due to Earth’s motion around the Sun. To obtain the precise distance to NGC 6397, Brown’s team em- ployed a clever method developed by astronomers Adam Riess, a Nobel laureate, and Stefano Casertano of the STScI and Johns Hopkins Univer- sity, also in Baltimore, to accurately measure distances to pulsating stars called Cepheid variables. These pul- sating stars serve as reliable dis- tance markers for astronomers to calculate an accurate expansion rate of the universe. With this tech- nique, called “spatial scanning,” Hubble’s Wide Field Camera 3 gauged the parallax of 40 NGC 6397 cluster stars, making measurements every 6 months for 2 years. The re- T his video zooms into a Hubble Space Telescope view of globular star cluster NGC 6397. [NASA, ESA, and G. Bacon (STScI)] lar clusters are used as references in stellar models to study the charac- teristics of young and old stellar populations. “Any model that agrees with the measurements gives you more faith in applying that model to more distant stars,” Brown said. “The nearby star clus- ters serve as anchors for the stellar models. Until now, we only had ac- curate distances to the much younger open clusters inside our galaxy because they are closer to Earth.” By contrast, about 150 glob- ular clusters orbit outside of our galaxy’s comparatively younger starry disk. These spherical, densely packed swarms of hundreds of thousands of stars are the first homesteaders of the Milky Way. The Hubble astronomers used !