Scientists of the University of British Columbia in Vancouver determined the motions of the stars
in the globular cluster 47 Tucanae, which is located about 16,700 light-years away in the southern constellation Tucana.
The analysis enabled researchers, for the first time, to link the movement of stars in the clusters with the stars' ages.
The two populations in 47 Tucanae differ in age by less than 100 million years.
"When analyzing the motions of stars, the longer the time baseline for observations, the more accurately we can
measure their motion," said Harvey Richer, who led the research team.
"These data are so good, we can actually see the individual motions of the stars in the cluster. The data offer
detailed evidence to help us understand how various stellar populations formed in such clusters."
Click on image to enlarge
The ancient globular cluster 47 Tucanae, a dense swarm of up to a million stars.
The image at left shows the entire cluster, which measures about 120 light-years across. Located in the southern constellation Tucana, the cluster is about 16,700 light-years away. The image is part of the Sloan Digital Sky Survey and was taken by the UK Schmidt Telescope at Siding Spring Observatory in New South Wales, Australia. The white rectangular box outlines the view taken by NASA's Hubble Space Telescope.
That image, shown at right, captures close-up views of thousands of cluster stars. The large, bright stars in the image are red giants. These stars have puffed up to several times their normal size because they have exhausted their nuclear fuel and are near the end of their lives. The image was taken by Hubble's Advanced Camera for Surveys.
Credit: NASA, ESA, Digitized Sky Survey (DSS; STScI/AURA/UKSTU/AAO), H. Richer and J. Heyl
(University of British Columbia), and J. Anderson and J. Kalirai (STScI)
47 Tucanae is 10.5 billion years old and one of the brightest of our galaxy's
more than 150 globular clusters. The cluster measures about 120 light-years wide.
This Hubble analysis supports previous studies of stars with varying chemical compositions and adds
the stars' orbital motion to the analysis.
Richer and his team used Hubble's Advanced Camera for Surveys to observe the cluster in 2010. They combined those
observations with 754 archival images to measure the change in position of more than 30,000 stars.
Using these data, they could discern how fast the stars move. The team also measured the stars' brightness
This stellar archaeology identified the two distinct populations of stars.
The first population consists of redder stars, which are older, less chemically enriched, and orbiting in random circles.
The second population consists of bluer stars, which are younger, more chemically enhanced, and moving in more elliptical orbits.
The lack of heavier elements in the redder stars reflects the initial composition of the gas that formed the cluster.
After the most massive of these stars completed their stellar evolution, they expelled gas enriched with heavier elements
back into the cluster.
This gas collided with other gas and formed a second, more chemically enriched generation of
stars that was concentrated toward the cluster center. Over time these stars moved slowly outward into more elliptical
In 2007, Hubble researchers found three generations of stars in the massive globular cluster NGC 2808.
But Richer's team is the first to link stellar dynamics to separate populations.