Globular Cluster 47 Tucanae
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). More ImagesAstronomers using NASA's Hubble Space Telescope have for the first time linked two distinct populations of stars in an ancient globular star cluster to their unique orbital dynamics, offering proof that the stars do not share the same birth date.
The analysis of the globular cluster 47 Tucanae shows that the two
populations differ in age by less than 100 million years. The cluster
resides roughly 16,700 light-years away in the southern constellation
Tucana.
Researchers, led by Harvey Richer of the University of British
Columbia in Vancouver, combined recent Hubble observations with eight
years' worth of data from the telescope's archive to determine the
motions of the stars in this cluster.
Previous spectroscopic studies revealed that many globular clusters
contain stars of varying chemical compositions, suggesting multiple
episodes of star birth. This Hubble analysis, however, goes a step
further, adding the stars' orbital motion to the analysis.
"When analyzing the motions of stars, the longer the time baseline
for observations, the more accurately we can measure their motion,"
Richer explained. "These data are so good, we can actually see for the
first time 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."
The Milky Way's globular clusters are the surviving relics from our
galaxy's formation. They offer insights into the early history of our
galaxy. 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.
Richer and his team used Hubble's Advanced Camera for Surveys in 2010
to observe the cluster. They combined those observations with 754
archival images to accurately measure the changes in positions of more
than 30,000 stars. Using these data, they could discern how fast the
stars are moving. The team also measured the stellar luminosities as
well as temperatures.
This stellar archaeology identified the two distinct populations of
stars. The first consists of redder stars, which are older, less
chemically enriched, and in random, circularized orbits. The second
population comprises bluer stars, which are younger, more chemically
enhanced, and in more elliptical orbits.
"The redder generation, which is deficient in heavier elements,
reflects the initial motion of the gas that formed the cluster," Richer
said. "These stars have retained a memory of their original motion."
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 towards
the cluster center. Slowly over time these stars have been moving
outwards, putting them on more radial orbits.
This discovery is not the first for Hubble in revealing multiple
generations of stars in globular clusters. In 2007 Hubble researchers
found three generations of stars in the massive globular cluster NGC
2808. Richer's team, however, linked stellar dynamics to separate
populations for the first time. Finding multiple stellar populations in
globular clusters has deep cosmological implications. Astronomers need
to solve future enigmas of these multiple generations to better
understand how stars formed in distant galaxies in the early universe.
The team's results are published in the July 1 issue of The Astrophysical Journal Letters.
CONTACT
Donna Weaver / Ray VillardSpace Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Harvey Richer
University of British Columbia, Vancouver, BC, Canada
604-822-4134
richer@astro.ubc.ca
