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Headlines Published on 17 September 2008

Title Astronomers investigate globular star cluster formation

An international group of astronomers has made a new discovery regarding the formation of globular star clusters. In their study, the scientists examined globular clusters outside the Milky Way Galaxy and found that they are more likely to form in dense areas, not from galaxy to galaxy. Their finding was recently published in The Astrophysical Journal.

Images taken by NASA/ESA Hubble Space Telescope showing four members of the Virgo cluster of galaxies © NASA/ESA
Images taken by NASA/ESA Hubble Space Telescope showing four members of the Virgo cluster of galaxies

Globular star clusters are what experts call a collection of stars orbiting a galactic core as a satellite. Thanks to the NASA/ESA Hubble Space Telescope, the astronomers were able to identify more than 11 000 globular clusters in what is believed to be Earth's closest galaxy cluster: Virgo. The scientists said the Hubble's Advanced Camera for Surveys (ACS) made the star clusters visible in 100 galaxies, differing in shape, size and brightness.

They said the capacity for Hubble to distinguish fuzzy globular clusters from stars in our galaxy and from other galaxies is due to its 'eye'. Dr Eric Peng from Peking University in China said, 'With Hubble, we were able to identify and study about 90% of the globular clusters in our observed fields. This was crucial for dwarf galaxies that have only a handful of star clusters.'

Dr Peng, lead author of the Hubble Space Telescope study, also explained the difficulty in distinguishing globular clusters from stars and galaxies when ground-based telescopes are used.

The astronomers noted that they had discovered globular clusters in the majority of dwarf galaxies within 3 million light-years of the cluster's centre. According to them, the giant elliptical galaxy Messier 87 is also found at the centre of the Virgo cluster of galaxies.

'Our study shows that the efficiency of star cluster formation depends on the environment,' commented Dr Patrick Côté from the Canadian-based Herzberg Institute of Astrophysics. 'Dwarf galaxies closest to Virgo's crowded centre contained more globular clusters than those farther away.'

Astronomers have long postulated that while Messier 87 hosts a large population of globular clusters, their origin is unknown. One theory is that the clusters may have been snatched from smaller galaxies that got too close to it, they said.

'We found few or no globular clusters in galaxies within 130 000 light-years from Messier 87, suggesting the giant galaxy stripped the smaller ones of their star clusters,' Dr Peng explained. 'These smaller galaxies are contributing to the build-up of Messier 87.'

Research into globular star clusters helps astronomers to better understand the early episodes that mark galaxy formation, the research team said. 'Star formation near the core of Virgo is very intense and occurs in a small volume over a short amount of time,' Dr Peng remarked. 'It may be more rapid and more efficient than star formation in the outskirts. The high star-formation rate may be driven by the gravitational collapse of dark matter, an invisible form of matter, which is denser and collapses sooner near the cluster's centre,' he added. 'Messier 87 sits at the centre of a large concentration of dark matter, and all of these globulars near the centre probably formed early in the history of the Virgo cluster.'

The Hubble Space Telescope project is being carried out by ESA and NASA.

More information:

  • European Space Agency
  • NASA

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