Signs of unrest in massive star cluster

By comparing two Hubble images taken ten years apart, astronomers have measured the tiny motions of hundreds of young stars in a massive star cluster, finding that they move in quite an unexpected way.

Hubble image of NGC 3603. Radiation streaming from the hot blue stars in the centre are responsible for carving out a huge cavity in the gas to the right of the cluster. Image: NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration.

The central cluster of star-forming region NGC 3603 is one of the most compact clusters in the Milky Way, with a mass of some 10,000 Suns packed into a volume with diameter of a mere three light-years.

Measuring star motions within a 20,000 light year distant cluster is extremely difficult, and relies on comparing images made years apart. Archival data from July 1997 was available for NGC 3603, taken with Hubble's Wide Field Planetary Camera 2, and was compared with images taken in September 2007, with the same instrument and same set of filters. Two years of painstaking work later, and reliable estimates for the motions of stars were extracted.

"Our measurements have a precision of 27 millionths of an arcsecond per year," says Boyke Rochau of the Max-Planck Institute for Astronomy in Heidelberg. "This tiny angle corresponds to the apparent thickness of a human hair seen from a distance of 800 kilometres."

The core of star cluster NGC 3603 is revealed in this WFPC2 image taken in 2007, which was compared with an image taken ten years earlier to detect the motions of individual stars. NASA, ESA and Wolfgang Brandner (MPIA), Boyke Rochau (MPIA) and Andrea Stolte (University of Cologne).

The team measured speeds for over 700 cluster stars of varying masses and surface temperatures and found that the motions had not yet 'settled down' – their velocities were independent of their mass and still reflect conditions from the time the cluster formed around one million years ago.

Stars are born when a giant cloud of gas and dust collapses, concentrating the matter into the cores of young stars. For massive star clusters like NGC 3603, this process is thought to be relatively rapid, and any left-over stellar ingredients are blown away by fierce stellar winds. Sometimes the cluster will then have insufficient gravity to stay bound together and will instead drift apart, while more massive clusters will remain tightly bound as globular clusters.

“This is the first time we have been able to measure precise stellar motions in such a compact young star cluster,” says team member Wolfgang Brandner. The results offer key insight into how clusters form and subsequently evolve.