Amateur astronomer working with ESO and FT on most massive star cluster

The Faulkes Telescope Project at the University of Glamorgan has helped professional and amateur astronomers to work together to unlock the mysteries of the most massive group of stars known in our local group of galaxies.

Astronomy Now equipment consultant and amateur astronomer Nick Howes was recently asked by members of a team working with the European Southern Observatory (ESO) to collaborate using the Faulkes Telescope South (FTS) on a project to image stars in the Westerlund 1 (Wd1) cluster.

Wd1 is the most massive compact young star cluster known in the local group of galaxies and is about 11,500 light years away from Earth. It was discovered by Bengt Westerlund in 1961 but remained largely unstudied for many years due to the high interstellar extinction (absorption by dust and gas) in its direction.

The cluster contains a large number of extremely rare, evolved high-mass stars including 6 yellow hypergiants, 4 red supergiants, 24 Wolf-Rayet stars, a luminous blue variable (LBV), many OB supergiants and an unusual sgB[e] star that has been proposed as the remnant of a recent stellar merger.

Besides hosting some of the most massive and least-understood stars in the galaxy, it is useful as an analogue to help astronomers determine what occurs within extragalactic “super star clusters”. Wd 1 is believed to have formed in a single burst of star formation, implying the constituent stars have the same age and composition.

X-ray observations have revealed the presence of the magnetar (an unusual type of highly magnetic neutron star) called CXO J164710.2-455216, a slow X-ray pulsar that must have formed from a high-mass progenitor star.

Using ESO’s Very Large Telescope, European astronomers had for the first time demonstrated that the magnetar was formed from a star with at least 40 times as much mass as the Sun. The result presents great challenges to current theories of how stars evolve, as a star as massive as this was expected to become a black hole, not a magnetar (neutron star).

This now raises a very fundamental question: just how massive does a star really have to be to become a black hole? The previous assumption was that stars with initial masses between about 10 and 25 solar masses would form neutron stars, and those above 25 solar masses would produce black holes.

“These stars must get rid of more than nine tenths of their mass before exploding as a supernova, or they would otherwise have created a black hole instead” says Dr. Simon Clark (Open University), one of the team members. “Such huge mass losses before the explosion present great challenges to current theories of stellar evolution.

”Nick was originally approached by Dr Ben Ritchie of the Open University, one of the project team working with the VLT, and asked to conduct additional observations using FTS.

“ESO were looking for a demonstration of the reddening towards the cluster, which is very large - something like 12 magnitudes in the V-band”, explains Nick. “Their predictions were that the I and R band images would show the cluster very clearly, but that by the B band it is down at about 20th magnitude, and we'd only see the foreground stars, with no sign of the cluster at all.

”Using 30s exposures from the U,B and V bands and 15s exposures for the R and I bands, Nick exceeded the researchers’ expectations with data from FTS, once again proving that it is an outstanding research tool that can be accessed by schools and amateurs.

FTP are now expanding the project to schools, working in collaboration with Dr Ritchie to observe Wd 1 over an extended period of time, and help to search for variable stars.















Image: The open cluster Westerlund 1, as seen in normal optical light (V, or visual, band; left) and in the near infrared (I, or Infrared, band; right). The infrared image clearly shows a far greater number of stars, which in the visual light image are hidden behind the dust and gas.

Further information:

The Wd1 cluster can be observed with FTS at these co-ordinates:

RA 16 47 04.00
Dec -45 51 04.9

Nick Howes obtained good results using 30s exposures in the U,B,V,R and I bands.

For more information on the FTP open cluster educational projects see:

Open clusters:

Massive stars:

A video showing the effects of extinction and reddening on this cluster can be found