Discovery helps to solve mystery in globular clusters
Posted: October 21, 2001

In a discovery which may help to solve a long-standing mystery, astronomers from the University of Manchester's Jodrell Bank Observatory and other members of an international team have found gas between the densely packed stars that make up the globular cluster, 47 Tucanae.

47 Tucanae is one of about 140 globular clusters associated with our galaxy, the Milky Way. As they contain up to a million stars packed into a relatively small space, the combined starlight near its centre would make night as bright as day! 47 Tucanae is one of the most spectacular globular clusters and can be easily seen with the unaided eye in the Southern Hemisphere. At a distance of 16,000 light years it is roughly the same size in the sky as the full moon. The stars in such clusters are very old, and many will have shed much of their mass into space during cataclysmic explosions at the end of their lives. For over 40 years, astronomers have looked for gas in these clusters without success but now, at last, it has been detected.

47 Tucanae
The globular cluster 47 Tucanae. The positions of 15 millisecond pulsars used in the reported study are superimposed on the optical image of the cluster. All pulsars are located near the centre. North is to the top and east is to the left. Background image copyright: Anglo-Australian Observatory (photograph by David Malin).
Interestingly, it is the death of the giant stars whose ejected gas has proved so elusive that has provided the means of its detection. Nature has offered some of them the chance of a second life as their iron cores have collapsed to form rapidly rotating neutron stars somewhat more massive than our Sun. Rotating several hundred times a second, they emit beams of radio waves which can be detected as regular pulses of energy as the beam sweeps across the Earth. These objects, known as "Pulsars", are thus massive cosmic flywheels and have been shown to be incredibly accurate clocks.

Using the 64-m Parkes radio telescope in Australia, the research team have discovered more that 20 of these exotic objects in 47 Tucanae. They have made very precise observations of the minute changes in the observed rotation rates due to the doppler shift caused by the gravitational pull of the cluster. This has enabled them to determine their positions within the cluster. As one would expect, some are on the far side and some on the near side of the cluster's centre. A further measurement made for each pulsar measures the amount of material and gas in the line of sight to us. To their delight, the team found that those on the far side of the cluster had more gas in front of them than those on the near side, thus proving the presence of gas within the cluster.

The international team is very excited about this discovery. Dr. Paulo Freire explains, "Astronomers have searched for indications of the expected gas at all frequencies across the electromagnetic spectrum. Finally we have now a solid detection using a radio telescope to observe pulsars which act as highly sensitive probes of the cluster environment."

Although it needs high precision measurements to detect the gas in the cluster, Dr. Fernando Camilo stresses, "the measured gas density is about 100 times larger than we would have expected from the interstellar medium surrounding 47 Tucanae."

Dr. Michael Kramer points out that the previously unsuccessful observations at other wavelengths are also important to understand their new result. From the previous measurements it can be inferred that the gas detected by the team must represent almost all the material existing in the cluster medium.

Astronomers had looked for gas because it is expected to be accumulated from the mass loss of the massive stars in the cluster and it has been a mystery where the cluster gas had gone. Several explanations had been put forward to explain the missing gas and amongst these were the winds from pulsars. It is somewhat ironic that the very same objects that may be responsible for the removal of most of the gas, have finally lead to its detection.

The members of the research team are Dr. Paulo Freire (University of Manchester, Jodrell Bank Observatory; now at the Arecibo Observatory), Dr. Michael Kramer and Professor Lyne (both at the University of Manchester, Jodrell Bank Observatory), Dr. Fernando Camilo (Columbia University, USA), Dr. Richard Manchester (Australia Telescope National Facility, Australia) and Dr. Nichi D'Amico (Observatorio di Bologna, Italy).

A pulsar is a neutron star which is the collapsed core of a massive star that has ended its life in a supernova explosion. Weighing more than our Sun, yet only 20 kilometres across, these incredibly dense objects produce beams of radio waves which sweep round the sky like a lighthouse, often hundreds of times a second. Radio telescopes receive a regular train of pulses as the beam repeatedly crosses the Earth so the objects are observed as a pulsating radio signal.

First discovered by radio astronomers at Cambridge, pulsars make exceptional clocks, which enable a number of unique astronomical experiments. Some very old pulsars, which have been "spun up" to speeds of over 600 rotations per second by material flowing onto them from a companion star, appear to be rotating so smoothly that they may be even "keep time" more accurately than the best atomic clocks here on Earth. Very precise timing observations of systems in which a pulsar is in orbit around another neutron star have been able to prove the existence of gravitational radiation as predicted by Albert Einstein and have provided very sensitive tests of his theory of General Relativity -- the theory of gravitation which supplanted that of Isaac Newton.

The Parkes radio telescope is part of the Australia Telescope, which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO (Commonwealth Scientific and Industrial Research Organisation). Paulo Freire received support from Fundacao para a Ciencia e a Teconologia through Praxis XXI fellowship BD/11446/97. Fernando Camilo is supported by NASA grant NAG 5-9095. Nichi D'Amico from the Italian Minister of the University and Technological and Scientific Research (MURST).