New light on dark clouds
Jan 10, 2001
Astronomers know that the dark clouds of molecules they see in space are 'star nurseries' - but the conditions that give rise to star birth have remained mysterious because existing techniques cannot provide sufficient information about the clouds. Now João Alves of the European Southern Observatory and co-workers have developed a new technique that can probe the structure of a cloud in unprecedented detail by analysing the light from the stars behind it (J F Alves et al 2001 Nature 409 159).
Traditional methods of studying dark clouds study the emission spectra of the molecules that make up the clouds. However, the signal of the main constituent of the clouds, hydrogen molecules, are very weak so astronomers must rely on the signals from other - much more scarce - molecules in the clouds. But these measurements have proved to be inadequate because astronomers do not know enough about the temperature or distribution of the dust inside the cloud. Instead, the new method uses sensitive infrared cameras to measure how the clouds block light from background stars. "The understanding of the physical structure of a cold dark cloud has always been the missing link in our current picture of star formation", Alves told PhysicsWeb.
Alves and colleagues chose a cloud known as Barnard 68 because they believed that its dense background of stars - which are mostly giant stars with well-known spectral ranges - would provide plenty of data on how the light is blocked out. Most importantly, the cloud currently shows no signs of star birth - although a third of similar clouds contain young stars - making it likely that the conditions inside it could give rise to new stars in the future.
The stars behind Barnard 68 cannot be detected at visible wavelengths, but the cloud becomes more transparent at longer wavelengths. Alves' team compared the light from these stars, which had been 'reddened' by the cloud, with the light from a group of nearby 'control' stars. They combined their existing knowledge of the cloud with established astrophysical laws to conclude that Barnard 68 is in equilibrium - in other words, its outward thermal pressure exactly counteracts its inward gravitational pull - and that it probably has a steady magnetic field. But the astronomers believe that over time various factors - such as cooling, dissipation of the magnetic field and any increase in external pressure - could destabilize the cloud and lead to the birth of low-mass Sun-like stars.
"These measurements constitute a major breakthrough in the understanding of dark clouds", says Alves. "For the first time, the internal structure of a dark cloud has been specified with a detail approaching our knowledge of stellar interiors".