OBSERVING R136A1
Despite its brilliance, this massive star is challenging to observe thanks to its crowded surroundings
Seen across 165,000 light years of space, R136a1’s brilliant light dwindles to an apparent magnitude well below naked-eye visibility at +12.3. Nevertheless, it remains within reach of all but the smallest telescopes. The real problem lies in its proximity to other stars. In the early 1980s, astronomers speculated that the central object might be a single bright star with a mass of 1,500 Suns, but many doubted that such a monster could exist. They were proven right in 1985 when R136 was confirmed to be a densely packed cluster.
As seen from Earth, the stars R136a1, R136a2 and
R136a3 are separated by approximately 0.1 and 0.5 arcseconds. While this might be a tiny angle, according to optical theory this is the kind of detail that should be resolvable through a telescope with a diameter of around a metre (3.3 feet) or more. The problem lies in Earth’s atmosphere, which complicates the theoretical behaviour of light in unpredictable ways. Moving masses of air act like tiny lenses, bending and warping the path of light rays while blurring and shifting the images seen from the surface. This is the phenomenon that causes stars to flicker and twinkle in the sky, while on long-exposure photographs it causes the images of stars to smear out from points into fuzzy-edged discs. Even at mountaintop altitudes above most of the atmosphere, it reduces the resolving power of even the largest telescope to about 0.5 arcseconds, blurring images of R136 and making its individual stars indistinguishable.
In the past two decades, astronomers have finally started to overcome this problem using the ingenious technology known as adaptive optics. The principle is simple – adjusting the configuration of the telescope itself to correct for changing atmospheric turbulence – but putting it into practice requires huge amounts of computing power. It was the adaptive optics system on the European Southern Observatory’s Very Large Telescope (VLT) that enabled a team of astronomers led by Paul Crowther of the University of Sheffield to produce the first resolved image of the cluster’s heart. Based on the brightness of its individual stars and a comparison with a relatively nearby monster star with better known properties, the team was able to calculate R136a1’s physical properties. They revealed the secrets of what remains, for the moment, at least, the heaviest star in the universe.