Astronomers have taken the first direct pictures of a gravitational ‘microlens’ in the Milky Way. The lens is thought to be a burnt-out white dwarf star, a type of object that could account for much of the missing – or ‘dark’ – matter that seems to exist in the universe. A team of astronomers led by Cailin Nelson of the Lawrence Livermore National Laboratory in the US found the faint object close to the line-of-sight of a distant star that was seen through a gravitational lens six years earlier (C Alcock et al 2001 Nature 414 617).
Astronomical bodies such as galaxies can often be detected – even if they are invisible – because they behave as lenses: their gravitational fields bend the light emitted by background stars towards Earth as they pass in front of the stars. When smaller objects – such as white dwarfs or neutron stars – create the same effect, it is known as microlensing.
In 1993, astronomers at the Mount Stromlo Observatory in Australia observed a number of stars in the neighbouring Large Magellanic Cloud galaxy. The light from each star was bent towards Earth by a gravitational lens – an effect that was detected as a rise and a fall in the intensity of the star’s light as the lens passed in front of it.
In 1999, Nelson and colleagues used the Hubble Space Telescope to re-examine one of these stars at very high resolution. They found a very dim red object close to the line-of-sight of the star – in exactly the same place that the gravitational lens would be now, according to calculations based on the earlier intensity observations.
Astronomers can calculate the mass of a gravitational lens from the degree to which it bends light, the relative motion of the Earth and the distant star, and the motion of the object passing in front of the background star. Together with spectra gathered at the Very Large Telescope, Nelson and colleagues deduced that the object is a white dwarf star typical of those found in the Milky Way.
The large-scale motion of galaxies suggests that they contain much more mass than astronomers can detect with telescopes. Astronomers have long struggled to understand what makes up this ‘dark matter’, which accounts for around 90% of the matter in the universe. One suggestion is ‘massive compact dark halo objects’ – or MACHOs – which include white dwarfs. Current estimates suggest that 8 – 50% of dark matter could be made up of MACHOs, but Nelson and co-workers are optimistic that their discovery will improve that estimate.