The Macho team surveys tens of millions of stars in the direction of the centre of our galaxy. The starfield is very crowded here and this increases the chances for seeing the rare gravitational microlensing events. The two events were also of exceptionally long duration, lasting 800 and 500 days respectively, which suggests that the lensing objects have a high mass. Hubble's identification of the lensed star allowed for an accurate estimate of the mass of the black hole. So far there have been more than 300 cases of gravitational microlensing seen towards the central regions of our galaxy to date.
Astronomers say they represent a rich harvest for future discoveries.
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