Caption

Astrometric microlensing occurs when a foreground object, like a neutron star, passes in front of a more distant background star. The neutron star’s gravity bends the distant star’s light, splitting it into multiple paths that reach the telescope. Although these distorted images can’t be resolved, their combined light appears brighter and slightly shifted from the distant star’s true position. As the alignment between the two objects changes over time, this apparent shift traces a small elliptical pattern on the sky. The size of that ellipse depends on how strongly the light is bent, meaning more massive objects produce larger shifts, allowing astronomers to directly measure the mass of the otherwise invisible neutron star.