Free-floating stars in the Milky Way's bulge

The path of a light beam is bent by the presence of mass, as explained by General Relativity. A massive body can therefore act like a lens—a so called "gravitational lens"—to distort the image of an object seen behind it. Microlensing is a related phenomenon: a short flash of light is produced when a moving cosmic body, acting as a gravitational lens, modulates the intensity of light from a background star as it fortuitously passes in front of it. About fifty years ago scientists predicted that if it ever became possible to observe a microlensing flash from two well-separated vantage points, a parallax measurement would pin down the distance of the dark object. The Spitzer Space Telescope, orbiting the Sun at the distance of the Earth but trailing behind the Earth by about one-quarter of the orbital path, had been working with ground-based telescopes to do just that until it was shut down last month by NASA as a cost-savings measure.

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