Astronomical distances are best measured by trigonometric parallax — using the annual shift in star position caused by Earth's motion around the Sun to derive distance. Now James Miller-Jones of the National Radio Astronomy Observatory in Charlottesville, Virginia, and his colleagues have used parallax to measure the first accurate distance to a nearby binary star system containing a black hole.
The authors took radio measurements at three-month intervals for a year and combined them with archival data. They reveal that the star–hole system, known as V404 Cygni, is just 2,390 parsecs from Earth — nearly half the distance previously thought. The group believes that the previous work underestimated the interstellar dust along the line of sight to the star system. The authors suggest that future parallax measurements will improve the understanding of how black holes form and behave.