V404 Cygni
V404 Cygni is a binary system consisting of a black hole with a mass of about 7001120000000000000♠12±3 solar masses and a late G or early K companion star of mass slightly smaller than the Sun in the constellation of Cygnus. The star and the black hole orbit each other every 7005559119456000000♠6.47129 d at fairly close range. Due to their proximity the main sequence star would be distorted into egg shape by the black hole's gravity and lose mass to the black hole. The "V" in the name indicates that it is a variable star, which repeatedly gets brighter and fainter over time. It is also considered a nova, because at least three times in the 20th century it produced a bright outburst of energy. Finally, it is a soft X-ray transient because it periodically emits short bursts of X-rays.
In 2009, the black hole in the V404 Cygni system became the first black hole to have an accurate parallax measurement for its distance from the Solar System. Measured by very-long-baseline interferometry using the High Sensitivity Array, the distance is 7000239000000000000♠2.39±0.14 kiloparsecs, or 7003780000000000000♠(7.80±0.46)×103 light-years.
Podcasts:
