V404 Cygni is a binary star system consisting of a black hole with a mass of about 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 two stars orbit each other every 6.5 d at fairly close range. Due to their close proximity the main sequence star would be distorted into egg shape by the black hole's gravity and lose mass to 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.
It has been suggested that the more massive component of the system might be a Quark star instead of a black hole.
On May 22, 1989 the Japanese Ginga Team discovered a new X-ray source that was cataloged as GS 2023+338. This source was subsequently identified as coincident in position with a previously known nova cataloged as V404 Cygni.
- "V404 Cyg: 10 years in quiescence" Elena P. Pavlenko, Yuliana G. Kuznetsova, Sergei Yu. Shugarov and Vladislav S. Petrov
- "The 1989 outburst of V404 cygni: A very unusual x-ray nova" R. M. Wagner, S. Starrfield, A. Cassatella, R. Gonzalez-Riestra, T. J. Kreidl, S. B. Howell, R. M. Hjellming, X. -H. Han, G. Sonneborn
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