天鹅座X-1（简称Cyg X-1）是一个银河系内位于天鹅座的双星系统，是著名的X射线源。它在1964年的一次火箭弹道飞行时被发现，是从地球观测最强的X射线源之一，其顶峰X射线通量为2.3×10 WmHz。天鹅座X-1是最先被广泛承认为黑洞的候选星体，也是同类星体中最受研究关注的。现在估计其质量为太阳质量的8.7倍，而其密度之高使黑洞成为唯一一种解释。如果如此，它的事件视界半径约为26公里。

天鹅座X-1属于一个高质量X射线双星系统，其距离太阳大约6,070光年，另一成员为一颗超巨星变星，编号为HDE 226868。两者相互围绕公转，距离为0.2天文单位，即地球和太阳间距离的20%。该星的星风为X射线源的吸积盘提供物质。盘的内部温度达到几百万K，因此辐射出X射线。两条垂直于吸积盘的相对论性喷流将被吸进的物质喷射出星际空间。

Cygnus X-1 (abbreviated Cyg X-1) is a well-known galactic X-ray source, thought to be a black hole, in the constellation Cygnus. It was discovered in 1964 during a rocket flight and is one of the strongest X-ray sources seen from Earth, producing a peak X-ray flux density of 6977229999999999999♠2.3×10 Wm Hz (7003230000000000000♠2.3×10 Jansky). Cygnus X-1 was the first X-ray source widely accepted to be a black hole and it remains among the most studied astronomical objects in its class. The compact object is now estimated to have a mass about 14.8 times the mass of the Sun and has been shown to be too small to be any known kind of normal star, or other likely object besides a black hole. If so, the radius of its event horizon is about 7004440000000000000♠44 km.