Deneb, carried in the old astrological lists as Deneb Adige, is Alpha Cygni, a blue-white supergiant of spectral type A2 and among the most intrinsically luminous stars visible to the naked eye, radiating perhaps a hundred thousand times the light of the Sun. Its distance is genuinely uncertain, of the order of one to two and a half thousand light years, which means its true brightness is only approximately known, but it is unquestionably a giant among stars. It marks the tail of Cygnus the swan, its name is from dhanab al-dajajah, the tail of the hen, and forms the northern apex of the Summer Triangle with Vega and Altair.
Deneb's uncertain distance is a genuine and still-active problem in astrometry: even the Gaia space telescope's exceptionally precise parallax measurements struggle with extremely luminous, distant supergiants like Deneb, because such stars are intrinsically variable and their parallax angle is vanishingly small at thousands of light years, so its true luminosity, somewhere in the range of Hundred thousand times the Sun's, is still stated with real caveats even in current research. Deneb's membership in the Summer Triangle alongside Vega and Altair makes it one of the most reliably identifiable stars in the northern summer sky, despite the real uncertainty that still surrounds its exact distance and luminosity.
Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Almagest (Mathematike Syntaxis)Claudius Ptolemy