Alpha Cygni variable

Alpha Cygni variables are variable stars which exhibit non-radial pulsations, meaning that some portions of the stellar surface are contracting at the same time others parts expand. They are supergiant stars of spectral types B or A. Variations in brightness on the order of 0.1 magnitudes are associated with the pulsations, which often seem irregular, due to beating of multiple pulsation periods. The pulsations typically have periods of several days to several weeks.

The prototype of these stars, Deneb (α Cygni), exhibits fluctuations in brightness between magnitudes +1.21 and +1.29. Small amplitude rapid variations have been known in many early supergiant stars, but they weren't formally grouped into a class until the 4th edition of the General Catalogue of Variable Stars was published in 1985. It used the acronym ACYG for Alpha Cygni variable stars. Many luminous blue variables (LBVs) show Alpha Cygni-type variability during their quiescent (hot) phases, but the LBV classification is generally used in these cases.

Deneb

Deneb (/ˈdɛnɛb/; α Cyg, α Cygni, Alpha Cygni) is the brightest star in the constellation Cygnus, it is one of the vertices of the Summer Triangle and forms the 'head' of the Northern Cross. It is the 19th brightest star in the night sky, with an apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous nearby stars. However, its exact distance (and hence luminosity) has been difficult to calculate; it is estimated to be somewhere between 55,000 and 196,000 times as luminous as the Sun.

Visibility

Deneb lies at one vertex of a widely spaced asterism called the Summer Triangle, the other two members of which are the zero-magnitude stars Vega in the constellation Lyra and Altair in Aquila. This formation is the approximate shape of a right triangle, with Deneb located at one of the acute angles. The Summer Triangle is recognizable in the northern skies for there are few other bright stars in its vicinity.

Deneb is also easily spotted as the tip of the Northern Cross asterism made up of the brightest stars in Cygnus, the others being Beta (Albireo), Gamma, Delta, and Epsilon Cygni. It never dips below the horizon at or above 45° north latitude, just grazing the northern horizon at its lowest point at such locations as Minneapolis, Montréal and Turin.