Deneb

Deneb /ˈdɛnɛb/ is a first-magnitude star in the constellation of Cygnus, the swan. Deneb is one of the vertices of the asterism known as the Summer Triangle and the 'head' of the Northern Cross. It is the brightest star in Cygnus and the 19th brightest star in the night sky, with an average apparent magnitude of 1.25. A blue-white supergiant, Deneb rivals Rigel as the most luminous first magnitude star. However its distance, and hence luminosity, is poorly known; its luminosity is somewhere between 55,000 and 196,000 times that of the Sun. Its Bayer designation is α Cygni which is Latinised to Alpha Cygni, abbreviated to Alpha Cyg or α Cyg. α Cygni (Latinised to Alpha Cygni) is the star's designation given by Johann Bayer in 1603. The traditional name Deneb is derived from the Arabic word for 'tail', from the phrase ذنب الدجاجة Dhanab al-Dajājah, or 'tail of the hen'. In July 2016, the Working Group on Star Names of the International Astronomical Union recognised the name Deneb for this star, and this name is entered in their Catalog of Star Names. An older traditional name is Arided /ˈærɪdɛd/, from the Arabic ar-ridf 'the one sitting behind the rider' (or just 'the follower'), perhaps referring to the other major stars of Cygnus, which were called al-fawāris 'the riders'. The 19th brightest star in the night sky, Deneb culminates each year on October 23 at 6 PM and September 7 at 9 PM, corresponding to summer evenings in the northern hemisphere. 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. In the southern hemisphere, Deneb is not visible south of 45° parallel south, so it just barely rises above the horizon in South Africa, southern Australia, and northern New Zealand during the southern winter. Deneb is located at the tip of the Northern Cross asterism made up of the brightest stars in Cygnus, the others being Albireo (Beta Cygni), Gamma Cygni, Delta Cygni, and Epsilon Cygni. It also lies at one vertex of the prominent and widely spaced asterism called the Summer Triangle, shared with the first-magnitude stars Vega in the constellation Lyra and Altair in Aquila. This outline of stars is the approximate shape of a right triangle, with Deneb located at one of the acute angles. Due to the Earth's axial precession, Deneb will be an approximate pole star (7° off of the north celestial pole) at around 9800 AD. Deneb's adopted distance from the Earth is around 802 parsecs (2,620 ly). This is derived by a variety of different methods, including spectral luminosity classes, atmospheric modelling, stellar evolution models, assumed membership of the Cygnus OB7 association, and direct measurement of angular diameter, but these methods give different distances and all have significant margins of error. The original derivation of a parallax using measurements from the astrometric satellite Hipparcos gave an uncertain result of 1.01 ± 0.57 mas that was consistent with this distance. However, a more recent reanalysis gives the much larger parallax whose distance is barely half the current accepted value. One 2008 calculation using the Hipparcos data puts the most likely distance at 475 parsecs (1,550 ly), with an uncertainty of around 15%. The controversy over whether the direct Hipparcos measurements can be ignored in favour of a wide range of indirect stellar models and interstellar distance scales is similar to the better known situation with the Pleiades. Deneb's absolute magnitude is currently estimated as −8.4, placing it among the visually brightest stars known, with an estimated luminosity nearly 200,000 L☉. This is towards the upper end of values published over the past few decades, which vary between L☉ and 196,000 L☉.