Deneb
Deneb is the proper name of the star Alfa Cygni (α Cyg / 50 Cyg), the brightest in the constellation Cygnus ("The Swan") and one of the brightest in the night sky, with an apparent magnitude in band B (blue filter) equal to 1.34 and in band V (green filter) equal to 1.25. Together with Vega (α Lyrae) and Altair (α Aquilae) they form the "summer triangle" asterism for observers in the northern hemisphere.
Name
The name Deneb derives from the Arabic dhaneb, "tail", understood as "the tail of the bird" (dhanab ad-dajājah), in clear allusion to its position within the alignment of stars that make up the figure of the Swan. It is a very common term in other stars, which indicates the caudal extremity of some constellations, such as Denebola in Leo or Deneb Kaitos in the Whale. Deneb Adige and Deneb el Adige are long forms of the same name.
Other traditional Deneb denominations, such as Arided —which appears in the Alfonsíes Tables—, Aridif and its variant Arrioph, are nowadays in disuse. Chicken was a name used by Johann Bayer to designate this star.
Observation
The 19th brightest star in the night sky, Deneb culminates each year on October 23 at 6 a.m. and September 7 at 9 a.m., corresponding to the evenings summer in the Northern Hemisphere. It never dips below the horizon at or above 45° north latitude, just skimming the northern horizon at its lowest point in places like Minneapolis, Montreal, and Turin. In the southern hemisphere, Deneb is not visible south of 45° south, so it barely rises above the horizon in South Africa, south of Australia, and north of New Zealand during the austral winter.
Deneb lies at the tip of the Northern Cross asterism formed by the brightest stars in Cygnus, the others being Albireo (Beta Cygni), Gamma Cygni, Delta Cygni, and Epsilon Cygni. prominent and widely spaced asterism called the Summer Triangle, shared with the 1st magnitude star Vega in the constellation of Lyra and Altair in Aquila. This star outline is roughly in the shape of a right triangle, with Deneb located in one of the acute angles.
Astronomers have observed the spectrum of Alpha Cygni since at least 1888, and by 1910 the variable radial velocity had become apparent. This led to the initial suggestion by E. B. Frost that this is a binary star system. By 1935, work by G. F. Paddock and others had established that this star was variable in luminosity with a dominant period of 11.7 days and possibly with other periods of lesser amplitude. In 1954, closer examination of the star's H and K calcium lines showed a stationary core, indicating that the variable velocity was instead being caused by the motion of the star's atmosphere. star. This variation ranged from +6 to −9 km/s around the mean radial velocity of the star. Other similar supergiants were found to have variable velocities, with this star being a typical member.
Pole Star
Due to Earth's axial precession, Deneb will be an approximate pole star (7° off the north celestial pole) around 9800 AD. C. The north pole of Mars points to the midpoint of the line connecting Deneb and the star Alderamin.
Preceded by | Polar star | Success |
---|---|---|
Alderamin | 8700 d.C. to 11000 d.C. | Delta Cygni |
Distance and physical characteristics
Deneb's distance from Earth is still controversial: due to its great distance, methods of determining distance using parallax provide a distance range of up to 3,200 light-years. The most precise measurements—carried out by the Hipparcos satellite—place Deneb only 1,425 light-years away, resulting in a luminosity 54,400 times that of the Sun. Its absolute magnitude is estimated at -7.2, so that, by way of comparison, its luminous power is such that in a single day it generates as much energy as the Sun in 140 years. At that magnitude, its brightness would be comparatively higher even than some industrial lasers.
Deneb is a rare type of white supergiant with spectral class A2Iae and a surface temperature of 8525 K. The direct measurement of its angular diameter —0.0025 seconds of arc— allows us to calculate its size, 210 times that of the Sun. If it were located in the center of the solar system, it would extend halfway around Earth's orbit, and in Earth's sky it would appear as a 55° 52' disk. of angular diameter approximately (considering the maximum radius of the star). Its projected rotation speed—30 km/s—results in a rotation period of approximately half a year.
A little over 10 million years ago, Deneb began life as a B or O-type star with a mass of 15-16 solar masses. A stellar wind blows from its surface, causing the star to lose mass at the rate of one millionth of the solar mass per year, 40 million times greater than that experienced by the Sun. At present Deneb has finished the fusion of hydrogen in its nucleus, but it is not known with certainty in what phase of its evolution it is; it may be in the process of becoming a red supergiant with an inert helium core, or, more advanced in its evolution, it may already have begun fusing helium in its core. In any case, it will end its life by exploding as a supernova within a few million years.
Deneb is the prototype of a class of variables called Alpha Cygni variables. Its surfaces experience nonradial pulsations that cause its brightness and spectral type to vary slightly.
As one of the brightest stars in the firmament, Deneb was a fair intermediate Pole Star (located 7° from the North Celestial Pole) 18,000 years ago, and will return to that position again around AD 9,800. C.