Shedir – Wikipedia
from Wikipedia, L’Encilopedia Libera.
| Shedir | |
|---|---|
 A photograph of Shedir; The NGC 281 nebula is visible on its left. |
|
| Classification | Orange giant |
| Ghost class | K0 III |
| Distance from the sun | 229 ± 9 light years |
| Constellation | Cassopea |
| Coordinate | |
| (All’Epoca J2000) | |
| Retta Ascension | 00 h 40 m 30.44 s |
| Declination | +56 ° 32 ′ 14.39 ″ |
| Physical data | |
| Medium range | 45 [first] R ⊙ |
| Pasta | |
| Rotation speed | 21 km/s |
| Temperature superficial |
4.625 K [2] (media) |
| Brightness | |
| Color index (B-V) | 1.17 |
| Metallicity | Fe/H −0.09 [3] |
| Estimated age | 100-200 million years |
| Observative data | |
| Size app. | +2,25 |
| Size Ass. | -1.99 |
| Parallax | 14.27 ± 0.57 but |
| Proper | Ar: 50,36 mas/anno Dec: -32,17 mas / year |
| Radial speed | -4.31 km/s |
| Nomenclature alternative | |
Coordinate: 
00 h 40 m 30.44 s , +56 ° 32 ′ 14.39 ″
Shedir (α cas / α cassopeiae / alfa cassopeiae, also known as Shdar , Screen ( Fire : / Sknar / [4] ), Schedir , Shadar O Shedir ) is the brightest star of the Cassiopea constellation. Its proper name derives from the Arabic صدر, şadr , what does it mean bust . It derives from the fact that it is placed right in the heart of this mythological figure, usually represented as a seat and with a mirror in hand, intent on combing her hair.
Although Shedir, with an apparent magnitude of +2.25, both for the most part of the time the brightest star of the constellation, is sometimes overcome by the variable range Cassiopeiae, which can in certain periods reach the apparent magnitude of +2, 15. In addition, Beta Cassiopeiae, although less bright, is so little (only 3 cents of magnitude), that the difference is difficult to appreciate with the naked eye.
In W -shaped asterism, formed by the brightest stars of the constellation, Shedir occupies one of the two tops below, being the other formed by Delta Cassopeiae. It is placed a lot of north in the celestial sphere, like all the constellation, so much so that it appears to be circumstances in Canada, in most of the United States, throughout northern and central Europe and in almost all of Russia. Post at 57ºN, Shedir is invisible to the south of the 33rd parallel of the southern hemisphere, that is, in Argentina, South Africa and in part of Australia.
Since Shedir is approximately 229 light years from the earth and given its apparent brightness, an intrinsic brightness can be inferred which amounts to about 885 times that of the sun. More than the temperature, this considerable brightness is given by the large radiant surface of the star . In fact, Shedir’s surface temperature is 4,625 K [2] , a little lower than that of the sun which is about 5,700 k. This temperature means that Shedir is assigned to the ghost class K0. Shedir’s radius is 45 times the sun, as has been from direct measurements carried out with interferometric techniques [first] . In fact, the breadth of Shedir’s album, seen from the earth, turns out to be 5.62 ± 0.06 mas, which from distance of 229 light years, gives such value. Such a great range is not surprising since Shedir is classified as a giant star and consequently the MMK IIIA class has been assigned. The letter a It means that it stands among the brightest giants, so much so that it is at the limit between class III and class II (i.e. that of the bright giants).
This brilliance suggests that Shedir’s mass is 4-5 times the sun. 100-200 million years old, Shedir started his existence as a class B star, however, he exhausted hydrogen in his nucleus, who contracting and warming himself made swell and cool the surface layers of the star. His final destiny is to become a white dwarf C-O.
Shedir’s metallicity, that is, the abundance of elements other than hydrogen and heel, is -0.09 [3] . This means that the abundance of metals in Shedir is about 80%of that of the sun. Since in the sun this abundance is 1.6%, in Shedir it is about 1.28%.
A curious aspect of Shedir is that it was classified in countless 19th century studies [5] as a variable star. It was classified as this in an article 1937, in which it was stated that it varies from apparent magnitude 2.2 to 2.8 [6] . However, since more sophisticated instruments of brightness are available, such as electronic ones, Shedir does not seem to change its brightness [7] .
There are two hypotheses: either the variations of brightness observed in the past were due to measurement errors, which is however strange given their quantity, or Shedir was truly a variable in the past, but it is no longer since the early decades of the twentieth century, Which is equally strange because usually a variable star does not stop being suddenly such, although it can change the length and periods of variability. All that remains is to continue to observe this star and see if its variability will reappear.
- ^ a b A. R. Hajian, J. T. Armstrong, C. A. Hummel, J. A. Benson, D. Mozurkewich, T. A. Pauls, D. J. Hutter, N. M. Elias II, K. J.. Johnston, L. J, Rickard, N. M. White, Direct confirmation of stellar limb darkening with the navy prototype optical interferometer , in The Astrophysical Journal , vol. 496, 1998, pp. 484-489, two: 10.1086/305388 . URL consulted on February 7, 2010 .
- ^ a b S. hedges, J. Meléndez, Precise radial velocities of giant stars. III. Spectroscopic stellar parameters [ interrupted connection ] , in Astronomy and Astrophysics , vol. 475, 2007, pp. 1003-1009, two: 10.1051/0004-6361: 20078233 . URL consulted on February 7, 2010 .
- ^ a b B. W. Carney, D. F. Gray, D. Yong, D. W. Latham, N. Manset, R. Zelman, J. B. Laird, Rotation and macroturbulence in metal-poor field red giant and red horizontal branch stars , in The Astronomical Journal , vol. 135, 2008, pp. 892-906, two: 10.1088/0004-6256/135/3/892 . URL consulted on February 8, 2010 .
- ^ Screen , in Dictionary of the physical sciences , Rome, Institute of the Italian Encyclopedia, 1996. URL consulted on January 27, 2018 .
- ^ For example, R. Snow, Observations on the variability of α Cassiopeiæ , in Monthly Notices of the Royal Astronomical Society , vol. 5, 1841, p. 107. It is W. R. Birt, Variability of α Cassiopeiæ , in Astronomical register , vol. 5, 1867, pp. 138-139.
- ^ L. Campbell, Variable stars: Variables star notes from the American Association of Variable Star Observers; The light variations of Alpha Cassiopeiae , in Popular Astronomy , vol. 45, 1937, p. 96. URL consulted on February 8, 2010 .
- ^ Y. V. Voroshilov, V. G. Metlov, On the Brightness Constancy of the Red Giant Alpha-Cassiopeiae = HD3712 , in Astronomicheski Tsirkulyar , vol. 1503, 1987, p. 5. URL consulted on February 8, 2010 .
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