  | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Cassiopeia[1] |
| Right ascension | 00h 40m 30.4411s[2] |
| Declination | +56° 32′ 14.392″[2] |
| Apparent magnitude (V) | 2.240[3] |
| Characteristics | |
| Evolutionary stage | horizontal branch[4] |
| Spectral type | K0-IIIa[5] |
| U−Bcolor index | 1.14[6] |
| B−Vcolor index | 1.170[1] |
| Variable type | Suspected[7] |
| Astrometry | |
| Radial velocity (Rv) | −4.31 km/s |
| Proper motion (μ) | RA: 50.88[2]mas/yr Dec.: −32.13[2]mas/yr |
| Parallax (π) | 14.29±0.15 mas[2] |
| Distance | 228 ± 2 ly (70.0 ± 0.7 pc) |
| Absolute magnitude (MV) | −2.01[8] |
| Details[4] | |
| Mass | 3.98 M☉ |
| Radius | 42.15+1.45 −1.55[9] R☉ |
| Luminosity | 734±52[10] L☉ |
| Surface gravity (log g) | 1.73 cgs |
| Temperature | 4,625±42[9] K |
| Metallicity[Fe/H] | –0.2 dex |
| Rotational velocity (v sin i) | 6.71[11] km/s |
| Age | 220 Myr |
| Other designations | |
| Schedar,α Cas, Alpha Cas,18 Cas,BD+55°139,FK5 21,GC 792,HD 3712,HIP 3179,HR 168,SAO 21609,ADS 561,CCDM J00405+5632,WDS J00405+5632A | |
| Database references | |
| SIMBAD | data |
Alpha Cassiopeiae is a star in the northernconstellation ofCassiopeia. It has the proper nameSchedar, pronounced/ˈʃɛdɑːr/);[12][13]Alpha Cassiopeiae is itsBayer designation, which isLatinized fromα Cassiopeiae and abbreviated Alpha Cas or α Cas. Though listed as the "alpha star" byJohann Bayer, α Cas's visual brightness closely matches the 'beta' (β) star in the constellation (Beta Cassiopeiae) and it may appear marginally brighter or dimmer, depending on whichpassband is used. However, recent calculations fromNASA'sWISE telescope confirm that α Cas is the brightest in Cassiopeia, with anapparent magnitude of 2.240.[citation needed] Itsabsolute magnitude is 18 times greater than β Cas, and it is located over four times farther away from theSun.
α Cassiopeiae (Latinised toAlpha Cassiopeiae) is the star'sBayer designation.
It bore the traditional nameSchedar, which was first encountered in theAlfonsine tables of the thirteenth century. It derives from theArabic word صدرşadr, meaning "breast" (as in chest), a word which is derived from its relative position in the heart of the mythological queenCassiopeia.[14]Johannes Hevelius used the nameSchedir in his writings, although there were additional traditional spellings of this Arabictransliteration such asShedar,Shadar,Sheder,Seder,Shedis, andShedir. In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[15] to catalogue and standardize proper names for stars. The WGSN approved the nameSchedar for this star on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.[13]
Al-Sufi andUlug Beg named the starAl Dhāt al Kursiyy (Arabic ذات الكرسي, meaning "the lady in the chair"), whichGiovanni Battista Riccioli changed toDath Elkarti.[14]
In Chinese,王良 (Wáng Liáng) refers to the ChineseasterismWang Liang, a famouscharioteer during theSpring and Autumn period. The stellar pattern consists of Alpha,Beta,Kappa,Eta andLambda Cassiopeiae.[16] Consequently, theChinese name for Alpha Cassiopeiae itself is王良四 (Wáng Liáng sì, English:the Fourth Star of Wang Liang).[17]
With a declination of 56° 32' North, α Cassiopeiae is principally visible in theNorthern Hemisphere. The star is detectable to most observers across the globe reaching as far south asPerth, Australia,Santiago, Chile and other settlements north ± 33° South latitude, albeit close to the horizon. α Cassiopeiae is located in line-of-sight of theMilky Waygalaxy, so there are other notable celestial objects that can be viewed close to this star—such as thePacman Nebula,NGC 436 andNGC 457.
Alpha Cas reaches itszenith above cities likeEdinburgh,Copenhagen andMoscow. It iscircumpolar throughout Europe, Russia, and as far south as Los Angeles, California on the North American continent as well as other locations around the globe having a latitude greater than ± 33° North. Since α Cassiopeiae is listed as asecond-magnitude star (equal to Beta Cas), it is easily observable to the naked eye as long as one's stargazing is not hindered by thelight pollution common to most cities.
The best time for observation is during the late autumn months of the Northern Hemisphere, when Cassiopeia passes the meridian at midnight, but given its circumpolar nature in many northern localities, it is visible to many of the world's inhabitants throughout the year.
With the advances inoptical interferometry in the 1990s, α Cassiopeiae'sangular diameter was measured in 1998 at variouswavelengths ranging from 500 to 850 nm. The result was alimb darkened angular measurement of 5.62 ± 0.06milliarcseconds (mas),[18] a diameter which equates to roughly 0.393AU or 42.3 R☉, assuming a parallax of 14.29 mas.[19] With theplanetMercury orbiting the Sun at approximately 0.4 AU, α Cassiopeiae'sphotosphere extends to roughly half the mercurial orbit.
α Cassiopeiae is ared giant star whosespectral classification isK0-IIIa, notably cooler than theSun.[5] However, because it is nearing the final stages of itsevolution, the photosphere has expanded substantially, yielding abolometricluminosity that is approximately 800 L☉. It is considered 98% likely to be ahorizontal branch star fusing helium in its core.[4]
According toHipparcos, the New Reduction (van Leeuwen, 2007), the estimated distance to the star is about 70parsecs or 228light-years.[2] Like all giant stars, α Cassiopeiae rotates slowly with an approximate velocity of6.7 km/s[11]—a speed which takes the star approximately 102 days to make one complete revolution on its axis.[citation needed]
α Cassiopeiae has been sometimes classified as avariable star, but no variability has been detected since the 19th century.[21] Also, three companions to the star have been listed in theWashington Double Star Catalog, but it seems that all of them are just distant line-of-sightoptical components.[22][23][24]
α Cassiopeiae is thought to be around 100 to 200 million years old, having spent much of that time as a blue-whiteB-type main-sequence star.[21]
In 1551,Gerardus Mercator, aFlemishcartographer, produced a celestial globe portraying the 48 traditionalPtolemaic constellations in addition to two others,Coma Berenices andAntinous.[20] On this globe, he represents Cassiopeia as the Queen of Ethiopia, punished for her boasting by being chained to a chair hanging upside-down. α Cassiopeiae is found near her left breast, reflecting its Arabic etymological origin.
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