Alpha Cassiopeiae

Alpha Cassiopeiae or α Cassiopeiae, also named Schedar (Template:IPAc-en),<ref>Template:MW</ref><ref name="IAU-LSN">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> is a second-magnitude star in the northern constellation of Cassiopeia. Though listed as the "alpha star" by Johann 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 which passband is used. However, recent calculations from NASA's WISE telescope confirm that α Cas is the brightest in Cassiopeia, with an apparent magnitude of 2.240. Its absolute magnitude is 18 times greater than β Cas, and it is located over four times farther away from the Sun.

Nomenclature

α Cassiopeiae (Latinised to Alpha Cassiopeiae) is the star's Bayer designation.

It bore the traditional name Schedar, which was first encountered in the Alfonsine tables of the thirteenth century. It derives from the Arabic word صدر şadr, meaning "breast" (as in chest), a word which is derived from its relative position in the heart of the mythological queen Cassiopeia.<ref name="ALLEN">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Johannes Hevelius used the name Schedir in his writings, although there were additional traditional spellings of this Arabic transliteration such as Shedar, Shadar, Sheder, Seder, Shedis, and Shedir. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)<ref name="WGSN">Template:Citation</ref> to catalogue and standardize proper names for stars. The WGSN approved the name Schedar for this star on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.<ref name="IAU-LSN"/>

Al-Sufi and Ulug Beg named the star Al Dhāt al Kursiyy (Arabic ذات الكرسي, meaning "the lady in the chair"), which Giovanni Battista Riccioli changed to Dath Elkarti.<ref name="ALLEN"/>

In Chinese, {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}) refers to the Chinese asterism Wang Liang, a famous charioteer during the Spring and Autumn period. The stellar pattern consists of Alpha, Beta, Kappa, Eta and Lambda Cassiopeiae.<ref>Template:In lang 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, Template:ISBN.</ref> Consequently, the Chinese name for Alpha Cassiopeiae itself is {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}, Template:Langx).<ref>Template:In lang 香港太空館 - 研究資源 - 亮星中英對照表 Template:Webarchive, Hong Kong Space Museum. Accessed on line November 23, 2010.</ref>

Visibility

File:Cassiopeia in Milky Way.png

Cassiopeia in the context of the Milky Way

With a declination of 56° 32' North, α Cassiopeiae is principally visible in the northern hemisphere. The star is detectable to most observers across the globe reaching as far south as Perth, Australia, Santiago, Chile and other settlements north ± 33° South latitude, albeit close to the horizon. α Cassiopeiae is located in line-of-sight of the Milky Way galaxy, so there are other notable celestial objects one can view close to this star—the Pacman Nebula, NGC 436 and NGC 457 being a few.

Alpha Cas reaches its zenith above cities like Edinburgh, Copenhagen and Moscow. It is circumpolar 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 a second-magnitude star (equal to Beta Cas), it is easily observable to the naked eye as long as one's stargazing is not hindered by the light 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.

Angular analysis

With the advances in optical interferometry in the 1990s, α Cassiopeiae's angular diameter was measured in 1998 at various wavelengths ranging from 500 to 850 nm. The result was a limb darkened angular measurement of 5.62 ± 0.06 milliarcseconds (mas),<ref name="HAJIAN">Template:Cite journal</ref> a diameter which equates to roughly 0.393 AU or Template:Solar radius, assuming a parallax of 14.29 mas.<ref name="NOTERADIUS group=note">To determine α Cassiopeiae's average radius in terms of solar units, the calculations begin with the formula for angular diameter as follows:

<math>\begin{smallmatrix} {\delta} = \frac{d_S}{D_S}\end{smallmatrix}</math>

where <math>{\delta}</math> equals the angular diameter of α Cassiopeiae in arcseconds, <math>{d_S}</math> the star's diameter in AU, and <math>{D_S}</math> the Distance from Earth in parsecs. If one knows the angular diameter and the Distance, then one can solve for <math>{d_S}</math> as follows:

<math>\begin{smallmatrix}d_S = \delta \cdot D_S = {0.00562} \cdot 70.0 = 0.393 AU\end{smallmatrix}</math>

To obtain α Cassiopeiae's radius:

<math>\begin{smallmatrix}R_S = {\left ( {\frac {d_S}{2}} \right )} = {\left ( {\frac {0.393}{2}} \right )} = 0.197 AU\end{smallmatrix}</math>

To convert 0.197 AU into Solar units, the math is straightforward. Since 1 AU = 149,597,871 km and the mean diameter of the Sun = 1,392,000 km (hence a mean radius of 696,000 km), the calculation is as follows:

<math>\begin{smallmatrix}d_S = {\left ( 0.197 AU \right )} {\left ( {\frac {149,597,871 km}{696,000 km}} \right )} = 42.31 R_{\odot} (rounded)\end{smallmatrix}</math>.</ref> With the planet Mercury orbiting the Sun at approximately 0.4 AU, α Cassiopeiae's photosphere extends to roughly half the mercurial orbit.

Properties

File:Cassiopeia starfield.jpg

Cassiopeia starfield showing α Cas, the orange giant, in relation to the other stars in the constellation

α Cassiopeiae is a red giant star whose spectral classification is K0-IIIa, notably cooler than the Sun.<ref name=perkins/> However, because it is nearing the final stages of its evolution, the photosphere has expanded substantially, yielding a bolometric luminosity that is approximately Template:Solar luminosity. It is considered 98% likely to be a horizontal branch star fusing helium in its core.<ref name=reffert2015/>

According to Hipparcos, the New Reduction (van Leeuwen, 2007), the estimated distance to the star is about 70 parsecs or 228 light-years.<ref name="LEEUWEN"/> Like all giant stars, α Cassiopeiae rotates slowly with an approximate velocity of Template:Val<ref name=hekker2007/>—a speed which takes the star approximately 102 days to make one complete revolution on its axis.Template:Cn

File:Mercatur Cassiopeia.jpg

CitationClass=web }} </ref>

α Cassiopeiae has been sometimes classified as a variable star, but no variability has been detected since the 19th century.<ref name="kaler">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Also, three companions to the star have been listed in the Washington Double Star Catalog, but it seems that all of them are just distant line-of-sight optical components.<ref>Template:Cite DR2</ref><ref>Template:Cite DR2</ref><ref>Template:Cite DR2</ref>

α Cassiopeiae is thought to be around 100 to 200 million years old, having spent much of its time as a blue-white B-type main-sequence star.<ref name="kaler"/>

Depiction

In 1551, Gerardus Mercator, a Flemish cartographer, produced a celestial globe portraying the 48 traditional Ptolemaic constellations in addition to two others, Coma Berenices and Antinous.<ref name="HARVARD"/> 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.

External links

• Harvard Map Collection The Mercator Globes
• The Internet Encyclopedia of Science: Shedar (Alpha Cassiopeiae)