Alpha Cassiopeiae

Visibility

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 that can be viewed close to this star—such as the Pacman Nebula, NGC 436 and NGC 457.

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.

Properties

α Cassiopeiae is a red giant star whose spectral classification is K0-IIIa, notably cooler than the Sun. However, because it is nearing the final stages of its evolution, the photosphere has expanded substantially, yielding a bolometric luminosity that is approximately 800 L_☉. It is considered 98% likely to be a horizontal branch star fusing helium in its core.

According to Hipparcos, the New Reduction (van Leeuwen, 2007), the estimated distance to the star is about 70 parsecs or 228 light-years. Like all giant stars, α Cassiopeiae rotates slowly with an approximate velocity of 6.7 km/s—a speed which takes the star approximately 102 days to make one complete revolution on its axis.

α Cassiopeiae has been sometimes classified as a variable star, but no variability has been detected since the 19th century. 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.

α Cassiopeiae is thought to be around 100 to 200 million years old, having spent much of that time as a blue-white B-type main-sequence star.

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 5.42±0.06 milliarcseconds (mas).

Later measurements have been made since then, for example, observations with the CHARA Array yield an uniform disk angular diameter of 5.502±0.023 mas in the H band and 5.529±0.007 mas in the K band. The limb-darkened angular diameter combining both wavelengths is 5.664±0.010 mas, yielding a radius of 43.14 ± 1.50 R_☉ (0.2006 ± 0.0070 au; 30,010,000 ± 1,040,000 km) at the star's distance.

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. 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.