Star Facts: Caph

Caph
Image Credit: The star Caph (Beta Cassiopeiae) by Anthony Ayiomamitis.

Caph (Beta Cassiopeiae) is a yellow-white Delta Scuti-class variable, F2-type giant star in the constellation Cassiopeia, with an apparent visual magnitude of +2.27. Along with the stars Alpheratz (Alpha Andromedae) and Algenib (Gamma Pegasi), Caph once formed a group of three bright stars known as the “Three Guides”. An imaginary line drawn due south from Caph, and passing through Alpheratz to the celestial equator marks the equinoctial colure, which is the point where the ecliptic intersects the celestial equator at each spring and autumnal equinox.

Quick Facts

• Constellation: Cassiopeia
• Coordinates: RA 00h 09m 10.68518s|Dec. +59° 08′ 59.2120″
• Distance: 54.7 light years
• Star Type: F2 III
• Mass: 1.91 sol
• Radius: 3.43–3.69 sol
• Apparent Magnitude: Variable (2.25 to 2.31)
• Luminosity: 27.3 sol
• Surface Temperature: 7,079K
• Rotational Velocity: 71 km/sec
• Age: 1.09–1.18 billion years
• Other Designations: Chaph, Kaff, Al Sanam al Nakah, 11Cas, Gl 8, BD+58°3, LHS 1027, HIP 746, CCDM J00092+5909

Visibility

While all the stars of the constellation Cassiopeia are best viewed from the northern hemisphere, southern hemisphere observers above latitude 33 degrees south can sometimes observe the constellation, albeit very low on their northern horizon. While Caph was once thought to have been a spectroscopic binary star with a faint companion in a 27-day orbit, no direct evidence of the companion star has been demonstrated definitively.

Physical Properties

Stellar Evolution

At its current state of evolution, Caph is about 28 times more luminous than the Sun, and at least three times as big as a result of it evolving into a red giant star with an absolute magnitude of +1.16. During its time on the main sequence, Caph was an A-type star with about twice the Sun’s mass, but like all stars evolving into giant stars, it has lost a significant percentage of its mass due to its energetic solar wind. Nonetheless, Caph’s corona is unusually weak, given its high rate of mass loss.

The star is almost certain to have used up all of its hydrogen fuel supply and its core is now shrinking and heating up, while its outer layers are expanding and cooling fairly rapidly. However, evolving stars only spend a relatively short time in this state, and are therefore relatively uncommon.

Variability

In terms of variability, Caph is the second brightest Delta Scuti variable star after Altair, and exhibits monoperiodic variations in brightness of between magnitudes +2.25 to +2.31 over a period of 2.5 hours. Typically, Delta Scuti variable stars are sub giant or main sequence stars that have stellar classifications of F5 to A0, are 1.5 to 2.5 times as massive as the Sun, and are close to exhausting their supply of hydrogen. On the H-R diagram, Delta Scuti variable stars inhabit the intersection of the helium instability strip with the main sequence, since their pulsations are related to those of classical Cepheid variables.

Rotational Velocity

Caph’s rotational velocity of 71 km/sec represents 92% of its critical speed, which is the speed at which it will start to fling off material into space. Its high spin rate means it completes 1.12 revolutions around its axis in a day, resulting in an oblate spheroid shape in which its equatorial diameter is 24% bigger than its polar diameter. This also results in the star having a higher temperature at the polar regions than at its equator, with this difference being about 1,000K.

History

One of the star’s traditional name derive from the modern Arabic word “kaf”, meaning “palm” in the sense that it represents the palm of a hand reaching out from the Pleiades. In pre-Islamic Arabic traditions, the star had the name “al-Kaff al-Khadib”, which referred to henna-stained hand represented by the five principal stars of the constellation. In this sense, the “stained hand” was a part of an old asterism called “Thuraya” that stretched from the Pleiades, through Taurus and Perseus into Cassiopeia, with the other “hand” being located in the constellation Cetus.

In China, the star Caph forms part of an asterism known as Wang Liang (after a famous charioteer), along with the stars κ Cassiopeiae, η Cassiopeiae, α Cassiopeiae and λ Cassiopeiae. Due to its position in this asterism, Caph is known in China as “Wáng Liáng yī”, which means “[the] First star of Wang Liang”.

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