Gamma Cassiopeiae

Lore & Background

γ Cassiopeiae (Latinized to Gamma Cassiopeiae, abbreviated Gamma Cas or γ Cas) is the object's Bayer designation, and it has the Flamsteed designation 27 Cassiopeiae. Although it is a fairly bright star with a combined apparent visual magnitude of 2.47, it had no traditional Arabic or Latin name. The Chinese name Tsih, 'the whip', is commonly associated with this star. The name however originally referred to κ Cassiopeiae, and γ Cassiopeiae was just one of four horses pulling the chariot of legendary charioteer Wangliang. This representation was later changed to make Gamma the whip. The IAU Working Group on Star Names approved the name Tiansi for γ Cassiopeiae Aa on 30 June 2020, after the older Chinese name. The star was used as an easily identifiable navigational reference point during space missions and American astronaut Virgil Ivan 'Gus' Grissom nicknamed the star Navi after his own middle name spelled backwards.

Reader's Guide

Gamma Cassiopeiae is an eruptive variable star. It is the prototype of the class of Gamma Cassiopeiae variable stars. In the late 1930s it underwent a shell episode and the brightness increased to above magnitude 2.0, then dropped rapidly to 3.4. At maximum intensity, γ Cassiopeiae outshined both Alpha Cassiopeiae (Schedar; magnitude 2.25) and Beta Cassiopeiae (Caph; 2.3). It has since been gradually brightening back to around 2.2. Observations by the TESS satellite in November 2019 showed amplitudes of no more than 2%. Gamma Cassiopeiae is a rapidly spinning star with a projected rotational velocity of , giving it a pronounced equatorial bulge. When accounted for its axial inclination, the true value is found to be . The equatorial radius is and the polar radius is . When combined with the star's high luminosity, the result is the ejection of matter that forms a hot circumstellar disk of gas. The emissions and brightness variations are apparently caused by this 'decretion disk'. The spectrum of this massive star matches a stellar classification of B0.5 IVe. The 'e' suffix is used for stars that show emission lines of hydrogen in the spectrum, caused in this case by the circumstellar disk. This places it among a category known as Be stars; in fact, the first such star ever to be so designated. A luminosity class of IV suggest it is a subgiant star that has reached a stage of its evolution where it is exhausting the supply of hydrogen in its core region and transforming into a giant star, although it is modelled to be only about a third of the way through its main-sequence life after a relatively brief 8 million years. The outer atmosphere has an intense effective temperature of , which is causing it to glow with a blue-white hue. It has 17 times the Sun's mass and is radiating as much energy as 19,000 Suns. Gamma Cassiopeiae exhibits characteristics consistent with a strong disordered magnetic field. No field can be measured directly from the Zeeman effect because of the star's rotation-broadened spectral lines. Instead, the presence of this field is inferred from a robust periodic signal of 1.21 days that suggests a magnetic field rooted on the rotating star's surface. The star's UV and optical spectral lines show ripples moving from blue to red over several hours, which indicates clouds of matter being held frozen over the star's surface by strong magneti

Did You Know?

• Gamma Cassiopeiae was observed with emission lines in 1866 by Angelo Secchi, though P Cygni had its emission lines noted earlier in 1865 by Secchi himself.
• The star was nicknamed 'Navi' by astronaut Gus Grissom, his middle name spelled backwards.
• It is the prototype of the Gamma Cassiopeiae variable stars.
• The system could contain a total of eight stars, including its common-proper-motion companion HD 5408.