Beta Cassiopeiae
A Delta Scuti variable star in Cassiopeia.
Beta Cassiopeiae is a Delta Scuti variable star in the constellation of Cassiopeia. It has the proper name Caph and is a giant star of spectral class F2, with a mean apparent magnitude of +2.28. It is one of the five stars forming the 'W' of Cassiopeia and is notable as the fourth brightest Delta Scuti variable in the sky.
Quick Facts
- Proper name
- Caph
- Spectral class
- F2 III
- Apparent magnitude
- +2.28 (variable, range +2.25 to +2.31)
- Absolute magnitude
- +1.3
- Variable type
- Delta Scuti
- Rotation period
- 1.12 rotations per day
Facts from the source article.
Lore & Background
The traditional name Caph derives from the Arabic word for 'palm', referring to the pre-Islamic Arabic term al-Kaff al-Khadib 'the stained hand', which originally described the five stars of Cassiopeia's 'W'. This term was later abbreviated and came to signify Beta Cassiopeiae alone. The star was also known as al-Sanam al-Nakah 'the Camel's Hump'. In Chinese astronomy, Beta Cassiopeiae represents Wang Liang, a legendary charioteer from the Spring and Autumn period, and is part of an asterism that includes four other stars representing his horses. Together with Alpha Andromedae and Gamma Pegasi, it was one of the 'Three Guides' marking the equinoctial colure, an imaginary line running south through Alpha Andromedae to the celestial equator.
Reader's Guide
Beta Cassiopeiae holds significance as a bright northern star and a key component of the distinctive 'W' of Cassiopeia. Its variability as a Delta Scuti star, with a period of 2.5 hours and a brightness range of +2.25 to +2.31, makes it a notable object for variable star observers. The star is rotating at about 92% of its critical velocity, giving it an oblate spheroid shape with an equatorial bulge 24% larger than the polar radius, and a temperature difference of about 1,000 K between the polar region and equator. It is viewed nearly pole-on, with its axis inclined about 20 degrees to Earth's line of sight. Historically, it was one of the 'Three Guides' used for marking the equinoctial colure, and in 1572, Tycho's Star (SN 1572) appeared about 5 degrees to its northwest. The star was once considered a spectroscopic binary but is now thought to be single.
Names Across Civilizations
Few stars carry as many names as Beta Cassiopeiae, reflecting centuries of astronomical observation across cultures. Its Bayer designation, derived from the Greek letter beta, yields the Latinized form β Cassiopeiae, commonly shortened to Beta Cas. The star's proper name, Caph, traces back to the Arabic word kaf, meaning "palm," evoking an image of a hand reaching out from the Pleiades. Other historical Arabic names include Chaph, Kaff, and al-Sanam al-Nakah, or "the Camel's Hump." In 2016, the International Astronomical Union's Working Group on Star Names formally approved Caph as the standardized proper name. The pre-Islamic Arabic term al-Kaff al-Khadib, meaning "the stained hand," originally described all five stars forming Cassiopeia's distinctive W shape, picturing a hand adorned with henna. In Chinese astronomy, the star belongs to the asterism Wáng Liáng, named after a legendary charioteer of the Spring and Autumn period, with the other four stars representing his four horses. Together with Alpheratz and Algenib, Caph was also recognized as one of the "Three Guides" marking the equinoctial colure.
A Star in Transition
Caph is a yellow-white giant star classified as F2 III, more than three times the diameter of the Sun and roughly twenty-one times as luminous, with an absolute magnitude of +1.3. It was once a hotter A-type star with approximately twice the Sun's mass, but it is now in a transitional phase: its core has exhausted its hydrogen fuel and is contracting and heating, while the outer hydrogen envelope is swelling and cooling in the direction of a future red giant. Stars occupy this intermediate stage for a relatively brief span, making such objects somewhat rare in the sky. The star's corona is notably weak compared to what one might expect. Caph spins at about 92 percent of its critical rotational velocity, completing a full turn in roughly 1.12 days. This rapid spin produces a pronounced oblate spheroid shape, with the equatorial radius exceeding the polar radius by about 24 percent. The resulting temperature gradient means the polar regions run approximately 1,000 kelvin hotter than the equator. From Earth, we view the star nearly pole-on, with its rotational axis tilted only about 20 degrees from our line of sight. Although once suspected to be a spectroscopic binary with a faint companion on a 27-day orbit, Caph is now regarded as a single star.
Rhythmic Pulsation
Beyond its steady glow, Caph is a Delta Scuti variable star, the fourth brightest member of this class visible to the naked eye, trailing only Denebola, Vega, and Altair. It behaves as a monoperiodic pulsator, meaning its brightness oscillates with a single dominant period of approximately 2.5 hours, shifting between magnitudes +2.25 and +2.31. Delta Scuti variables are subgiants or late main-sequence stars of spectral types F5 through A0, with masses ranging from 1.5 to 2.5 solar masses, all approaching the end of their core hydrogen-burning lifetime. The physical mechanism driving their pulsations is tied to the same helium instability strip on the Hertzsprung–Russell diagram that governs classical Cepheid variables; Delta Scuti stars simply occupy the point where that strip intersects the main sequence. This connection places Caph in a fascinating evolutionary window: it is a star that has already left the main sequence behind, yet its pulsations still echo the same helium-driven instability that powers some of the most important distance indicators in astronomy. The short 2.5-hour period and modest amplitude make it a relatively gentle pulsator by the standards of its class.
A Landmark of the Northern Sky
With a mean apparent magnitude of +2.28 in the V-band, Caph anchors one corner of the iconic W-shaped asterism that defines Cassiopeia, sitting just beside the slightly brighter Schedar. Because it shines in the deep northern celestial hemisphere, the star is a familiar sight to observers across Europe, North America, and northern Asia, yet it remains largely invisible to those far south: in Tasmania the constellation never rises above the horizon, and even from Cairns in northern Australia it barely clears it. Caph also holds a place in the history of observational astronomy. In 1572, the supernova now called Tycho's Star erupted roughly five degrees to the northwest of Caph, a brilliant new light that appeared near this well-known star and drew the attention of Tycho Brahe. Additionally, Caph was one of three bright stars—along with Alpheratz in Andromeda and Algenib in Pegasus—traditionally called the "Three Guides." These three marked the equinoctial colure, an imaginary great circle running due south from Caph through Alpheratz down to the celestial equator, precisely where the Sun's annual path crosses the equator at each spring and autumn equinox.
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