Beta Cephei variable

Lore & Background

The variation in radial velocity of Beta Cephei was discovered by American astronomer Edwin Brant Frost in 1902, who initially concluded it was a spectroscopic binary. Paul Guthnick first detected a variation in brightness in 1913. Beta Canis Majoris and Sigma Scorpii were found to be variable not long afterward. Vesto Slipher noted in 1904 that Sigma Scorpii's radial velocity was variable, and R.D. Levee and Otto Struve concluded this was due to the star's pulsations in 1952 and 1955 respectively. These variables were often called Beta Canis Majoris variables because Beta Canis Majoris was the most closely studied example in the first half of the 20th century, though its location in the southern sky hampered observations. However, Beta Cephei was the first member of the class to be discovered, so they are generally called Beta Cephei variables despite the similarity of name with Cepheid variables.

Cecilia Payne-Gaposchkin and Sergei Gaposchkin catalogued 17 probable members of the class in their 1938 Variable Stars, though they classified them with Delta Scuti variables. 16 Lacertae was another star extensively studied before 1952. The number known jumped from 18 to 41 in 1966. Otto Struve studied these stars extensively in the 1950s, but research declined after his death. Christiaan L. Sterken and Mikolaj Jerzykiewicz classed 59 stars as definite and 79 more as suspected Beta Cephei variables in 1993. Stankov listed 93 members in a 2005 catalogue, plus 77 candidates and 61 poor or rejected stars. Six stars—Iota Herculis, 53 Piscium, Nu Eridani, Gamma Pegasi, HD 13745 (V354 Persei), and 53 Arietis—had been found to exhibit both Beta Cephei and SPB variability.

Reader's Guide

Beta Cephei variables are notable for being among the brightest stars in the sky, including Beta Crucis, Beta Centauri, and Spica (though Spica mysteriously stopped pulsating in 1970). Their pulsations are driven by the kappa mechanism and p-mode pulsations, where at a depth of 200,000 K an abundance of iron increases opacity rather than decreasing it, leading to energy buildup and cyclic expansion and contraction. This is known as the Fe bump or Z bump. A small number of stars with periods shorter than one hour form a short period group, designated BCEPS in the GCVS. In 2021, Beta Crucis became the first star of any kind to have its pulsation modes identified using polarimetric asteroseismology. The class remains an important laboratory for understanding stellar pulsation and interior structure, with ongoing cataloguing efforts distinguishing definite members from candidates and rejected stars.

Did You Know?

• Beta Cephei variables are also known as Beta Canis Majoris stars.
• Spica was classified as a Beta Cephei variable but mysteriously stopped pulsating in 1970.
• Their variation in brightness is much greater—up to 1 magnitude—in ultraviolet wavelengths.