Sheliak — ことの変光星

Sheliak, Beta Lyrae, is the prototype of an entire class of eclipsing binary stars — the Beta Lyrae variables. The system consists of two massive stars so close that they have warped into teardrop shapes by mutual tidal forces, with a stream of gas flowing from one component onto the other through an accretion disk. The primary is a B7 bright giant and the unseen secondary is a more massive early-B star buried in a thick disk. Total mass is about 15 Suns, distance roughly 960 light-years, and the orbital period is 12.94 days.

Sheliak's total brightness varies continuously between magnitudes 3.3 and 4.4 on the 12.94-day cycle — a change anyone can follow with the naked eye over two weeks. Compare it to nearby Gamma Lyr (magnitude 3.3) and Zeta Lyr (4.3). Sheliak lies right next to the famous Ring Nebula (M57) — they fit together in the same low-power telescope field, making this one of the sky's great photo opportunities. Best observed June through October.

The name Sheliak comes from the Arabic "al-shiliyāq," meaning "the harp" or "the lyre." The variability was discovered by John Goodricke in 1784 — only five years after his discovery of Algol's variability. Goodricke died at age 21, just days after his election to the Royal Society. The orbital geometry was worked out in the 20th century through extensive spectroscopic and photometric analyses.