Nusakan — かんむりの二重星

Nusakan, Beta Coronae Borealis, is a magnetic chemically-peculiar (Ap) star of spectral type A8V located 114 light-years away in the constellation Corona Borealis. It is part of a binary system with an F2V companion in a 10.5-year orbit. The primary's surface is pockmarked with unusual abundances of chromium, strontium, and europium, held in chemically-concentrated spots by a strong global magnetic field — a tell-tale signature of the Ap class.

Nusakan marks the western end of Corona Borealis's graceful semicircular crown, west of Alphecca (the brightest crown star). It is a naked-eye object at magnitude 3.68. The binary companion is too close (< 0.3 arcseconds) to split visually, but the primary rotates once every 18.5 days, and over that cycle its surface element-patches produce measurable magnitude and color changes — tiny but real variability. Best observed April through September.

The name Nusakan comes from a now-obscure Arabic astronomical text, possibly from "al-nasaqān," meaning "the two rows" or "lines," referring to the shape of the crown. The Ap peculiarity was first noted in the 1920s, and the strong magnetic field was mapped in detail by Zeeman-Doppler imaging in the 1990s and 2000s.