15 Cancri

Lore & Background

15 Cancri was originally designated Psi Geminorum in Johann Bayer's Uranometria, but when constellation borders were set in 1930, it was moved to Cancer and that designation was dropped. Radial velocity measurements taken at the Dominion Astrophysical Observatory in Victoria, British Columbia, Canada in 1918 and 1919 revealed it to be a single-lined spectroscopic binary system. The first orbit was calculated in 1973 by Helmut Abt and Michael Snowden with a period of 585 days, though later measurements indicated an orbital period of 635 days.

The visible component, 15 Cancri A, is an Ap star—a chemically peculiar star with an over-abundance of iron peak elements, particularly silicon, chromium, and strontium, in its spectrum. Like all Ap stars, it possesses a strong magnetic field that varies as the star rotates. In 1968, the star's visual brightness was shown to vary regularly over about four days, and it was given the variable star designation BM Cancri in 1972 as a member of the α² Canum Venaticorum class. The period has since been measured more accurately and is believed to be the rotational period of the star.

Reader's Guide

15 Cancri holds significance as a representative of the α² Canum Venaticorum class of variable stars, which are characterized by strong magnetic fields and chemical peculiarities. Its variability, first noted in 1968, is tied to its rotation and magnetic field, making it a valuable object for studying stellar magnetism and chemical stratification in Ap stars. The star's history includes a change in constellation assignment—from Gemini to Cancer—reflecting the formalization of constellation boundaries in 1930. As a single-lined spectroscopic binary, it also contributes to the understanding of binary star dynamics, though the orbital period remains somewhat uncertain, with initial calculations giving 585 days and later measurements suggesting 635 days. Its faint naked-eye visibility (apparent magnitude about 5.6) and location roughly 700 light years away place it within reach of amateur telescopes for photometric monitoring. The star's designation as BM Cancri, assigned in 1972, formalizes its variable star status within the established α² CVn class.

Did You Know?

• Radial velocity measurements from 1918 and 1919 at the Dominion Astrophysical Observatory revealed it to be a single-lined spectroscopic binary.
• The orbital period was first calculated as 585 days in 1973, but later measurements gave 635 days.