CQ Ursae Majoris

Lore & Background

The star was identified as chemically peculiar by W. P. Bidelman in 1964, who noted strong strontium lines in its spectrum, a feature earlier observed by W. E. Harper in 1937. A. P. Cowley and C. R. Cowley later found abnormalities in both strontium and chromium lines. In 1972, E. W. Burke, Jr. and J. T. Howard discovered luminosity variability with a period of 1.706 days, but S. C. Wolff and N. D. Morrison in 1975 reported a longer period of 2.451 days, double the earlier estimate.

W. K. Bonsack in 1974 noted peculiarities with europium lines and found that lines of ionized calcium and chromium are variable. Z. Mikulášek in 1978 showed that ionized europium lines varied in antiphase with ionized chromium lines. In 1981, Y. V. Glagolevskij measured a large magnetic field, interpreting it as variable and reversing. Mikulášek fitted the observations to an oblique rotator model with a concentration of europium-titanium near the north magnetic pole, a patch of chromium-strontium near the southern pole, and a belt of enhanced iron along the magnetic equator.

CQ Ursae Majoris is classified as an Alpha2 Canum Venaticorum variable with stellar classification A2IVpSrCr. It is a magnetic Ap star where variability is modulated by the rotation period. The star spins with a period of 2.449967 days and has an estimated age of about 537 million years. It has double the mass and twice the radius of the Sun, radiating 19.5 times the Sun's luminosity from its photosphere at an effective temperature of 8,620 K. The magnetic field is inclined at 60° with a dipole field strength of roughly .

Reader's Guide

CQ Ursae Majoris holds significance as a well-studied example of a magnetic Ap star, where chemical peculiarities and variability are linked to rotation and a structured magnetic field. The discovery of its strontium and chromium anomalies, along with the antiphase variation of europium and chromium lines, provided early evidence for surface chemical patches on such stars. The oblique rotator model developed for CQ UMa, with distinct elemental concentrations at the magnetic poles and equator, has become a paradigm for understanding the distribution of elements in magnetic chemically peculiar stars. Its relatively bright magnitude (near naked-eye visibility) has allowed detailed spectroscopic and photometric monitoring over decades, contributing to the refinement of rotation periods and magnetic field measurements. The star's variability, with a period of about 2.45 days, is a classic example of rotational modulation in Ap stars, and its reversing magnetic field underscores the complex magnetohydrodynamics at work. As a reference object in the Alpha2 Canum Venaticorum class, CQ Ursae Majoris continues to inform models of stellar magnetism and chemical stratification.

Did You Know?

• CQ Ursae Majoris was found to be chemically peculiar by W. P. Bidelman in 1964, with strong strontium lines.
• The star's variability was discovered in 1972 by E. W. Burke, Jr. and J. T. Howard with a period of 1.706 days.
• Z. Mikulášek in 1978 showed that ionized europium lines vary in antiphase with ionized chromium lines.
• The star has a magnetic field inclined at 60° and a dipole field strength of roughly .