HD 54879


HD 54879 is a bluish-hued star in the southern constellation of Canis Major, close to the border with Monoceros. It is too faint to be seen by the naked eye, having an apparent magnitude of 7.65, but can be readily observed using a pair of binoculars. The star is located some distant according to Gaia EDR3 parallax measurements, and is moving away from the Solar System at a heliocentric radial velocity of.

Stellar properties

This is an O-type main-sequence star with a stellar classification of O9.7V, indicating that it is near the border between the spectral types O and B. It radiates 28,200 times the luminosity of the Sun from its photosphere at an effective temperature of, over five times hotter than the Sun. Its mass and radius are not well-constrained due to uncertainties in the star's distance, but simulations show that it likely formed with a mass of 16 ± 1 about 5 million years ago.

Magnetic field

The star was found to have a strong magnetic field in 2014 with an average strength of 2,954 G and a dipole strength of 3,939 G, thousands of times stronger than Earth's magnetic field. Its strength slowly varies with a period of 7.41 years. This is related to the extremely slow rotation of HD 54879 with a period of 7.02 years, the second longest recorded in an O-type star after HD 108.
Since the magnetic fields trap stellar winds to an Alfvén radius of ≳12 times the stellar radius, HD 54879 experiences very little mass loss, estimated at 6.3 /yr, compared to the ~1 /yr if the star did not possess a magnetic field. The entrapped gas forms a disk around the star in the equatorial plane of the magnetic field, which is probably the source of the Hα emissions seen in the star's spectra.

Element distribution

In 2021, a spectroscopic study proposed the possibility that the stellar atmosphere was inhomogeneous in elemental makeup, with helium lines forming at much higher altitudes than the emission lines of oxygen and silicon. This, however, was refuted in 2024, and a more uniform chemical composition is now favored.

2019 controversy

In 2019, a research team announced that a sudden spike occurred in the strength of the magnetic field, the spectral type of the star shifted from O9.7V to B2V, and that the star displays radial velocity variations indicative of a long-period binary. These findings were later retracted, the apparent changes attributed to an inadequate signal-to-noise ratio related to instabilities in the data reduction pipelines. However, a follow-up study by another team was unable to confirm such instabilities and instead accredited the faulty results to human error.