Xi<sup>1</sup> Canis Majoris is a blue-white variable star in the constellation Canis Major. Its name is a Bayer designation that is Latinized from þ<sup>1</sup> Canis Majoris, and abbreviated Xi<sup>1</sup> CMa or þ<sup>1</sup> CMa. This star is visible to the naked eye with an apparent visual magnitude of +4.3. Based on an annual parallax shift of , it is located at a distance of approximately from Earth.
The stellar classification of Xi<sup>1</sup> Canis Majoris matches an early-type B-type star. It has generally been assigned a luminosity class of III (giant) or IV (subgiant), for example B1III or B0.5IV. Comparison of its properties with model evolutionary tracks suggest that it is a main sequence star about three quarters of the way through its main sequence lifetime. Its estimated age is 11 million years. This is a massive star with 14 times the mass of the Sun and 8 times the Sun's radius. It is radiating 30,900 times the luminosity of the Sun from its photosphere at an effective temperature of 27,000 K.
þ<sup>1</sup> Canis Majoris is classified as a Beta (ò) Cephei variable that ranges from magnitude +4.33 to +4.36 with a period of 5.03 hours. This period has remained very stable, changing by no more than one second per century. Its pulsations cause its radius to vary by 1.0% to 1.5%. At the same time its effective temperature varies by about above and below its mean temperature.
It has the longest known rotation period of any B class star, taking around 30 years to complete one revolution on its axis. This is thought to be due to magnetic braking; þ<sup>1</sup> Canis Majoris has the strongest magnetic field of any ò Cephei star and would be expected to spin down completely in around four million years. It also has the strongest and hardest X-ray emission of any ò Cephei star. The X-ray emission varies in phase with the optical pulsations. The stellar wind from þ<sup>1</sup> Canis Majoris is very weak with a terminal velocity of ; the star is losing less than ÷yr<sup>âÂÂ1</sup>.
This star forms a naked eye pairing with þ<sup>2</sup> Canis Majoris a little less than a degree away. The Washington Double Star Catalog lists two 14th magnitude companions at an angular separation of about . In addition, an unseen close companion is suspected due to some faint emission lines in the spectrum that are best explained by a Be star that is invisible against the brighter primary.