Miaplacidus is the second brightest star in the constellation Carina, at magnitude 1.68. It is also thus known as Beta Carinae, and it lies southeast of Canopus, the constellation's brightest star (and the 2nd-brightest in the sky).
Miaplacidus is the brightest star in an asterism known as the Diamond Cross, marking its southwestern end. Miaplacidus' position in Carina puts the star in the hold of the mythological ship Argo Navis, the vessel used by Jason and his Argonauts. Its name likely derives from the Arabic and Latin words for "Placid Waters", and first appeared on star maps in Elijah Hinsdale Burritt's 1856 "Geography of the Heavens".
Properties
Physically, β Car is a relatively common, class A2 subgiant star with an average surface temperature of 9100 K. At a distance of 111 light years, Miaplacidus has a luminosity of 210 suns, which yields a radius 5.85 times the Sun's. A direct measure of its angular diameter (1.59 milliarcseconds) agrees well, giving a radius of 5.83 times solar. A rapid rotational velocity of at least 139 km/sec gives it a rotation period under 2.1 days.
Evolution
Current stellar evolutionary theory implies that Miaplacidus has a mass between 3 and 3.1 times the Sun's, in accord with its spectral class. At an age of 350 million years, its core hydrogen fusion is ending, leaving a core of nearly pure helium. Helium fusion to carbon will begin in about 2.5 million years, when the star becomes a red giant. There is no evidence of any companion, and Miaplacidus seems devoid of any dusty disks that would imply planet formation has taken place.