Home PageStar of the Month PageArticle Page


April's Star of the Month: "The Pup"


At evening dusk during early April, Sirius, the Dog Star, shines somewhat west of due south. But as the month parades onward, the night sky's brightest star slowly sinks into the western dusk, a sure sign of spring coming into summer. Eventually to fade into the sunset, Sirius' disappearing act during the summer is said to usher in the notorious "Dog Days" of helter-swelter.

Sirius is a binary star, or two stars orbiting a common center of mass. Sirius' companion star, Sirius B, also called "The Pup," is much smaller and dimmer than its "parent" star. Although faint, "The Pup" would be fairly easy to see through a telescope or good binoculars, if it weren't for the overpowering glare of nearby Sirius. "The Pup" is a white dwarf star, a stellar corpse that was once a red giant. It's thought that our Sun some billions of years into the future will also swell up to become a red giant, then collapse afterwards to become a white dwarf, an evolutionary scheme pictured for middle-of-the-road stars.

White dwarf stars are smaller than many planets, but are nonetheless much more massive. According to the astrophysicist Chandrasekhar, no white dwarf can exceed 1.4 solar masses. If the star were any more massive, it would shrink even further, either into a neutron star or a black hole. In a close binary star system, a white dwarf sometimes can pull enough material from its companion star to exceed the 1.4 solar mass limit and explode as a supernova. "The Pup," however, is not expected to explode, because it's too far distant from Sirius.

Cosmologists claim that their observations of white dwarf supernovae (called Type Ia) prove that the expansion rate of the universe is accelerating.


Your Weight on "The Pup"

April Feature Article