Procyon B is a dim white dwarf star. A white dwarf is a stellar remnant composed mostly of electron-degenerate matter.
Procyon B is difficult to observe as it has a visual magnitude of only 10.82 (15000 times fainter than Procyon A) lying at just 5 seconds of arc from its primary Procyon A.
Procyon B is only 0.0006 as luminous as our Sun and only 0.02 its diameter (half the size of Earth). Because it is a white dwarf it still has 0.6 of our Sun’s mass having a density estimated to be around 0.33 tonne per cubic centimeter.
Procyon B is separated from Procyon A by an average of only 14.9 au roughly the distance between Uranus and though the eccentric orbit carries them as close as 8.9 au and as far as 21.0 au. The two components orbit each other with a period of 40.8 years.
With a surface temperature of 7740 K deduced from its spectrum Procyon B provides evidence of its lesser mass and greater age.
Procyon B is the degenerate remnant of what was once a much larger brighter and hotter star that eventually shed its outer envelope to leave behind the compact carbon-helium core.