University of Wisconsin

Space Astronomy Laboratory



OBJECT: Alpha Orionis (Alf-Ori)
Other names: HD 39801
ID # 3606

UW Astronomer: Joni Johnson

Alpha Ori is the brightest star in the constellation of Orion; the right shoulder of Orion, the Hunter. It is a "red supergiant", with a luminosity of 14,000 Suns and a size that would engulf Mars' orbit if it were superposed on the Solar System. It is at a distance of 590 light years making it the closest of the red giants. As it swells and contracts it varies in brightness from 0.2 to 1.8 magnitude over a 5.7 year period. Stars become red supergiants when they have run out of their primary fusion fuel, hydrogen, just prior to fading into a white dwarf or going supernova. (Our sun will do the former in about 5 million years).

To locate Orion, turn toward the south in winter in the northern hemisphere. Orion will be the brightest constellation in the sky. From the southern hemisphere, it is visible from October to March, but upside down.

Recent ground-based polarimetric observations of red supergiants have shown a puzzling variety of polarization effects. The polarization from a starspot should rise dramatically toward the ultraviolet wavelengths where WUPPE will observe. Because these very red stars are about 100 - 1000 times fainter in the ultraviolet than in visible light, a rather long exposure is required on a visibly brighter star.