“The Hubble picture reveals a huge ultraviolet atmosphere with a mysterious hot spot on the stellar behemoth's surface. The enormous bright spot, more than 10 times the diameter of Earth, is at least 2,000 degrees Kelvin hotter than the star's surface. Credit: Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA”

This information obtained from http://hubblesite.org/newscenter/archive/1996/04/.

Rigel:

According to the Macmillan Encyclopedia, “the star β Orionis…at magnitude 0.18 …is the seventh brightest star in the sky, and though designated β is actually the brightest star in Orion. Betelgeuse (β Orionis), now an irregular variable, may have been the brighter star when the Bayer letters were allocated in 1603” (6). According to data listed in MegaStar version 5.0, Rigel (Beta Orionis, 19 Orionis) has a spectral type B81a, which means its spectrum should show neutral helium and some hydrogen (4).

Rigel has an enormous luminosity estimated from 40,000 to 700,000 times that of the Sun (see below discussion), and it has a high surface temperature of 12,000K. Rigel is also a variable star and is a spectroscopic binary as well as having a companion 9.5 arcseconds away. The companion is easily visible in a small telescope. Rigel is part of a large association of bright, blue stars, including the stars of Orion's Belt, the Orion Nebula and its embedded stars, and many of the other hot blue-white stars in Orion, except Bellatrix which is a closer foreground star (5).

The Middle Three – Alnitak, Alnilam, and Mintaka

Alnitak, Alnilam, and Mintaka comprise the belt of Orion. They are supergiant blue stars all about second magnitude in brightness, easily visible from most city and suburban locations. Alnitak will eventually become a red supergiant somewhat like Betelgeuse (5). Though all three stars have similar colors, classes, and temperatures, Alnilam in the middle of Orion’s belt is by far the most luminous. Even though it is the farthest one away, it is the brightest visually. From Alnilam's somewhat uncertain distance of 410 parsecs, it is estimated to have luminosity up to 375,000 times that of the Sun, and it has a very high surface temperature of 25,000 K. It illuminates a faint nebulous cloud. “Alnilam has served for many years as a ‘standard star’ against which to compare others. The star will shortly turn into a magnificent red supergiant far more luminous than nearby (on the sky) Betelgeuse, its only fate someday to explode” (5).

Alnitak, the eastern star in Orion’s belt, is the brightest O star in the sky. It has a companion, a blue B star about three seconds of arc away. The region around Alnitak contains several dusty clouds of interstellar gas, including the Horsehead Nebula (IC434) to the south and NGC2024 to the north (4).

Mintaka is part of a multiple star system, and Mintaka itself is a spectroscopic double star, consisting of a hot (30,000 K) B giant star and a somewhat hotter class O star, each radiating about 70,000 times the solar luminosity, and each having masses somewhat over 20 times the Sun (5). “Mintaka is most famed …as a background against which the thin gas of interstellar space was first detected, when the German astronomer Johannes Hartmann in 1904 discovered absorptions in the star's spectrum that could not be produced by the orbiting pair” (5).

The Leftovers – Bellatrix, Meissa, and Saiph

Bellatrix was once thought to be part of the physical association of stars in Orion, but it is much closer than the other bright stars in Orion. It is now considered to be independent of the other stars, and it may be an incidental “foreground” star (5). Bellatrix was at one time assumed to have a steady brightness, but it has also been found to vary a small amount over an irregular period (5). Stable stars with no variations in brightness over long periods are not common, and we are lucky the Sun, a G2 star in middle age, is so stable (7).

Back | Next |