MINTAKA (Delta Orionis). Orion is defined by his great belt, three bright second magnitude stars in a row that the ancient Arabs called "the string of pearls," which is the meaning of the name of the middle star, Alnilam. The two flanking stars, eastern Alnitak and western Mintaka, both come from Arabic phrases that mean "the belt of the Central One," the Central One the Arabic personification of our Orion, a woman lost to history. Though (at magnitude 2.21) Mintaka is the seventh brightest star in Orion, and the faintest of the three belt stars, it still received the Delta designation from Bayer, who lettered the belt stars in order from west to east before dropping down to Orion's lower half to continue the process. Of the sky's brightest stars, first through third magnitude, Mintaka is closest to the celestial equator, only a quarter of a degree to the south, the star rising and setting almost exactly east and west. The star is wonderfully complex. A small telescope shows a seventh magnitude companion separated by almost a minute of arc. At Mintaka's distance of 915 light years (very nearly the same as Alnitak at the eastern end of the belt), the faint companion orbits at least a quarter of a light year from the bright one. In between is a vastly dimmer 14th magnitude component. The bright star we call Mintaka (whose solo magnitude is 2.23) is ALSO double, and consists of a hot (30,000 Kelvin) class B, slightly evolved, giant star and a somewhat hotter class O star, each radiating near 90,000 times the solar luminosity (after correction for a bit of interstellar dust absorption), each having masses somewhat over 20 times the solar mass. This pair is too close to be separated directly. The duplicity is known through the star's spectrum (its rainbow of light), which detects two stars orbiting each other every 5.73 days, and also because the stars slightly eclipse each other, causing a dip of about 0.2 magnitudes. Mintaka is most famed. however, 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. From this discovery, and others that followed, we now know that all of the Galaxy's interstellar space contains an enormously complex medium of gas and dust that is the birthplace of new stars. Mintaka will also, to some distant generation of astronomers, be famed in death, as each of its components is so massive that their only fate is to explode violently as supernovae.
- - Written by Jim Kaler