THETA-1 ORI (Theta-1 Orionis). Dropping nearly straight down from Orion's brilliant three-star Belt (from right to left Mintaka, Alnilam, and Alnitak, or Delta, Epsilon, and Zeta Orionis) is the Hunter's three-star "Sword," in the middle of which is one of the great sights of the telescopic sky, the Orion Nebula. Also known as Messier 42, the Nebula is an immensely complex cloud of dusty gas 1400-1500 light years away and 20 light years across (depending on just where you draw the ill-defined boundaries) that is made to fluoresce by the hot stars of Theta-1 Orionis, which are situated directly in front of it. (The Orion Nebula is a "blister" on the face of the great Orion Molecular Cloud, which lies behind the Hunter and hosts several sites of active star formation. Though invisible to the eye, it glows brightly in the radio spectrum). Even a small telescope shows Theta-1 to be a quartet, which carries a group name, the "Trapezium," from west to east labelled Theta-1 A, B, C, and D. Clumped within a span of 22 seconds of arc (10,000 Astronomical Units), all are hot class O and B stars that together make an apparent "single star" of magnitude 5 (4.7). Taken separately, from A through D, they are of magnitude 6.7, 8.0, 5.1, and 6.7 and blue spectral classes B1, B0, O6, and B0.5. All contribute to the energetic ultraviolet light that energizes (ionizes, stripping electrons from atoms) the Orion Nebula. By far the leader of the pack is Theta-1 C, a great 40-solar-mass star with a temperature of 40,000 Kelvin (making it the hottest "naked eye" star, though the 4 are inseparable without optical aid), a huge luminosity 210,000 times that of the Sun (85 percent of the Trapezium's total), and a 1000 kilometer/second wind with 100,000 times the flow rate of the solar wind. The power of the star is such that it is evaporating dusty disks around nearby new stars that in other settings might form planets. The other members of the Trapezium pale only in comparison with "C," all containing over 10 solar masses. A main interest lies in their multiplicity. Theta-1 A is an eclipsing double also known as V 1016 Ori. Every 65 days, the star dips by a magnitude as a star still in the process of formation just one Astronomical Unit away passes in front of the bright component, the whole thing watched by another companion 100 AU off. Theta-1 D seems to have a companion as well. The champion in this contest, however, is Theta-1 B, which has a companion 60 AU away called "B1." "B" itself is another eclipser (known also as BM Ori) that drops by nearly a magnitude every 6.5 days, the companion probably much like the Sun. Since "B1" is also double, Theta-1 B is quadruple. Adding them all up (and including fainter Theta-1 E, which lies close by), the Trapezium is a complex multiple of 11 stars! And that is not really the end, as the Trapezium is really the core of an incredibly dense star cluster that seems to fill the background of the Orion Nebula, all of it born less than a million years ago. Most multiple stars are hierarchical, a distant star going around a close double (like Theta-1 B), or two close doubles going around each other (like Mizar or Epsilon Lyrae), which gives great stability. The Trapezium, on the other hand, is gravitationally unstable, the stars all too close together. As a result, one after the other will be ejected from the group. After only a few million years, the leader of them all, Theta-1 C, will inevitably explode as a great supernova, the others probably doing so as well, all lighting the dusty gases of interstellar space, all providing shock waves that will promote new star formation within the local molecular clouds. (Read more about Theta-1 Ori in Jim Kaler's "Hundred Greatest Stars").

Update 2008: New parallaxes of young stars believed to be embedded in the Orion complex give a distance of 1350 light years, close to that used in the above analysis. The new measures would decrease the above luminosities by about 10 percent, but have little other effect. Note, however, that the Trapezium stars could be at somewhat different distances than the young stars used in the new measure.
Written by Jim Kaler 12/31/04. Last updated 7/25/08. Return to STARS.