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.