Theta1 Orionis - The Trapezium

The Trapezium, θ1 Orionis, is a tight open cluster of stars that lies in the very heart of the Great Orion Nebula (M 42). It is one of the best known multiple stars in the entire sky.

Discovery and Components

The Trapezium was discovered by Galileo in February of 1617, who sketched three of the stars (A, C, D), but somehow missed the surrounding nebulosity. The fourth component (B) was later identified by several observers in the 1670s. Several more components were discovered by 1888, for a total of eight.

The most identifiable Trapezium stars are the brightest four. They are designated A, B, C, and D, in order of increasing right ascension. They lie clumped within a span of 22" or 10,000 AU. All are hot class O and B stars that together make an apparent "single star" of magnitude 4.7. Taken separately, from A through D, they are of magnitude 6.7, 8.0, 5.1, and 6.7; and spectral classes B1, B0, O6, and B0.5. The brightest and hottest is component C (θ1 Ori C), a great 40-solar-mass star with a temperature of 40,000 K, a huge luminosity of 210,000 suns (85% of the Trapezium's total), and a 1000 km/sec stellar wind with a flow rate 100,000 times the Sun's. This star is so powerful that it is evaporating dust disks around nearby stars which might form planets in other settings.

Both θ1 Ori A and B have been identified as eclipsing binaries. θ1 Ori A is also known as V 1016 Ori; every 65 days, the star dips by a magnitude, as a still-forming companion just 1 AU away passes in front it. Both are watched by another companion 100 AU off. More complex, however, is θ1 Ori B. Also known also as BM Ori, it drops by nearly a magnitude every 6.5 days; its companion is probably much like the Sun. θ1 Ori B has another companion 60 AU away called "B1." Since B1 is also double, θ1 Ori B is a quadruple system. θ1 Ori D seems to have a companion as well. Adding them all up (and including fainter θ1 Ori E, which lies close by), the Trapezium is a complex multiple of 11 stars!

Several other faint stars appearing inside the Trapezium are physically related to the cluster. One of these is 4' southeast of C, and another is 4' north of A. Within 5' of the Trapezium are more than 400 faint, young stars, recently condensed from the giant cloud in which they are embedded.

Formation and Evolution

The Orion Nebula is an immense, complex cloud of gas and dust about 1350 light years away, and 20 light years across. It is a blister on the face of the great Orion Molecular Cloud, which hosts several other sites of active star formation. The Trapezium is a relatively young star cluster that has formed directly out of the Orion Nebula. Its five brightest hot stars are situated in front of it, and have masses on the order of 10 to 30 times the Sun's. They produce most of the ultraviolet light that energizes the surrounding nebula and causes it to fluoresce. The Trapezium cluster is estimated to be less that 100,000 years old.

The Trapezium may be a sub-component of the larger Orion Nebula cluster, a grouping of about 2,000 stars within a diameter of 20 light-years. About half the stars within the cluster have been found to contain evaporating circumstellar disks, a likely precursor to planetary formation. In addition, brown dwarfs and low-mass "runaway stars" have been identified within the nebula.

Most stars are multiple or hierarchical (like θ1 Ori B), a configuration which gives great stability. The Trapezium stars, on the other hand, are too close together, and thus gravitationally unstable. As a result, one star after the other will be ejected from the group. After only a few million years, the most massive, θ1 Ori C, will explode as a supernova, the others probably doing so later. All of these supernovae will provide shock waves that promote new star formation within the interstellar molecular clouds.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]