M42 Orion (Great) Nebula, M43 De Mairan's Nebula and Trapezium Stars

M42, the Orion (Great) Nebula

The M42 (NGC 1976) is the most-studied and is the brightest (Mag. 4.0) diffuse nebula in the sky. It is a prominent feature and forms as part of the gaint gas cloud complex spreading across a large portion of the Orion constellation. This cloud complex also includes the Horsehead Nebula, M43, M78, and the Trapezium open cluster. Althought it is about 1600 light-years away, the cloud complex stretches over several hundred light years across and appears more than 50 times the diameter of the Moon when view from Earth. The M42 itself extends about 60 x 60 arc minutes, covering four times the area of the full Moon.

(Credit: HST: NASA, C.R. O'Dell and S.K. Wong (Rice University))

Image above composed of a mosaic of 15 separate fields taken from the HST and covers a portion of M42, 2.5 light-years across or only 5% the area covered by the full Moon. Nevertheless, it contains almost all of the light from the bright glowing clouds of gas and a star cluster associated with the nebula. The prominant feature of this open star cluster is the Trapezium (Theta Orionis), consists of four hottest and most massive stars located at the center of the nebula (four pinkish bright spots - two more obvious, the other two less obvious - at the center of the image above). In fact the nebula's details are illuminated (and heated) by these four stars. They also out-shined and their total mass exceed those of other combined 700 young stars that made up of the rest of the star cluster.

Credit: Left: J. Bally, D. Devine & R. Sutherland, D. Johstone (CITA), HST, NASA. Right: M. McCaughrean (Max-Planck-Institute for Astronomy), C. R. O'Dell (Rice University), HST, NASA

Image on the left above shows the HST image close-up of the Trapezium stars. Note that fine details of cloud structure are also revealed. The nebula contains many young stars with protoplanetary disks or structures in the early stages of forming a solar system. These disk structures which were discovered with the HST are called 'proplyds'. Image on the right above shows four such proplyds. The red glow in the center of each disk is a young, newly formed star, about one million years old (our Sun is about 4.5 billion years old). The disks may eventually form planetary systems like our Solar System

Image above shows the brightest portion of the M42 Orion Nebula. Above it is the M43 De Mairan Nebula (NGC 1982). The M43 is a diffuse nebula that actually forms a part of the Orion Nebula. Its visual magnitude is about 9.0 with an apparent dimension of 20 x 15 arc min. It is illuminated by a star (HD 37061) of BIV type (6.5-7.6 mag.) and contains its own small cluster of stars.