With an average stellar age of only one million years, the Trapezium is one of the youngest clusters known. Most of its member stars are hidden by dust or by the glare of the nebula but are visible at infrared wavelengths.
The four brightest stars (A, B, C, and D), which form the vertices of a trapezium and give the cluster its name, can be seen easily with a small telescope. Two fainter stars of eleventh magnitude, E and F, show up in moderately sized amateur instruments under good seeing conditions. A further two, of 16th magnitude, G and H, are only visible in very large amateur scopes.
Theta1 Orionis A, also known as V1016, is an eclipsing binary with a period of 65.432 days and a magnitude range of 6.72 to 7.65. Theta1 B, also known as BM Orionis, is another eclipsing binary, spectral type B2-B3, with a period of 6.471 days and a magnitude range of 7.90 to 8.65. An infrared companion has also been discovered in the Theta1 A system, making it a triple star, and Theta1 B is now known to be a quadruple system, with three components detected separately in the near-infrared. Infrared observations have that shown Theta1 C, which is actually the brightest star in the cluster, is a close binary as well. [Thanks to Karl Menten, Max-Planck-Institut fuer Radioastronomie, for a correction to this paragraph.]
The following history of the first observations of the Trapezium was kindly provided by Jim Mosher and Tom Pope:
Although not published in his lifetime, Galileo Galilei made a clear drawing of this region in a manuscript dated February 4, 1617. His drawing very accurately depicts not only the three brightest stars of the Trapezium but also the two field stars (Theta 2 Ori A & B) along the extreme lower left edge of the photo shown here. Galileo's drawing, and accompanying text, appeared on p. 880 in Vol. 3 of Antonio Favaro's 20-volume edition of the Collected Works of Galileo, published in the 1890's. A facsimile of this page, including Galileo's drawing is accessible via the internet here. The existence of this drawing was mentioned in an article by Czech amateur Leo Ondra in the July 2004 Sky and Telescope (p. 72), who, in turn, had found mention of it in a 1949 article by Italian science writer Umberto Fedele.
Previously, it was suggested on this page that the Trapezium was first drawn as a triple star (A, B and C) by Giovanni Hodierna before 1654 and described by Christen Huygens in 1656. Mosher replies to this as follows:
The reader may wish to review Hodierna's drawing of the Orion Nebula (M42) here.Star D was independently discovered by Jean Picard and Christiaan Huygens in 1684, while E and F were discovered by William Struve and John Herschel in 1826 and 1830, respectively. Star G was found in 1888 by Alvan Clark while testing the 36-inch refractor that he made for Lick Observatory, and Edwin Barnard discovered H later in the same year with the same instrument.
Related category• NEBULAE AND STAR CLUSTERS
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