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Trapezium (Theta1Orionis)

Trapezium. Credit: ESA
The Trapezium. The brightest star is Theta1 Orionis C. Credit: John Bally, Dave Devine, and Ralph Sutherland, STScI, NASA
A star cluster at the heart of the Orion Nebula consisting of more than 1,000 young stars crowded into a space about four light-years in diameter (roughly the distance between the Sun and the next nearest star). A few of these stars are hot O and B stars. The majority, however, are later-type, cooler, less luminous, and less massive pre-main sequence stars of a solar mass or less.

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.

The reader may also be interested in Mosher and Pope's comparison of Galileo's drawing of the Trapezium to a modern photograph taken through an optically-similar telescope here and here.

We do not know if Galileo's drawing is the first, but it is a remarkably accurate early representation of how the Trapezium appears through a small telescope. In such a telescope by the way, as shown by our modern photos, the star patterns are much more prominent than the nebula, which is faint and hard to see.

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.

Hodierna's map of M42, color-coded by Jom Mosher
Color-coded version of Hodierna's M42 map
Personally, I think the claim that Hodierna depicted the Trapezium is based on a misinterpretation of his drawing. I believe he is merely trying to indicate the size and location of the nebula relative to Orion's Sword stars and does not show the Trapezium at all. At the right end of the oval he uses to represent the M42/M43 nebula he does indeed show a grouping of three stars, but these are much too widely spaced to represent the Trapezium as seen through a 20X Galilean telescope, and in fact he shows other stars just as closely spaced at the far right end of his diagram. In an attempt to understand Hodierna's drawing I generated the following image of this area using Patrick Chevalley's Carte du Ciel star mapping program. It shows an approximately 2-degree-wide section of Orion's Sword with all stars down to a magnitude of 9, which is about the faintest Hodierna is likely to have seen through a small Galilean refractor. It also includes the modern outline of the M42/M43 nebulae. The stars have been color-coded to highlight the possible correspondence between Hodierna's stars and the modern map.
color-coded Carte du Ciel map of M42 region
Color-coded Carte du Ciel map of M42 region
If this identification is correct, then the blue, yellow, and red stars in Hodierna's map represent Theta2 Ori B, Theta2 Ori A, and Theta1 Ori, respectively – the same stars drawn by Galileo, but with the Trapezium unresolved. While other interpretations of the stars in Hodierna's print are possible, in no event would he seem to be illustrating the Trapezium, which would have appeared to him, as it did to Galileo, as little more than one large star with two much smaller ones very closely attached.
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.

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