The history

M42 is one of the better known Messier's objects, and certainly the most spectacular one. As a star of the Orion's sword, it was known since the beginning of the recorded astronomy; Ptolemy, Tycho Brahe and Bayer bring it as a mag. 3 star and the latter named it "q Orionis" nella sua Uranometria, in 1611.

The stars of the Orion's Belt and of the Orion' sword were among the preferred targets of the Galileo's telescope; he never mentions, anyway, any nebulosity around q Orionis.

M42 is easily visible to the naked eye; is therefore somewhat remarkable that its nebulous kind couldn't be never recorded (as we know) since its discovery by the French amateur Nicholas-Claude Fabri de Peiresc, in 1610. The jesuit astronomer Cysatus (1588-1657) indipendently rediscovered it, in 1618.

The early "telescopic" drawing of this nebula is due to G. B. Hodierna, it is in his booklet "De Systemate Orbis Cometici, deque Admirandis Coeli Caracteribus", printed in Palermo in 1654.

To find M42

M42 is easily visible to the naked eye, as a nebulous starfrayed, in the middle between the three traditional members of the Orion's sword.

To observe M42

M42, commonly known as "The Great Orion's Nebula", is the brightest nebula in the sky: easily visible with the naked eye, it can be well observed through any optical instrument, from smallest binoculars to Hubble Space Telescope (HST).

M42 is the brightest region of a larger cloud of gas and dust spreading out along a large part of the constellation (over 10°, and several light years). The long-exposures photos shows this wide nebulosity including, besides the Orion's nebula in its center, several other well known objects, as the dark "Horsehead Nebula" in the "Orion's Belt" and the reflection nebula around M78. An image of the whole constellation, taken (like the preceding one) by Gianni Tumino and Jean Marc Lechopier of AIDA (Associazione Iblea Divulgazione Astronomica, Ragusa) shows the widespread nebulosity of this region, M42 and the neighbouring objects, the Barnard' Ring and, on the upper left, in the Monoceros coinstellation, the "Rosette Nebula".

The angular size of M42 is over 3,600' square, more of the full Moon. His linear extension is some 30 light years.

A huge, dark lane crosses the northern edges of the nebula; it is easily visible in the photos. On the northern boundaries of M42 there are also some reflection nebulae, just reflecting, weakly, the light of the Great Nebula. They are NGC1973-5-7, too faints for the Messier's optical instruments.

The studies on M42

In 1888 Huggins showed, through the spectral analysis, that the stars are actually surrounded in a cloud of glowin gas. The different values of radial velocity (measured by Campbell and Moore from 1915 to 1918) found in different regions of the nebula are sign of turbulent motion within it.

R. J. Trumpler in 1931 discovered that the trapezium is associated with a rich cluster of red stars, visible only in infra-red photography. Also associated with the nebula are several hundred irregularly variable stars. The cluster around the trapezium appears to be expanding at the rate of about one part in 300,000 each year (Strand, Deaborn Observatory). Therefore, the would be about 300,000 years old: the youngest yet known.

Fessenkov, in the context of his studies on the critical density of gas required for star formation, has found that in some regions of M42 the density is more than the critical one.

Hubble Space Telescope recently showed us the live broadcast of the star birth in M42, but already in 1954 seven stars was found in a small group that, in 1947, contained only five of them.