The Great Nebula in Orion and the Tip of the Sword

March 16, 2018

William Herschel called it an "unformed fiery mist, the chaotic material of future suns. Here is found every variation of celestial doings...stars, multiple stars, blue stars, clusters, nebula, gas, dust..." The Orion Nebula region is the most observed, photographed, and studied region of the sky, and generally considered to be the most wonderful and beautiful object in the heavens.


For the casual observer there is no other region in the entire sky better for low power wide field telescopic viewing. The night should be clear and the observer well away from man-made lighting and with their eyes dark adapted and shielded from stray light. For a dramatic effect, position the telescope ahead of the nebula and then let it drift slowly into view. The 17.5" Hawkins telescope at the Veen Observatory can show many of the details as seen in photographs, with much detail viewed as bright branches, dark rifts, and bays, all on a mottled yellow-green background with 6 stars at its very center. The first person to discover the diffuse nebula in Orion was Nicolas-Claude Fabri de Peiresc on November 26, 1610, when he made a note of observing it with a telescope borrowed from Guillaume du Vair. However, the first published observation of the nebula was done so by the astronomer/mathematician Johann Baptist Cysat in 1619.


Photo courtesy of Joe McBride, taken at the Veen Observatory using a 8" Newtonian and a ASI071 camera; 42 minutes of data taken at different integration times and HDR processing.


In 1880 the Orion Nebula, also called M42, was the first nebula to be successfully photographed, by Henry Draper using an 11" refractor and a 51 minute exposure. Before that, William Huggins did specroscopic observations of planetary nebulas NGC6543 in Draco and M42 and concluded that light from these objects comes from a rarified gas excited to luminescence, which would hold true for pretty much all diffuse nebulas and planetary nebulas.


The Great Orion Nebula is a small brightly illuminated portion of a vast cloud of gas and dust called the Orion complex 22 whose immensity is well beyond comprehension. Even though it is one of the nearest objects of its type, the exact distance is still difficult for us to precisely determine. The minimum distance using color and magnitude measurements puts the cluster out to an estimated 1300 light years away, while proper motion measurements done by K.A. Strand in 1957 pushed the nebula out to an estimated 1900 light years away. In 1969-1970, measurements at Palomar Observatory came up with a distance of 1600 light years away, so the generally accepted range of distance is 1600-1900 light years.


Most of the light coming from the nebula is fluorescence, which is produced by strong ultraviolet radiation coming from the Theta Orionis star system. Early spectra of the nebula produced some unidentified lines, which were considered to be the hypothetical element called nebulium. They were later found to be produced by oxygen, nitrogen, neon, and helium, but under conditions not found on Earth.


Two strong emission lines of doubly ionized oxygen at 5007 and 4959 angstroms give the nebula its strong greenish color. Even though other colors are present,  the naked eye is not sensitive enough to pick them up through a telescope.  A long exposure photograph made at the Palomar Observatory by W.C. Miller revealed a wide range of  red, blue, and purple hues with details not seen in the eyepiece of any telescope.


As with most nebulae, hydrogen is the most abundant, followed by helium, carbon, oxygen, etc.  A calculation of one cubic foot of the nebula consists of:


Hydrogen: 2.5X10^7 atoms

Helium: 2.5x10^6 atoms

Carbon: 1.5x10^4 atoms

Oxygen: 6.25X10^3 atoms

Nitrogen: 5X10^3 atoms

Sulfur: 9X10^2 atoms

Neon: 2.5X10^2 atoms

Chlorine: 5X10^1 atoms

Argon: 3.8X10^1 atoms

Fluorine: 3 atoms


Much of the detail and structure captured in photographs can be made out visually in a 8" or larger telescope. The naked eye is better for this than the camera in some respects when viewing the nebula, as it can see both bright and faint portions of the nebula, whereas a camera can blow out or over-saturate bright portions in an effort to capture faint areas with a long exposure. When this happens, detail is lost. The bright central area of the nebula, sometimes called the Huyghenian Region, is easily observed with high magnification and shows a lot of detail in any telescope 4" and larger. In my own experience, under excellent conditions and using  a 22" telescope at 80x, this region looked highly mottled with a yellow-purple color and detail that went out of the field of view and was easily traceable for several fields in every direction.


Iota Orionis is the bright blue star just south of the Orion Nebula. Its Arabic name is "Na'ir al Saif," meaning simply "Bright One of the Sword." It is a triplet system and is easily observed with small telescopes. Its magnitude is listed at 2.76 and spectral class of O9, and modern measurements estimate it to be at a distance of 1300 light years. Older estimates suggested that it was as far away as 2000 light years. Its spectral attributes are those of a giant with an absolute magnitude of -6, which equates to a luminosity of 20,000 times that of our Sun. Iota's closest companion (11" separation) is of spectra class B9 and has a magnitude of 7. Its other companion (50" separation) is 11th magnitude. Iota itself is also a spectroscopic binary with a period of 29.1 days. J. Pearce in 1953 calculated that the orbit has a rather high eccentricity and modern theoriess conclude that the second star may have been captured. Parallax measurments indicate that Iota is a member of the NGC1980 star cluster that resides about 2 degrees to the north. There are 10 or so 6th-8th magnitude stars including the wide double star Struve 747, and numerous fainter stars scattered about can give the observer an impression of a sparse open cluster with blue-white stars. This in itself gives a great view in any telescope, but to get the Orion Nebula included in the same field of view is something to keep you coming back for more.


This time of year is perfect for observing or photographing this bright nebular region. As February turns to March the sky can clear and reveal the Orion Nebula and surrounding stars along with other gems of the Winter sky without the frigid temperatures of December and January... but you better hurry because by late April, most of this area will be gone to the Sun until mid-September in the morning hours.

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