The telescopic history of the Orion Nebula and the M43 region is quite an interesting story and contains one of the greatest conundrums in astronomy, worthy of a decent detective mystery. First realised by Alexander von Humboldt, none of the observers before Galileo had noted the particularly nebulosity, although to the naked eye today it is obviously bright and definitely not stellar.
The first written observations, like those of Al Sufi, published in 986 A.D. were highly detailed - even noticing the Andromeda nebula (M31), but he does not mention or show any nebulosity in Orion’s Sword. M42 is obviously not a star, even with the naked eye, though the explanation of this discrepancy nebula may not have been as bright during this time. By 1603, Bayer made his highly detailed star chart of the region that also does not show any nebulosity in Orion’s Sword. Yet Bayer noted the two stars in the Sword as an optical double, and named them Theta (θ1) Orionis and Theta (θ2) Orionis.
Amazingly, Galileo never mentions it. Galileo was
first to make telescopic sweeps of the area. Extensively
observing the constellation of Orion in some detail in
December 1610, he merely noted an increase in the number
of stars in the region. Added an additional eighty stars
or so within the sword, and made the subsequent drawn
figure in his Sidereus Nuncius (The Sidereal
Messenger). Nicholas Peiresc in November 26th,
1611 then made the
first known telescopic observations, which continued
between December 6th to 10th. The written observations of
Peiresc states he was ‘surprised’ to
find “a small illuminated cloud”;.
Johann Baptist Cysat (Cysatus), a Swiss Jesuit priest, made the next known observation in 1618, while comparing the nebulae with a close-by comet that was visible at the time.
Christian Huygens gave the first detailed
description using a Galilean telescope in 1656. Huygens
thought he had discovered M42 for the first time, and
subsequently published observations in his Systema
Saturnium, a few months after the original
observation. It contained a full description and a
drawing, and told of the remarkable star Theta (One)
Orionis in the very heart of the nebula. His original
sketch, contains seven stars inside the nebula, with five
lying beyond it. The stars of the Trapezium, he curiously
only notes three stars, though for the telescope aperture
used, he should have easily seen four.
On the same night in March 1769, Messier placed the Orion's Great Nebula in his famous catalogue of 103 objects; as M42 and M43. He separated the nebula into two parts because he though they were different objects, and even made a drawing of them.
In March 1774, Sir William Herschel observed M42, and later in the successive years of 1801, 1806 and 1810. His theory, written in the Nebula Hypothesis, realising for the first time the true nature of the Orion nebula. He wrote on M42;
“...which would see here an unformed fiery mist, the chaotic material of future suns.”
Noting that the observations made by him were different from Huygens, William Herschel stated in December 1810 the belief that the nebula had actually changed. Some thirty-seven years later, he was absolutely convinced that several stars had actually dissipated their nebulosities. By comparing his sketches, he also noted changes in the appearance of the nebula’s internal structures. It seems, by our knowledge of the meticulous detail of his observations, it is likely that these changes were real. Later other were to question the same thing about other nebulae like Eta (η) Carinae.
Compared to modern observations, several prominent and obvious stars of the Trapezium are missing. Ie. The ‘E’ and ‘F’ stars. Wilhelm Struve, as late as 1826, discovered the fifth ‘E’ star of the Trapezium. His magnitude estimations, show it has brightened considerably in the last 170 years. Some have speculated that many of these stars are very very young. The 110 stars discovered in the inner portions of the nebulae each show embryonic material, and proto-planetary disks which are being “shredded” of over a short time. In 1936, observations the variable star FU Orionis was found to have brightened by at least six magnitudes over a short period of several months! Ever since this star shows small fluctuations in brightness but has remained at an elevated average of 10.3 magnitude. Another two fainter variables of V1057 Cyg and V1517 Cyg also have shown similar phenomena.
It is very possible that these stars may have switched to a more energetic process, or that the star has suddenly ejected the incipient nebulosity that surrounds them. This may be the case with other stars observed in this region. Observation using infra-red wavelengths and the Hubble Space Telescope (1994-95) have revealed many observational details of the formation and cause of such brightening.
Sir John Herschel in 1826 made observations, whose
descriptions remain as the standard, and even today are
still quoted in most of the astronomical texts. His most
poignant description written in 1837, says;
“I know not how to describe it better than by comparing it to a curdling liquid, or to the breaking up of a mackerel sky where the clouds of which it consists beginning to assume a cirrus appearance.”
Reverend Thomas Webb in his ‘Celestial
Objects for Common Telescopes’. (1859)
“The Great Nebula, one of the most wonderful objects in the Heavens;...”
R.A. Proctor also in the late 1870’s, described it as follows;
“... the thought that seemed so impressive, so thrilling, as to surpass even the feeling of awe with which is the solemn darkness of night we see some mighty group of suns sweep into the field of view of the telescope ... that here on this tiny square inch of shoreline, with its thin film of chemical sands, has been traversing the solemn depths of space... Here we have mirrored by Nature herself that marvellous round of milky light below* Orion.....” (* as observed from the northern hemisphere.)
Mary Proctor, daughter of R.A. Proctor, continued with her apt description;
“ This is an irregular square with a star in each corner, and is situated in the midst of a dark gap in the nebula, within in which the 4 stars gleam brightly... (The Trapezium) The radiant mist surrounding them has a greenish tinge, revealing the vast stellar cloud known as the ‘Nebula in Orion’ with its Isles of light and silvery streams, and gloomy gulfs of mystic shades.”
E.J. Hartung in ‘Astronomical Objects for Southern Telescopes’ in 1968 describes the Orion Nebula as thus;
“One of the most attractive objects in the sky, this great nebula is too well known to need description;...”
These possible changes in the whole area are certainly curious. It is hard to reconcile how the observers before Pieresc could have missed such an obvious object, unless there was a sudden brightening of M42/M43 at the time of the introduction of telescopic observations. It is possible, if this hypothesis is correct, that the nebula started rapidly to brighten a few years before the observations of Huygens. An explanation of the mechanism could be the simultaneous “throwing off” of the embryonic nebulosity of the stars in the Orion Nebula. This would cause the stars to brighten suddenly, like FU Orionis. They would then expose the surrounding nebulosity of the whole region to the intense UV radiations of the stars. In a period of a few decades, the material contained in the nebula begins to shine by its own light, becoming the brightest of all emission nebulae.
It is truly compelling to think that the Orion Nebula may in fact be a very new object, which has actually has evolved even further since the invention of the telescope four-hundred odd years ago.
Now at the beginning of the twenty-first century, the Orion Nebula or NGC 1975-1980 is visually the brightest of the nebulae seen in the sky. This magnificent object is a bright emission nebula that is still spectacular even in light-polluted skies. The most modest telescopes can make out the general shape, and its greenish colour. Using even the largest of telescopes, we can observe something new that we have not noticed before. Commonly we call it the Great Nebula in Orion M42 is found lying in the middle of the sword of Orion, or the handle of the saucepan, as seen from the southern hemisphere. In even moderate telescopes it is a truly grand object and words cannot adequately describe this nebula as it truly appears.
My own impression of the Orion Nebula is firmly fixed in my brain. As the one of the first objects that I observed with a small 60mm refractor, I can still recall being fascinated by the faint, but highly delicate nebulosity. A sharp boundary to darker portions contrasts the ‘bowl’ of the nebula. The most obvious feature is the boundary of light call the Bright Bar. Naturally, when observing it with a much larger telescope, the subtle differences in the surface brightness become obvious, with a mottled surface inter-dispersed with faint filaments.
One of the first astronomical photographs taken was of this object, allegedly made by American spectroscopist, Henry Draper, at the Harvard College Observatory in 1880. It is obviously a favourite among those interested in astrophotography and is truly the most photographed object in the sky! A small telescope, with the short time in guiding, produces truly magnificent results. For the amateur astrophotographer it is likely the first object attempted. It remains a popular photograph in astronomical periodicals, advertising materials and in books. For example, it has been in each issue of the magazine Sky and Telescope for the past seventeen years!
We know it as the same nebulosity illuminated by stars and making the individual parts luminescent. In reality, this nebula is so bright because of the molecules and atoms contained within the nebula. The excitation of this matter is by many nearby hot stars that are radiating strong UV radiation and causing the fluorescent glow. The main portion of M42 is possibly illuminated by the star known as Becklin's object, which is unseen due to absorption of light by the bulk of the nebula.
In true size of the nebula covers about 1.25 degrees of the sky, being equivalent to be between seven and eight times the apparent area of the Moon. M42 is 66′x 60′ in size, while to the north is M43 that measures some 20#8242; x 14#8242;. The combined magnitude of the nebula for M42 is equal to a 2.0 magnitude star, while the star known as θ1 Orionis has the combined visual magnitude of 2.9. This star is also known as the multiple star HJ 4581 (h.4581) which was first catalogued by John Herschel.
Astronomers have estimated that the nebula’s total masses, is somewhere between 3 000 ΣM and 5 000ΣM. The distance to the nebula has been presently been estimated to be between 370pc. and 490pc or 1 200 ly. and 1 600 ly. However most of the literature often tends towards the higher values. In size, the bright or ‘fan-shaped’ section of M42 is estimated to be about 8 to 15 ly. across, while the maximum size revealed by the photographic process, measures to about 30 to 35 ly across! We have measured the nebula to approach the Earth at 25 kms-1 (8 kms-1 less than stated by Hartung), while the Trapezium multiple is travelling at a slower velocity of -11 kms-1.
No one knows the future of the nebula of M42, but the stellar creation process is still going on. Some estimate that it will become even more brilliant perhaps sometime within the next million years or so.