Orion Nebula - Houdini Of The Sky

One of the most impressive and bright deep sky objects in the sky, it fascinates and captures even the most casual telescopic observer. But if it is so obvious to the eye, why wasn't it recorded by the multitude of observers who scanned the skies and noted anything prominent or unusual.

In the tenth century, the prominent Persian astronomer Al-Sufi published his momentous work, Suwaru'l-Kawakib (Uranometry). He mentioned The Andromeda Galaxy, M31, and plotted its position correctly on his charts, calling it the "Little Cloud". However, he failed to identify the Orion Nebula which appears brighter than M31.

The Chinese astronomers noted everything of importance in the sky, certainly any unusual grouping of stars or obvious oddity. They divided the sky up into 28 Zodiacal constellations along the ecliptic (lunar mansions) and 283 individual figures (officials) based on a total of 1565 recorded stars. In the Wen Miao Temple (Confucian Temple of Literature), many stone engravings were uncovered. Dating from the thirteenth century, the information contained in this catalogue was gathered with instruments that were the best of their type in the world at that time.

The Chinese charted seven stars in the Pleiades, and divided Orion up into three separate groups. They also plotted the three stars in the Hunter's Sword, including many stars down to the sixth magnitude, but no sign of the Orion Nebula is recorded.

Moving forward in time some five hundred years brings us to the celebrated inventor and observer, Galileo Galilei. Having heard of the invention of the telescope, Galileo constructed his own early on in 1609 and turned it on the heavens in the summer of that year. Galileo was monitoring Jupiter, which was going through its retrograde motion, as it neared opposition on December 7th, near Bellatrix in Orion.

Drawn to nearby Orion, Galileo felt compelled to thoroughly investigate this region, intending to show it as it appeared in a telescope. However, the star densities in this area are overwhelming and Galileo decided to concentrate on the sword and belt region. He plotted three belt stars and six sword stars and added another eighty unknown stars.

Galileo's charts have been criticised for their accuracy, but not so their completeness. Most stars down to the sixth magnitude were recorded but no mention of any nebulosity was made. He was certainly aware of the term nebulosity as he demonstrated that the Lambda Orionis region was composed of many faint stars and not the nebulous cloud many supposed.

Galileo made hundred's of telescopes, though he admitted that only a handful were suitable for astronomical usage. Using only the best telescopes, the fifth and sixth magnitude stars he observed appeared to the eye as stars of the first magnitude. Galileo was never slow in declaring any new discovery so it must be assumed that he never saw this vast cloud of gas.

It was left to France's most prominent astronomer, Nicolas Peiresc, to discover this wonderful apparition. Nicolas studied under Galileo during the period 1599-1602 though his serious interest in things astronomical came about some years later in 1610, when Galileo discovered the attendants of Jupiter. Nicolas' observatory soon had five good quality telescopes, the first delivered to him in November 1610.

Only one day after using the telescope for the first time, he turned it onto Orion's sword and resolved the middle star into a "small illuminated cloud". He was clearly astonished by this discovery, not expecting to see such an obvious object so readily. Eight years later, in 1618, a Jesuit astronomer, Johann Baptist Cysatus observed:

"...another of these phenomenon in the heavens is the congeries of stars at the last star in the Sword of Orion, for there one can find a similar congestion of some stars in a very narrow space and all around and in between the stars themselves is a diffused light like a radiant white cloud".

It seems that Cysatus obviously mispositioned the nebula, putting it around the last star rather than the middle star, which is curious in itself.

Another interesting character who figures large in the history of the Orion Nebula is Christiaan Huygens (1629-1695). Huygens first resolved Saturn's rings and detected Saturn's brightest satellite Titan in the year 1656. He believed that he was the first to see the Orion Nebula, not knowing about Peiresc or Cysatus' observations.

He sketched only three Trapezium stars in 1656, the missing star appears to be the eclipsing binary BM Orionis, an EA (Algol) type variable star. Sketches made in 1684 showed four Trapezium stars, all stars shown on his charts can be identified on today's sky maps. Interestingly, the book Star Names: Their Lore and Meaning by Richard Allen further complicates this issue.

“Huygens noted the triplicity of theta 1 when he discovered the nebula; the 4th component was first seen in 1684; the 5th was discovered by Robert Hooke in 1664, but forgotten and rediscovered by Struve in 1826”.

If Hooke saw the 5th (E) star in 1664, then this 11th magnitude star pre-dates the three magnitude brighter BM Orionis by 20 years ! He believed the nebula to be undergoing rapid changes, a theory believed by the next famous astronomer to study this remarkable region.

Sir William Herschel, the greatest amateur astronomer and a meticulous and careful observer, was convinced that active, dynamic changes were taking place within the nebula. He began his observations of this area with M43, a nebulous haze surrounding an eighth magnitude star, on March 4 1774. He also noted two similar though smaller nebulous stars, one each side of M42. Reports in 1783 showed that M43 was faintly joined to M42, though he found it not connected one year later.

In the years 1801, 1806 and 1810, he convinced himself that M43 was disconnecting itself from the main body of M42. In December 1810, he found that the two nebulae either side of M42, seen in 1774, had disappeared, the two stars only seen. Wanting to compare the appearance of M42 on March 13, 1811, to its earlier structure in 1774, he selected a telescope of similar light gathering ability and turned it onto this enigmatic glow. His drawings when compared with his earlier sketches showed changes in its structure.

Another curious result was his observation of the Trapezium stars. He failed to notice the two faint eleventh magnitude stars, now known as E and F. They are both seen in as little as a six inch aperture on a steady night, so it is not easy to see how he could have managed to fail to spot these two stars. I cannot believe that Herschel missed these stars if they were at their current separations and brightness. Herschel took great interest in double stars, using magnifications of between 220 to 6000 to study these stellar systems. Herschel spent many years studying M42 and its environs and yet these stars eluded him. Star E was first seen by F Struve in 1826 and star F was discovered by Sir John Herschel in 1830. Perhaps these stars were too faint and beyond telescopic detection for William Herschel.

Thomas Harrison states that these stars are of later spectral types than the four bright Trapezium stars and are currently at about the same absolute magnitude as the FU Orionis prototype star. If they are of this class, they may have been five to six magnitudes fainter, so not detected by Herschel.

What is an FU Orionis star? Named after the prototype FU Orionis three degrees northwest of Betelgeuse, only a handful are known, including V1057 Cygni and V1517 Cygni. Generally of spectral type A, F or G, these pre-main sequence stars undergo sudden luminosity increases within a very short time span.Indeed, FU Orionis underwent such a change in 1936, brightening by up to six magnitudes over a period of three months.

Usually at sixteenth magnitude, it rose to about tenth and stayed around that brightness, dropping to eleventh magnitude by 1960. Currently, it is around twelth magnitude. The reason for these spectacular outbursts has yet to be found though the two main explanations are (a) changes in the stellar structure as put forward by Herbig, among others, and (b) the loss of a dense surrounding stellar dust envelope as championed by Cohen and Woolf (1971). A possible third theory could be interactions in a newly formed binary system.

The four bright Trapezium stars are of an earlier spectral type, O and B. Harrison suggests:

"...it is quite possible that one or more of these may have undergone a similar degree of sudden brightening. Not all the stars making up the Trapezium would necessarily have had to brighten at the same time, since any one of the hot stars, depending on its proximity to the cloud, could supply the necessary ultraviolet radiation to cause the nebula to fluoresce. Only one star is thus necessary to bring the cloud to its present level of luminosity, and that would be the star closest to or more totally embodied by the cloud".

In his conclusion, Harrison writes:

"...If the nebula did suddenly rise to its present level of brightness in the seventeenth century, then we propose that the FU Orionis mechanism may be responsible. This sudden brightening of the pre-main sequence stars, while not theoretically predictable, has been observed on the part of at least three spectral A-F stars. If it occurs for the earlier class stars as well, then as the wave of ultraviolet light spreads across the dark nebula it would brighten over a period of time determined by the speed of light and the size of the visible cloud. The historical evidence could be interpreted to support such a scenario".

He continues:

"...It would be fascinating to believe that the Great Nebula in Orion, perhaps one of the most impressively beautiful deep space objects viewed through a telescope and unique to all the constellations of the heavens, actually came into view at the same time that the astronomical telescope was introduced as a scientific instrument, revolutionising science and launching astronomy as a bona fide scientific enterprise. It would be a truly sentimental marriage between the heavens and the Earth".

In 1973, Dr Lohsen of Germany undertook a photoelectric study of the Trapezium region. Measuring the four stars in ultraviolet, blue and yellow light, Lohsen could now use three of the stars as comparisons to the eclipsing variable BM Orionis. On October 10 and 11, star A was around one magnitude fainter than normal. Star A is now known as an eclipsing binary with a possible pre-main sequence companion of low surface brightness, maybe similar to the companion of BM Orionis.

How could this star’s variability of one magnitude be missed by the thousands of amateur astronomers that gaze upon this region in awe? How could Galileo have missed one of the finest nebulae in the heavens?

REFERENCES:

The Orion Nebula: Where in History is it? - Thomas G. Harrison Q. Jl R. Astr. Soc. (1984) Vol. 25

Burnham's Celestial Handbook Vol. 2 - Robert Burnham Jr

An unexpected new variable in Orion - Sky And Telescope July 1975 pg 12

The FU Orionis Phenomenon - Gunnar Welin Protostars and Planets: I Gehrels (Editor)

Star Names Their Lore and Meaning - Richard Hinckley Allen

Orion Nebula Image - NASA, ESA, M. Robberto (STScI/ESA) and The Hubble Space Telescope Orion Treasury Project Team