Slings and Arrows: Hunting the Hunter

Continuing my efforts to grasp with my feeble mind the things that incredibly clever people have worked out using only a few basic instruments, in order to bring more meaning to my own blundering observations of the heavens, I now turn my attention to the constellation of Orion.

Orion Nebula as drawn by Charles Messier 1771

The great hunter currently bestrides the firmament just above the shed roof, bow in hand; one of the most instantly recognisable features of the winter sky. As such it has always enjoyed a special significance in every culture that has pondered the stars and sought to discern their own destiny in those distant, glimmering points of light.

In Ancient Egypt the constellation of Orion was associated with Osiris; the resurrected lord of the underworld. Within the Great Pyramid of Khufu, two ‘star shafts’ are angled upwards from the main burial chamber leading to the outside of the pyramid. The southern shaft is aligned with the centre of the constellation of Orion, which is associated with Osiris, at the same time as the northern shaft is aligned with Alpha Draconis; the star which at the time that the pyramids were constructed would have constituted the celestial pole. It is perhaps coincidental but the function of these shafts may have been to allow the spirit of the dead pharaoh to set out on and return from its celestial journeys.

For the Maya too the constellation of Orion represented a major deity; Hunhunahpo, the Great Father, who is sacrificed following a celestial ball game and whose blood fertilises the earth. Like Osiris the myth of Hunhanahpo is bound up with the cycle of sowing and reaping and sowing anew; of life and death and resurrection. The Maya were keen observers of the heavens with structures at various Mayan sites tentatively identified as observatories. Within the constellation of Orion the Maya traced a triangle between the two stars which form the ‘feet’ Rigel and Saiph and the first star of the belt Alnitak. This triangle was described as the celestial hearth and at its centre the Maya identified a feature that they referred to as the smoke of the hearth; making them the first people to record an observation of the Orion Nebula.

The Observatory at Tulum as drawn by Frederick Catherwood 1844

The Mayan civilisation was long fallen into ruin by the time that the first European observer recorded a sighting of the Orion Nebula. Nicolas Claude Fabri de Peiresc was the archetypal enlightenment gentleman amateur scholar with a fascination for everything from the fossils beneath the earth to the stars in the heavens. His sprawling country home near Toulon featured an enormous garden filled with exotic plants and a menagerie of animals. By the end of his life he had amassed a correspondence of over ten thousand letters with around five hundred intellectuals of his day from all over Europe. Inspired by the exploits of Galileo and keen to see for himself the moons of Jupiter described in Sidereus Nuncius, De Peiresc had an observatory constructed and obtained one of the new fangled telescopes. In 1610 De Peiresc made a discovery of his own when he beheld through his new telescope the Orion Nebula. Indeed he coined the term ‘nebula’ to describe the cloudy phenomenon.

Nicolas Claude Fabri de Peiresc

Having established himself as an astronomer of note De Peiresc then turned his attention to one of the vexing problems of the age; the accurate calculation of longitude. Seizing upon the opportunity of a lunar eclipse in 1635, De Peiresc on his own initiative dispatched agents to Rome, Cairo and Aleppo in order to record simultaneous observations of the event. From his resulting calculations De Peiresc was able to provide a revised estimate of the length of the Mediterranean, reducing the previous figure by an astonishing 1000km, not that they had kilometres back then! In the following year he produced the first recorded map of the surface of the moon.

The nebula that De Peiresc had discovered attracted the fascination of other notable astronomers. Christiaan Huygens produced a diagram of the nebula in 1656 and it came to the attention of the great cataloguer of nebulous objects Charles Messier in 1769. Messier's drawing of the nebula, which he designated M42, appears at the top of the post. Messier was primarily concerned with the search for comets and his cataloguing of other deep sky objects was intended to assist in this enterprise. The question remained however; just what were these nebulae made of? In 1814 the invention of the spectroscope by the expert lens maker Joseph von Fraunhofer presented the means to investigate the nature of nebulae.

Joseph von Fraunhofer demonstrates the spectroscope

Combining the spectroscope with a telescope allowed the emission spectra; that is the emission of light by hot gasses at specific wavelengths corresponding to their chemical composition, of various celestial bodies to be studied. In 1865 English astronomer William Huggins, working alongside his wife Margaret, used this method to study the Orion nebula and declared it to be composed of ‘luminous gas'. Huggins was puzzled by the emission spectra that he obtained from his studies of nebulae as some of the emission lines did not correspond to any known element. He therefore proposed the existence of an entirely new one – Nebulium. Sadly it was later shown that the mysterious spectra corresponded to doubly ionised oxygen and that nebulas are not made of nebulium. Sometimes the truth can be so disappointing. In the course of his studies Huggins also observed the phenomenon of ‘doppler shift’ in the spectral lines of celestial bodies either towards the red or blue end of the spectrum depending on whether they were moving closer to or further away from the earth, which would later be seized upon as evidence of the fact that the universe was expanding.

Sir William Huggins

The constellation of Orion also boasts another notable feature in the red giant star Betelgeuse. This behemoth of a star became in 1920 the first star, other than the sun, to have its size successfully determined. Now maths was never my strongpoint and years of reliance on spread sheets and calculators have caused my mathematical ability to regress to the point that I would probably fail a maths test for ten year olds, so I am not going to get into this too much but suffice to say some very clever chaps worked it all out.

The measurement was achieved through the use of an interferometer; a device which allows a light source to be split into separate beams and then re-converged in a single image. This allowed the observers to overcome the spurious interference which blurred and distorted the image of the star in the telescope.

Hooker Telescope with Michelson interferometer 1920

Making use of our old friend the Hooker Telescope at Mount Wilson Observatory California, Albert Michelson and Francis Pease mounted a twenty foot wide beam onto the front of the telescope upon which four six inch mirrors were arranged; two in front of the aperture angled at 45 degrees and two placed parallel opposite them at either end of beam. By observing Betelgeuse through the telescope and adjusting the distance between the two outlying mirrors until the image generated was free of interference, Michelson and Pease were able to accurately measure the angular diameter of its photosphere. From this, knowing its distance from the earth, they were able to determine its actual size at some 240 million miles across. See the links below for a full explanation. To put that in perspective, as the earth is 93 million miles from the sun if Betelgeuse was at the centre of the solar system it would engulf Mercury, Venus, Earth and Mars and extend almost to Jupiter. Blimey!

Slings and Arrows

Wednesday 8 January 2014

Hunting the Hunter – Observing Orion

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