THE WONDERFUL NEBULA OF ORION

Wanganui Chronicle, 7 January 1947, Page 7

THE WONDERFUL NEBULA OF ORION

(By “Sky Pilot.”) A few months ago, when the Great Square of Pegasus and Andromeda constellations were visible in the evening sky, I drew attention to the great Andromeda nebula which, incidentally, I pointed out, is not a nebula in the sense that we use that term, but another great galaxy like our own. It is often called an extragalactic nebula. With the constellation Orion well placed in the evening sky now for observation the opportunity is presented to consider one of the most striking of galactic nebulae thrown around Theta Orionis in the Hunter’s Sword. In considering this nebula which is visible to the nake: eye as a hazy star we are considering the most outstanding of the groun of diffuse or irregular-shaped nebulae. There are two types of diffuse nebulae: Luminous (to this group belong the most beatiful objects in the sky) and obscure of dark nebulae. Famous examples of the first are the Trifid Nebula in Sagittarius (2000 light years distant) and the network or veil nebula in Cygnus—“like an airy gossamer scarf too fragile for aught but fairy hands, it throws its fine spun beauty across the sky.” (Draper and Lockwood). Examples of the second are the famous Coalsack, next to the Southern Cross, and the great lanes in the Milky Way and around and in Sagittarius. Other famous nebulae of the diffuse type are the American Nebula in Cygnus and the Crab in Taurus as well as the looped nebula in the Larger Magellanic Cloud. The person not familiar with astronomical terms may ask what is meant by the word nebula? Nebula, is a Latin word meaning “cloud” or “mist,” and is well suited to describe the misty and cloudy patches of light seen by the astrenomer. So, a nebula is quite distinct from a cluster of stars. The Orion nebula, W best of these diffuse nebulae, is indescribably beautiful, "an explosion of glory in the blackness of space.” It is a greenish-blue cloud which used to be considered some 600 light years (or 3,600,000,000,000,000 miles; away, with a width of 60,000,000,000,000 miles or some ten light years). It is now generally considered to be at least 1000 (whilst many astronomers •say 1800) light years away, and with a diameter of 25 light years. The spectroscope reveals that, the nebula is a condensation of interstellar gas in the region of the constellation of Orion, the density of which is estimated to be somewhere in the neighbourhood “of one thousand of a billionth of the density of the air we breathe and therefore an extremely fine vacuum.” Cosmic dust is spread throughout, and the nepvia is really a part of a great extension of nebula right over the constellation. What was once considered to be nebulium is now found to be nothing more than our familiar oxygen “radiating under different conditions than can be obtained in the laboratory.” CAUSE OF LUMINOSITY. The famous nebula is the middle star of the sword. “Once you have found the great nebula in Orion you have seen without doubt one of the most superbly beautiful and splendid objects the heavens have to show. (Draper and Lockwood). The cause of the luminosity of the nebula is due to the ultra-violet light radiated from nearby stars as in the case with all diffuse nebulae. When the stars are of medium temperature the nebulous matter then only reflects the light, but when the stars are hot. blue stars, they excite activity within the nebula which causes it, as in the case of Orion, to emit a light of its own. The

energy is absorbed by the atoms and then re-radiated. The Orion nebula shines by the light of the many bright stars found in the famous Trapezium (Theta Orionis) which range from eighth magnitude to below fifth and which shine as precious jewels in their colours of white, lilac, garnet and reddish light. Dr. V. M. Slipher found in 1912 that the Pleiades are likewise enveloped in nebulous material which causes them to appear like the glow round street lamps on a foggy night. Obviously the nebula is lit up by the stars nearby and within. If anything were to happen that the stars ceased to give their light the nebula would immediately become dark. Luminous galactic nebulae are composed, then, of gas and dust, and reflect or emit a light caused by nearby stars of great heat. The Orion nebula teems with great energy and is the seat of violent currents in great turmoil. Campbell and Moore found velocities in their central parts of from 6 to 14 miles per second. “Tremendous streams of nebulous matter are being intermingled in chaotic fashion.” (Fath). The whole region of Orion is covered with this filmy nebulous material of which the 1 Orion nebula is only a small part. When the photographic plate is put on the region it reveals great areas like this. DARK NEBULAE LIKE RIFTS. In addition to luminous nebulae there are large sections of obscure or dark nebulae like dark lanes or rifts in the Milky Way from Cygnus to Centaurus and large patches in Scorpio and Orphiucus. These were once thought to be “holes” in the sky or vast areas of ’empty space where there was an absence of luminous matter. They are now believed to be dark nebulae obscuring vast portions of the starry realm. There composition is the same as the luminous nebulae, gas and fine dust, but they are not near enough to powerful suns to illuminate them. Sometimes stars are seen appearing in them but these are actually much nearer to us than the nebula. Barnard discovered that the Coalsack and other dark nebulae are clouds of nebulous matter of extremely tenuous gas with cosmic dust intermingled. In places the dark nebulae are fringed by light nebulae and vice versa. The great rift in the Milk Way due to dark nebulae from Cygnus to Centaurus covers a range of about 120 degrees or a third of the distance round the sky. The Horse's Head dark nebula near Zeta Orionis is a magnificent example, an enormous cosmic “horse” rearing its great mane and head in the sky. The famous nebula in Orion is well seen through the Wanganui telescope and is a grand sight especially on a very dark night. Looking at it, we are viewing an object as it looked at least 1000 years ago. A very great deal may have happened since. Light leaving 'it to-night and travelling at the rate of 186,000 miles per second will not reach us for at least 1000 years. This is a fine study for amateurs, with even a small telescope, for it can be picked up so easily. The nebula was first observed in 1611 by the French naturalist, Nicholas Peiresc, of Provence, and appeared to be lost to viewers again until 1656 when Christijn Huyghens gave it diligent study and Sir William Herschel, who was very much interested in the nebula, watched it for hours. It has ever since commanded the attention of all astronomers. During the summer and autumn occasion should be taken, especially as the days shorten, to see it from the Wanganui Observatory. It is a memorable sight and worthy of much study.