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Southern Cross - February 1997

• Constellation of the Month - Orion

Constellation of the Month - Orion

John Morland

Orion the Hunter is the brightest and grandest constellation in the sky - and it is shared by all as it lies on the celestial equator. Orion is one of the first (if not the first) constellation I learned about in high school. I remember distinctly that Orion, Canis Major, Crux and Scorpio were the first constellations I recognised in the sky, a sight and feeling I remember to this day - and yes, they really looked like what they are supposed to represent! From there, I just "turned left at Orion" and proceeded to recognise more constellations - an action I still subconsciously do today.

Orion is depicted with a club and a shield, facing a charging bull (Taurus); a hare (Lepus) is also nearby. Orion sets as Scorpio, the animal that finally killed him, rises.

Orion is just crammed with double stars, clusters and nebulae. It is home to the best emission nebula in the sky, home to the Orionids - a meteor shower that is active from October 16th to 26th, when up to 20 meteors an hour can be seen originating near the border with Gemini - and home to the famous Horsehead Nebula, the photo of which is in every astronomy book I have seen (other than specific books on, say, the Sun or Moon) and which is the object on the Society's John Morland Nebulous Award Shield.

Orion's impressiveness is due to that it is an area of star formation in a nearby arm of our galaxy, centred on the famous Orion Nebula with the imaginative name of M42-43 (or if you prefer NGC 1976 and 1982). Where do I start in describing the multitude of objects visible in even a small scope? Which ones do I describe? Let us keep with convention and start at describing some of its stars.

Rigel (the giant's leg - Beta Ori), the brightest star in Orion, is a blue-white supergiant at magnitude 0.1 at 1050 light years away. It is really a triple star system. Rigel A and B are a spectroscopic double, rotating around each other in less than 10 days! Rigel C appears as a 7th magnitude close companion (9.4 arc seconds) which I can separate in my 60 mm at moderate power, in the glare of the primary. My 12.5" Newtonian, stopped down to 5.5" for unobstructed aperture (this converts the image of a "spiky" star to a neat point with a subtle Airy disk - far better for objects like the Moon, planets and close binaries), at higher power shows the companion clearly. Rigel A and B are consuming their hydrogen at a ferocious rate by using the CN cycle (refer to my article on the Sun in this Journal) and will last only a tiny fraction of the life of our Sun. Rigel is referred to as Beta Ori even though it is the brightest star in Orion; why it is not referred to as Alpha is may be because Betelgeuse (Alpha Ori) - a variable star - appeared brighter when Bayer applied his designations.

Mintaka (the belt - Delta Ori), located on Orion's belt on the Rigel side, is 1500 light years away and appears fainter at mag. 2.2. There is a wide companion at mag. 6.9. The primary appears blue-white and is an eclipsing binary varying by about 0.2 magnitude every 5.7 days.

Alnitak (the girdle - Zeta Ori), located on the Betelgeuse side of the belt, at magnitude 2 has a close companion of mag 4.2 and a wide companion at mag 10. It is 1100 light years away.

Eta Ori is another complex multiple - variable at 1500 light years away; it has two very close (1.6 seconds of arc) stars of mag. 3.7 and 5.1. I can resolve this with my 12.5" stopped down to an unobstructed 5.5" at moderately high power. The 9" Oddie refractor resolves it very clearly at around 570x. The brighter component also is a spectroscopic triple, the two closest being an eclipsing binary rotating around each other every 8 days varying by 0.1 magnitude, the third star taking a period of 9.5 years around both. Speckle interferometry has resolved this third star.

Lambda Ori is another blue-white star, at mag. 3.7 it has a close companion (4.4 arc seconds) of 5.6 mag. This required fairly high magnification to resolve it.

And the best I reserved till last, Sigma Ori. It is one of the most impressive multiple stars I have seen, I call it the "mini solar system" - because that is what it looks like! It is 1500 light years away and its combined magnitude is 3.7. It has two further stars, one a bluish at mag 6.5 the other a reddish at 7.2, closer still is a 10th magnitude companion, the colours are enhanced when I remove the 5.5" aperture stop to reveal the full 12.5". Close by in the same field is a triple star, Struve 761, in a form of a triangle of 8 and 9 mag. stars. Its designation Sigma, meaning "sum off", is very appropriate! Together these stars form a lovely sight to be viewed again and again.

Moving on to the faint fuzzies, with the 5.5" aperture stop off to maximise light gathering, the first nebula that is a must see is M42/43.

M42 is THE nebula to show at public viewing nights; it never fails to impress, it is the most celebrated of Orion's deep sky wonders and it is the finest diffuse nebula in the sky. It was the first nebula to be photographed, in 1880 by Henry Draper. This nebula appears as a luminous fan spanning about degree in diameter; it is about 1500 light years away and up to 30 light years across. It is the birthplace of a star cluster, the nebula being excited by the intense radiation of these new stars. Behind the nebula, there is even a larger dark cloud forming even more stars. The larger the aperture of the telescope, the more complex become the wreaths and swirls of gas. Embedded in the nebula is Theta Ori known as the Trapezium - an average night brings out four stars, a good night brings out 5 and excellent night brings out 6 stars in a small to moderate size telescope. A dark cloud (known as the Fish Mouth) separates M43 from M42, which are really part of the same cloud. M43 is centred on a mag. 9 star.

M42 responds well to nebular filters, there is certainly no shortage of light! A Deep Sky filter enhances the contrast, reveals clearer detail in the centre of the fan (or the "bowl"), sharpens its edges and extends the fan - particularly if viewing close to city lights. A UHC (ultra high contrast) filter brings out the complex swirls and further sharpens the edges of the fan, and brings out "the sword" - a curved razor edge appearing like a cutless at the edge of the nebula. An OIII filter nearly cuts out the stars but dramatically sharpens the nebula edges and "the sword", and the swirls now become vivid - a magnificent view to be returned to time and again! Even a H-Beta filter has its purpose, once you get used to the dramatic loss of light, "the sword" all but disappears but "the bowl" becomes prominent, and different wreaths and ridges appear. I find it very interesting to view the nebula using a little bit of oxygen then a little of hydrogen. For an overall view, keeping some starlight, I feel the UHC is the best filter for this object - after a deep sky site of course.

NGC 1977 is an elongated nebula, just to the north of M42. It is centred on the 4th mag. star 42 Orionis. It is 1500 light years distant, and is lovely nebula but completely overshadowed by its close proximity to M42.

Just a little further to the north is NGC 1981 which is a scattered star cluster. On the southeast side of the cluster lies Struve 750 - a double star with components of 6 and 7 magnitude.

Also a little north, right next to Zeta Ori, is the Flame Nebula (NGC 2024). This nebula is reasonably large (at least 10 x 4 minutes of arc) and detailed. It starts to show well in 6" or larger telescope. It has a broad dark band that begins to bisect it before splitting up inside it. Unfortunately 2nd magnitude Zeta Ori interferes, so if you wish to improve your view you require reasonable power, remove Zeta Ori out the field, and use a UHC filter.

M78 is a small (about 4 minutes of arc) elongated nebula centred on a wide pair of stars that look like eyes staring out. I can make out a slight wispy structure in my 12.5" at moderate power with a Deep Sky filter. Upon closer inspection you may find that the whole field has faint nebulosity.

Barnard's Loop is very faint nebula about 5 x 0.5 degrees. Roughly it arches from Rigel, sweeps in a broad arc to the east around M42 and the Horsehead (which are within the arc), and back around to a point half way between the belt and Betelgeuse, around NGC 2111 (a sparse open cluster). To see it you need very good conditions away from city lights, a telescope/ binoculars with a wide field, and an H-Beta filter or UHC filter would not go astray. If you see this loop then a further challenge awaits - the fainter Lambda Ori nebula. This nebula lies to the north around, you guessed it - Lambda Ori. This nebula has a vague shape a bit over a degree in diameter.

And last but not least is the Horsehead Nebula. For some reason which escapes me, this Society has irrevocably associated my name with this nebula! It is notoriously difficult to see, even though it is a relatively easy object to photograph. This is because the background nebula emits only in hydrogen light, most of which is hydrogen alpha, deep in the red part of the spectrum where our eyes are insensitive at those light levels. However it also emits some light in the next excitation level - hydrogen beta, which is in the green-blue part of the spectrum where our eyes are far more sensitive. The whole nebula is really a glowing area of gas up to 1/2 degree wide surrounding the star Zeta Ori (but not incorporating the Flame Nebula). Running south is a strip of nebulosity, and where the nebula becomes slightly brighter, a dark cloud (shaped like a horse's head) indents it. The Horsehead is small but not tiny (about 5 minutes of arc) and has very low contrast.

The first time I have really seen it clearly was through Peter Bobroff's 20" with a H-Beta filter at his Michelago property. Once seen, then it becomes surprisingly easier to see in smaller telescopes (this has led to stories where some have failed to see it in a 40" whilst others have seen it in a 5" refractor - or even in 10x 70 binoculars with H-Beta filters tapped to each eyepiece). To see it, you must pick your nights, use low magnification and stick on a H-Beta filter (or very least a UHC). I have seen it through Graeme Tapper's 12.5" at Tidbinbilla, and through Eric Pozza's and my own 10" at Michelago. I have seen it from my backyard in Curtin through my 12.5" and even the 6" Takahashi, with an H-Beta filter on very clear, dark settled nights. On one particular night I even glimpsed it using no filter at all in my backyard! Others have also achieved this with Denver Baines' 12.5" at his Bellmount Forest property. My next goal is to see it in a 4".

To see it - pick your night, use a reasonable size scope, an H-Beta (it really does work - in fact it is the only nebula really suitable for that filter!), get Zeta Ori out of the field, and keep the magnification low (exit pupil no smaller than 4 mm). If you don't have a filter, get away from the city, and use a low magnification that is just high enough to block out the natural sky glow. Just let the photons flow in, and what the heck, why not also use a bit of imagination!