Scorpius the Scorpion

Scorpius the 'Scorpion' was the slayer of Orion who can be seen fleeing in the west as the scorpion rises in the east. It is one of the oldest constellations, known since antiquity and recognised by many cultures as a scorpion or snake, and with the early Chinese, part of the mighty 'Azure Dragon'. The Babylonians gave it many titles including 'The Lord of the Seed', 'King of Lightning' and 'Lusty King'. The astrologers called it the 'accursed' constellation, responsible for war and disharmony and saw it as the birthplace of warmonger Mars. Pliny thought that any appearance of a comet in this region predicted a plague of reptiles and locusts.

Scorpius lies immersed in the rich starfield of the Milky Way, covering an expansive area of 497 sq.° and containing many beautiful deep sky objects and double stars. This region is one of the best for inexperienced observers to tackle as most objects are near bright stars, therefore easily found.

The brightest star in Scorpius is alpha or Antares from the Greek 'Rival of Mars'. Shining at magnitude 0.9 with a small irregular brightness variation, this M-type red supergiant has a diameter around 960 million kilometres though its mass is only about 10 to 15 times that of our Sun. Therefore Antares is relatively cool. Antares lies around 420 light years distant and has a luminosity (brightness) of 9,000 Suns.

In April, 1813, Professor Burg at Vienna observed an occultation of Antares by the Moon and reported:

"...I observed the emersion of a star 6.5 mag, which, about 5 seconds after suddenly appeared to me like a star of the first magnitude, and it is from this transition that I have dated the time of emersion. Perhaps Antares is a double star, and the first observed small one is so near the principal star that both, viewed even with a good telescope, do not appear separated..."

Antares is indeed a double star with a magnitude 5.4 B-type companion only 3.0" away, a good resolution test for 15cm mirrors and upwards. In moments of settled seeing, a vivid emerald green dazzle of light is seen buried within the glare of orange Antares, the green colour an effect of contrast against the orange. The best time to observe is when the Moon occults Antares, the re-appearance coming on the Moon's unlit side, the companion emerging first followed by Antares seconds later.

There are a number of globular clusters scattered about Scorpius and NGC 6093 (M80) is one of the more compressed available to the telescope. Discovered by Charles Messier on Jan. 4th, 1781, it lies halfway along a line joining alpha and beta Scorpii. Messier saw it as:

"....round, the centre brilliant and it resembles the nucleus of a little comet..."

Sir William Herschel described it as:

"May 26, 1786. 20 feet telescope. The 80th of the Connoissance des Temps is a beautiful, round cluster of extremely minute and very compressed stars about 3' or 4' in diameter, by the increasing compression of the stars the cluster is very gradually much brighter in the middle...the richest and most condensed mass of stars which the firmament can offer the contemplation of the astronomer."

M80 is extremely condensed and a 20cm mirror at high power will start to resolve the outer edges only.

For contrast, look for the globular cluster NGC 6121 (M4) 1.3° west of Antares. Discovery is often credited to Cheseaux in 1746 who recorded it as:

"close to Antares...white, round and smaller than the preceding ones. I do not think it has been found before..."

Messier observed it in May 1764:

"cluster of very small stars; with an inferior telescope it appears more like a nebula; this cluster is situated near Antares and on the same parallel..."

M4 is one of the loosest globular clusters in the sky, appearing similar to an open cluster. There is a curious bar of bright stars running across centrally, standing out amongst the fainter scattered members. Star counts to magnitude 19.3 have shown around 10,000 stars in a spherical halo, many fainter stars can be assumed.

NGC 6144 can be found 0.6° northwest of Antares, this globular having a luminosity similar to M4 but is smaller and heavily obscured by the blanketing dust and gas that pervades this region. Discovered by Sir William Herschel in 1785, he recorded it as:

"a very compressed and considerably large cluster of the smallest stars imaginable, all of a dusky red colour, the next step to an easily resolvable nebula."

NGC 6144 is very loose and appears rather faint and small, needing a medium mirror or larger and Antares needs to be kept out of the field of view to make it obvious.

NGC 6281 is an open cluster east of the naked eye double star mu Scorpii. The field is very beautiful with a central knot of stars in a pyramid shape as well as several pairs and 2 bright orange stars. A 20cm mirror will resolve around 25 stars and larger mirrors with high magnification many more fainter members.

The next open cluster is situated in a most beautiful region. NGC 6231 is the first open cluster I ever observed and the delight and excitement is re-experienced whenever I view it. Here we have a knot of bright diamonds inlaid upon hundreds of fainter members. All the bright stars are giant or supergiant types of intense brilliance. NGC 6231 was discovered by Giovanni Hodierna, appearing in his catalogue of 1654. This is an exceptionally young open cluster, around 3.2 million years old. Belonging to the Scorpius OB association, the hottest star in this cluster is of spectral type O8.

A recent proper motion study by R. Rees & K. Cudworth* suggests that NGC 6231 formed around the time that NGC 6397, a globular cluster in nearby Ara, passed through this region of the galactic disk and thereby triggering the formation of NGC 6231. Disk passages of globular clusters triggering stellar formation was proposed by Wallin et al. (1996, ApJ, 459,555).

South of NGC 6231 and in the same low power field lie zeta1 and zeta2 Scorpii. The magnitude 4.8 B-type hypergiant zeta1 is thought to be a member of NGC 6231, lying around 2,600 light years from our Sun. Zeta1 is well immersed into the depths of the Milky Way, the cloaking gas and dust dimming the star by 2 magnitudes. Zeta1 weighs in at 36 solar masses and has an actual luminosity 850,000 times that of our Sun.

With a short lifetime expectancy of around 10 million years or so, zeta1 is roughly halfway through it's life cycle, this hypergiant will explode in a fiery supernova event. Mass loss is currently running at 1.55 × 10^–6 solar masses per year, the equivalent of the Sun's mass every 640,000 years.

Zeta2 is a magnitude 3.6 orange K-type giant, a lot closer than zeta1 at 150 light years.

North of NGC 6231 is Tr 24 (H12), an open cluster connected to the former by a chain of scattered stars. This whole field is littered with lines of radiating stars that outline a spiral arm between our own spiral arm and the centre of the galaxy. This is a superb area for astrophotography, the resulting images revealing the faint emission nebula IC 4628 which is involved with these stars, just a brighter part of an extended faint gas cloud that envelops this region. Large mirrors combined with a UHC filter may just discern this nebulosity if sky conditions are excellent.

West of NGC 6231 is the large scattered open cluster NGC 6124, containing many orange stars and pairs, discovered by Lacaille in the early 1750s. The age of this cluster is computed at 51 million years.

NGC 6388 is a globular cluster, bright and round lying 1.7° south of the bright star theta Scorpii. Discovered on June 5, 1826 by James Dunlop, he recorded it as:

"...a beautiful round nebula, about 5' diameter, with a bright round well defined disk or nucleus, about 15 arcseconds diameter, exactly in the centre; this has the appearance of a planet surrounded by an extremely faint diluted atmosphere; there is a small star involved in the faint atmosphere: the atmosphere is at least 6' diameter". It is a well compressed group of suns, brighter toward the middle and unresolved in mirrors below 40cm.

Situated halfway along and north of a line joining the 'stinger' stars lambda and upsilon with mu Scorpii is NGC 6302, commonly called the 'Bug Nebula'. This remarkable bi-polar nebula has a high surface brightness so don't be afraid to use high power to extract detail. Two irregular lobes of gas extend either side of a brighter central mass. The explosive process that formed NGC 6302 was particularly violent, spewing out material at speeds around 400 km/sec., most planetary nebulae have gas outflows in the region of a few tens of km/sec.

Moving back to lambda, go east to the star G Scorpii, a 3rd magnitude K-type orange giant star. In the same low power field is a globular cluster, NGC 6441, in lovely contrast with the orange star. Bright and round, it is extremely compressed and can appear slightly purple in colour with large mirrors.

Around 1.5° to the west northwest of kappa Scorpii lies the bright 4th magnitude star Q Scorpii. Southwest of this star sits an extremely faint and difficult globular cluster. NGC 6380 was discovered by Sir John Herschel from the Cape of Good Hope in 1834. It wasn't until the 1950s that is was found to be a globular cluster by A Thackeray on photographic plates taken with the 74 inch telescope at Radcliffe Observatory.

Why is this cluster so difficult? A bright magnitude star sits on its southern edge, overwhelming this small and unobtrusive glow. I may have spotted it once in 1990 when using a 55cm telescope from Mt Roskill, Auckland, NZ. It was extremely difficult, particularly from a suburban site though a brightening of the background against the glow of the orange star was suspected. This is definitely one for a darker sky.

Ton 2 is another extremely difficult globular cluster lying close to Q Scorpii, once again the glare from the star drowning it out. Discovery is credited to Paris Pismis who identified it on Schmidt plates taken at the Tonantzintla Observatory, Mexico in 1959. Note that Ton 2 is erroneously plotted on many atlases. It actually lies very close to Q Scorpii and only the largest mirrors and superb sky conditions will pull this one in. Not for the fainthearted.

The glorious open cluster NGC 6405 (M6) is at the apex of an elongated triangle with G Scorpii and lambda. Large and bright, this very rich cluster is superb, resembling a butterfly with open wings, hence its charming name 'Butterfly Cluster'. Among the many bright stars are numerous groups of fainter stars. In the northwestern region is a v-shaped chain of faint equal magnitude stars, particularly delightful. High power will draw out many fainter members.

Finally, move back to lambda then sweep northeast to capture the stunning naked eye open cluster NGC 6475 (M7). A low power will show off 80 or so stars brighter than 10th magnitude within its 1° diameter along with many fainter members. The spectra types are mainly B and A and the age of this group is around 260 million years. Many observers are so taken by this majestic group of brilliant, dazzling jewels that they miss one of Scorpius' hidden surprises. With care, a small and faint 11th magnitude globular cluster, NGC 6453, can be seen on the cluster's northwestern outskirts.

There are many other objects in Scorpius worth tracking down, mainly open clusters and faint planetary nebulae that need a dark sky. This region is an observers dream.

Clear skies and good hunting ~CosMos

*Did The Globular Cluster NGC 6397 Trigger the Formation of the Young Open Cluster NGC 6231?: R. Rees & K Cudworth (Yerkes Observatory).