The Southern Cross, more properly known as Crux
Australis or the official name Crux, is
one of the brightest and romantic constellations of the
sky. Crux, from the Latin term of Cross, is symbolised on
our national flag, so all Australians and New Zealanders
knows of its existence. Symbolised defiantly on the Eureka
Flag, and made the official ‘Made in
Australia’ labels through 1947 to 1974 for most
goods manufactured in this country. The Australian Cricket
team sing proudly in their irreverent version of
“Under the Southern Cross I stand...”,
which still evokes pride and nationalism among all
Australians. Anyone, with even a minor interest in
astronomy is automatically drawn to this region. Having
any interest in astronomy can draw your close friends and
acquaintances to join your interest in the sky. The most
frequent questions for most of the southern inhabitants of
the world is ”What star is that?“
followed by ”Where is the Southern
Cross?“
In the sky the constellation of Crux does not really look
like a cross but perhaps more resembles a flying kite. The
faintest of the five stars of this brilliant stellar
asterism makes a decided dent against its symmetry. To the
east are the Southern Pointers, often just named
The Pointers of the two stars Alpha and Beta
Centauri that aim slightly north of Crux’s true
centre, following the Cross in the eternal circle of the
celestial sphere.
We are drawn to this area because of the concentration of
bright stars, mixed with the diverse range of bright and
interesting celestial objects. This area is an ideal place
for the beginner who has obtained an optical telescope,
and start his astronomical searching through the skies.
This section gives a history of the region, a suitable
map, and general descriptions of objects the observer may
wish to gain some experience.
Crux was likely named by either the sailor A. Royer in
1679, or by the French explorer Abbé de La Caille
in 1670. In ancient times it was included with the
constellation of Centaurus. As time passed-by some of the
western world migrated south, first to the southern
Africa, South America. Soon the constellation was give an
individual identity.
In the 1930’s the International Astronomical Union (IAU), ratified Crux as a unique constellation instead of being the subsidiary of the surrounding Centaurus. Crux has the smallest area of the eight-eight constellations, being a mere 68.477 square degrees, some 0.16% of the entire sky, and rates closely behind the other two northern minnows of Sagitta and Vulpecula. Due to its position it culminates on March 30th at midnight, or May 14th at 9 pm. From 33 degrees South latitude to the South Pole, Crux is circumpolar, so its outline never sets. In Sydney, it just scrapes the southern horizon generally around 9 p.m. during the beginning of Spring. In the city sky-glow it always can just be seen, even when upside-down and near the horizon, where atmospheric refraction distorts the familiar shape to appear much larger in size than normal. In the northern hemisphere, the Cross can be seen scrapping just above he southern horizon during spring from the southern United States, North Africa and Southern Asia.
Its circumpolar motion also means Crux can be used as a direction finder for true south, as long as you are south of the equator by 5 degrees or so. Just proportion four-and-a-half times the longest length, between Gamma and Alpha Crucis, to the point where the South Celestial Pole (SCP) lies. Dropping down from the SCP to the horizon indicates the potion of true South. Boy Scouts and farmers in their agricultural fields use this method to find their way home on dark clear nights.
Crux is wholly embedded within a very bright portion of the southern Milky Way and whose size has the smallest area of any of the established eighty-eight constellations. Conversely, it also contains three of the most prominent night-time first magnitude stars that are all placed within the Top 25 of stars. These luminaries are Acrux, Mimosa and Gacrux; being respectively ranked as the 15th, 21st and 25th in apparent brightness which is more properly in termed visual magnitude.
Crux until the last few centuries has not always been
recogmised as a constellation in its own right. It was
once placed as a small subsidiary of the surrounding
constellation of Centaurus that now appears around three
sides of the Cross to the east, west and north. Crux
didn’t really become its own unique constellation
until it was finally agreed and ratified in the
1930’s by the now world-wide and officially
recognised International Astronomical Union (IAU)
controlling the naming, discovery and termonology of
astronomical bodies.
In total area Crux covers merely 68.477 square degrees of
the sky. This small area is only about 0.16% of the entire
sky, rating very closely behind the other two minnow
constellations of Sagitta and Vulpecula in the northern
Milky Way.
Only five (5) main stars make out the outline of the Cross, and like most of the other eighty-seven constellations, have given names based on a combination of the Greek alphabet - whose order in nearly all cases signifies decreasing brightness, followed by the constellation’s name. For Crux, each of the brighter five stars just happens to descend in brightness in a clockwise manner, hence the five brightest stars in the outline are named Alpha, Beta, Gamma, Delta and Epsilon. Next is added the constellation name, and this is written in its genitive form of possession, following its Latin-like syntax. So instead of calling the star ‘Alpha of Crux’ it becomes to be called instead Alpha Crucis. Astronomers have further summarise this by sometimes using a later agreed three-letter abbreviation - so Alpha Crucis can be written just as Alpha Cru or even only as α Cru. Once a four-letter sequence was suggested also in the 1930’s, but this was mercifully rejected after a while as being unnecessarily too complicated. Diven or proper names for stars are usually only reserved for only the brightest of stars, but our Crux’s case, only Alpha, Beta and Gamma Crucis have them. They may have had names in the past but none has remained in popular usage today.
Interestingly, the Greek nomenclature of these stars around the beginning of the 15th Century were introduced quite differently. One of the first formulations of these was created by the German protestant lawyer, Johann Bayer (or Philolaus) (1572-1625) who labeled the four brightest stars as the Centaurus stars ε [epsilon], ζ [zeta], υ [upsilon] and ξ [xi]. Ie. Where α=ζ, β=ξ, γ=ε and δ=υ - as also given in Bayer’s attached catalogue. (Crux he describes as moderis crux)
Alpha Crucis is usually called Acrux - this being a clever combination of its Greek letter and constellation name. The modern usage of the name was likely first coined by the American astronomical popularist Eliijah H. Burritt somwhere between 1833 and 1856, where the name appears next to this star in his own star atlas that sold over 250 000 copies. Acrux to the eye has a distinct cold steel blue tint but is paler than Virgo’s 1st magnitude star Spica - some 53o further north of its position. As mentioned before, Alpha Crucis was part of Centaurus which marked the place of the right rear hoof of legs of the Centaur. Prior to cartographer Bayer, the star assigned the Greek letter ζ. Oddly, ζ Centauri is now recognised as the shoulder of the Centaur!
Beta Crucis has been properly called Mimosa though many sources, mainly expressed by northern hemisphere observers, continue to improperly claim the name Becrux. This modern name never been recognised as such and has been contrived without much thought or humble reverence. Similar in colour to Acrux the star exhibits the same cold-bluish white tint. Its brightness is also about half a stellar magnitude fainter than Acrux. Mimosa is the most western of the stars in the main asterism being close to the famous open star cluster called the Jewel Box. The star’s name has an interesting history, although it is strangely not mentioned in many references - even among the pages of the in the classic 1890’s book ‘ Star Names - Their Lore and Meaning’ by Richard Allen, Mimosa original name derives from from the 25mm to 50mm globe-like flower stucture of the fern-like plant known as Mimosa pudica. Found primarily in Micronesia in the Southern Pacific, but in recent times almost everywhere, its unique flower is shaped like a star with hundreds of rays pertruding from its slightly more darkened centre. Many of these flowers are typically either, purplish-blue to purple in colour and are tipped with a white anther placed on the ends of the ray tips. The family of these plants are the Mimosacees, which is relation of the Acacia tree. A similar plant with similar shaped flowers is the Japanese species called Mimosa microphylia. Although first found in Brazil, the plant has a strong Australian connection, as the plants all first originate from Australia’s some 700-odd species that includes the springtime’s golden-yellow wattle. Although Mimosa pudia is considered a tropical weed / shrub throughout the world because of is prickly stems, it is easily recognised by its by its leaves which close-up automatically when lightly shaken or touched. (In some places of the world it is called the ‘Sensitive Plant’
Indeed the name is quite appropriate as its flower does look like some dazzling bright star in the sunlight, agreeing well with this Southern Star of the Cross.
Gamma Crucis name Gacrux was added in the early 20th Century, and also mimics Burritt’s formulation of the name for the brighter star Acrux. According to Richard Allen’s “Star Names: There Lore and Meaning”. Lying at the top or more northerly part of the Cross, Gacrux is visually different in colour to the the other principal blue stars, being a vivid orangery-red or reddish-orange tint. To my eyes this colour is much paler than Antares / αScorpii red colour but is certainly richer than the northern 1st-magnitude orange star Arcturus. Gamma Crucis is roughly about 0.5 magnitudes fainter than Mimosa and almost a whole magnitude fainter than Acrux. Both Gacrux and Acrux point in the direction of the South Celestial Pole and are presently separated (2005) by some 6.0o.
Delta Crucis as yet this star has not been yet blessed with a decent proper name. At 2.7v magnitude, almost exactly two (2) magnitudes fainter than Acrux, it lies on the eastern side of the Cross. Like Acrux and Mimosa the star appears as a gorgeous pale blue which is perhaps just a little paler than both of them.
Epsilon Crucis is the faintest of the five prominent stars, but has no given name. Its main contridiction of our brilliant stellar asterism is that it makes a decided dent against the constelation’s overall symmetry. Epsilon Crucis in fact was just simply ignored by the early stellar cartographers, and this can be clearly seen within Bayer’s 1603 Map of this region of sky. The star is much fainter than the other four principle stars, being just over 3 magniudes fainter than Acrux - being listed as 3.8v magntiude.
Often Epsilon’s faintness confuses many novices in finding the Cross. It is also different in colour, appearing to my eyes as a washed-out almost ‘dirty’ orange star.
To the east by some degrees are the two Southern
Pointers, often called simply The Pointers as
they point towards the Cross. These brilliant southern 1st
magnitude stars are commonly known just as Alpha
Centauri and Beta Centauri but do not lie
within Crux’s boundaries. Both stars are presently
separated in distance of some 4.4o degrees,
which is similar in distance to the northern zodical
constellation of Gemini whose 1st magnitude stars, Castor
and Pollux are separated by 4.5o. These
‘Northern Pointers’ however are fainter than
the southern ones.
α and β Cenauri are within the large
constellation of Centaurus the Centaur being the mythical
creature of the Ancient world that combined both the
intelligence of Man and with the speed and grace of the
horse.
Visually this bright stellar duo are conviently are
aligned to point directly to the southern cross, even
though their true aim is actually just slightly north of
Crux’s true centre - roughly 50' north of γ
Crucis.
The Pointers always chase the Cross in its eternal circle
around the celestial sphere, but never catches them.
Alpha Centauri is position in the sky lies some 15.6o east of Acrux. The star is unmistakeable because of its sheer brightness - the third brightest in the night-time sky - and gorgeous ruch golden-yellow colour that contrasts will with the cold steel-blue of its neighbour, Beta Centauri, and the other blue stars of the Cross. It is perhaps the most famous star of all being our closest stellar neighbour. Its distance is about 1.3 parsecs or 4.3 light-years, simply meaning that the light we see from the star has taken 4.3 years to reach us. It is also the 3rd brightest star in the sky, placed only behind the Sirius in Canis Major and southern Canopus in Carina. Although classically deemed as 1st magnitude stars, all three have modernly determined visual brightness that are negative values. Sirius is presently given as -1.46 magnitude followed by -0.72 for Canopus. α Cen is -0.06.
For many years Alpha Centauri was placed behind the 4th brightest naked-eye star, Arcturus (α Boo), in the northern constellation of Böotes. Although a close-knit race, Arcturus is -0.02 magnitude, being pipped only by only a miniscule -0.04 magnitudes! This is further complicate by the fact that Alpha Centauri is a moderately close binary star which is unresolvable to the naked-eye. Visual brightness then must be based on a combined magnitude. Some sources claim the visual magnitude is -0.06, or express them separately as +0.1 and +1.3. Others use the Hipparcos satellite’s “V” magnitudes, which is a slightly different method in measuring brightness. If we add these values together, the individual -0.01 and 1.35 magnitude stars becomes -0.28! Arcturus according to Hipparcos is the fainter magnitude of -0.05
Through my eyes I think Arcturus certainly looks fainter the α Cen merely by the fact that Arcturus is more orangery-red in colour compared to α Cen’s intense rich yellow. This is probably due to the human eye being less sensitive to redder objects, thus making such reddened stars appear fainter.
α Cen proper name is Rigel Kentaurus or sometimes abbreviated as Rigel Kent. This is a relatively new name was coined by the early aviators during the early 20th Century who used the star for navigation purposes. During the 1st Millennium A.D, its Arabic name was Wazn, but this usage has not continued or passed on into modern times even though β Centauri has!
Beta Centauri happens to be about magnitude fainter than Alpha Centauri and lies between it and the Cross - some 4.4o from α Cen and 9.4 West of β Crucis. At the present time, β Centauri lies some 28' further north than α Centauri. Although the star is just slightly fainter than α Centauri the true distance is some 525 light-years - being 122 time further away. This clearly suggest that β Centauri is far more luminous than the Sun, and is more akin with the other blue luminous stars of the Cross.
β Centauri is named either Hadar or Agena. Hadar is an Arabian name certainly the older name that dates back to 1st Millennium A.D.
C R U X in history was not recognised as the same famed
constellation that we commonly acknowledge today. We do
know that in ancient times Crux was only included among
the stars of the constellation of Centaurus. In Roman
times the Cross were given only as status as a small
sub-group of stars or asterism of Centaurus that
was known as Thronis Caesaris in veneration of
their emporor and god, Augustus Caesar. This is suprising
considering the brightness of the stars, which has been
tentatively an not very convincingly due to the
star’s proximity to the southern horizon.
The modern constellation of C R U X †has often been
claimed to have been named by either the French explorer
Abbé de La Caillé in 1672-3 or the sailor
Augustine Royer in 1679. Of these perhaps Royer’s
claim should be perhaps acknowledged first, as he was
first to coin “Crux Australis” for the
Southern Cross defining it as a seperate constellation.
Another recent found image of the Cross appear on a
celestial globe that was engraved by Jodocus Hondius and
created from drawings by M. Emerie Mollineux (Molyneux) of
Lambeth in 1592 in Tudor England, whose ‘new’
constellation name was then adopted by Bayer in 1603. This
clearly preceeds Royer claim. By since this time the
“stellar” name of Crux has been found in
several earlier historical references. One of the earliest
significant known references is from a private letter
written in 1503 by the Italian navigator, Amerigo
Vespucci, who described “four magnificent
stars” Another by the sailor Antonio Pigafetta
in 1515 said while travelling of the Globe, wrote;
“... a wonderful cross, most glorious of all the
constellations in the Heavens.”
Modern historians have more recently have assigned Crux’s original derivation to the Italian, Andrea Corsali that was found in an old twenty-page letter and manuscript entitled; “Lettera di Andrea Cosali allo illustrissomo Principe Duca de Medici, venuta Dellindis del mese di October nel XDXVI”; (The original of can be found in the National Gallery of Australia and is available in the Internet accessable Archives Section. Here we can clearly see the small drawn figure on the first written page of the manuscript clearly showing Crux and its five principal stars near the zenith, but also seen are the two Small and Large Magellanic Clouds and the position of the South Celestial Pole (polo anlatico) The whole page can be viewed by selecting the thumbnail mentioned in the page above. It seems this document is yet to be properly translated. Corsali wrote in 1516 that Crux was; “...is so fair and beautiful that no other heavenly sign can compare to it”
One of the first instances of the Southern Cross in the historical literature was by Dante’s (1265-1321) in the famous trilogy “Paradisio”, “Inferno” and “Purgatoio”. Here in the last book, Purgatio, he describes only the four principle southern stars of the Cross - naming them after the admirable virtues of Justice, Prudence, Temperance and Fortitude. Dante seems once to have seen the Cross low on the southern horizon and likely missed faint Epsilon Crucis in haze or poor seeing.
Yet of all of these claims it remains quite uncertain if these individuals had any influence with its popularist name of today. In my own opinion the true derivation of Crux’s name will likely to remain forever unknown to us.
As Western World and its civilization expanded, European
populations began to migrate ever southward. It was at
this time that the bright stars of Crux gained new respect
and individual identity. This can be seen from the
writings of these first arrivals either as new explorers
and travellers to the then unknowm new lands of the South,
which began in earnest somewhere between the 13th to the
early 15th Century. Each person was venturing forward in
the hope or promise of finding new opportunities in the
‘New World’ and perhaps to fullfill their own
dreams or to make their fortunes. Most first journeyed
either Southern Africa or South America where this
wonderfully bright and conspicuous constellation could be
seen. In time this was to be expanded across the world to
the lands of the East Indies, the islands of the Pacific,
the North and South islands of New Zealand, and
‘Terra Australis’ or ‘New Holland’
- today’s Australia.
During these early days of southern hemisphere
exploration most of the European voyagers and settlers
made the Cross their familiar and nightly friend - a
companion of sorts thwarting the real hazards when making
crossings of an often unpredicatable ocean over a long
distance.
Crux to these explorers also became a strongly recognised and true symbol of their own religion and mysticism. For example, The early Portuguese sailors and navigators perceived Crux as the manifested Christian symbol of their religious faith. This certainly gave them great comfort regarding their own personal safety during the often long and dangerous voyagers through the then mostly uncharted southern oceans. Even today, many international travellers who journey on ship to the Lands of the South are sometimes highlighted by viewing the Cross from the ship’s deck - and perpetuating the origin of Crux’s charisma. I have even heard stories from sailors in the Second World War while stationed in the Pacific Ocean. While writing this newer version of this article, I recieved an charming e-mail from one such person, Mr. Ron Dragotta, who presently resides in Santiago, Chile. He describes, and I quote;
“...on viewing my favourite constellation in the southern hemisphere, the Southern Cross, I remember viewing this many times while sailing back and forward across the Pacific, too many years ago while in the U.S. Navy. Quite a sight on a dark night at sea though!”
An example of such sentiments was that of Alexander van Humboldt who himself saw the glorious Southern Cross in 1799 lying above the lonely waters of the mid-Atlantic. He expressed the following famous quotation;
“For several days the lower regions of the air was full of water vapour, but the night of the forth to the fifth of July, 1799 in the sixteenth latitude we saw the Southern Cross for the first time. It was steeply inclined and appeared from time to time between the clouds, the centre of which flickering sheet lightning, shone with silvery radiance. If a traveller may mention his personal feelings, I may remark that on this night one of my dreams of my earliest youth was fulfilled.”
Another famous quote was made by Charles Darwin in his book The Voyage of the Beagle. At the end of his lengthy voyage of discovery, he summarises the journey by saying;
“Among the other most remarkable spectacles which we have beheld, may be ranked the Southern Cross, the clouds of Magellan, and the other constellations of the Southern Hemisphere, water spouts..., glaciers..., active volcanoes.... etc.”
This romance continues today. Over the years I have seen many amateur astronomers, sitting and talking around a night-time campfire on bitterly cold winter nights. By warming themselves during these short break from the cold air and their long observation sessions, they soon forget all about the many deep-sky wonders they see through their telescopes, and just as in the days of old, simply look and ponder at the southern night sky. Sometimes there falls an almost awed silence, as even their own hearts have become warm and comfortable by these familiar stars like the Cross.
I have many memorable experiences of my own the Cross.
One was seeing the Southern Cross in the wee morning
hours from Banka Island, which is an island off western
Sumartra 100 kilometres due south of Singapore, and a
little south of the equator but a few hundred kilometres.
My view occurred on 18th March 1988 at about 4 a.m. while
sitting as a passenger in the back of a four-wheel drive
while travelling to a the site to see a dawn total solar
eclipse. A group of us had stayed in a Tin mine which was
in the centre of the island but too far north from placed
where we wished to be. Banka island is split into two
geographical area divided by a several short but deep bogs
covering several kilometres. Here all vehicles have to
traverse. It wasn’t much fun in the daylight but
night was a heart-pumping challenge in the dark. I saw the
Cross peaking in and out of the trees as the vehicle moved
along the dirt road. It was sadly inclined 40o
to the horizon and placed slightly west of south. It
struck me how beautiful these stars were and how much
larger it seemed from the refraction through the tropical
atmosphere.
Among the Australian Aborigines. Maori and the Pacific Islanders there are many legends associated with this region of the sky. Most are either depicted on cave or bark paintings or having been passed down through the generations by simple word-of-mouth. For example, one Aboriginal tribe explained that the fish Alakilijia once originated from the depths of the Coal Sack, which was imagined as a deep hole in the river of the Milky Way. Two brothers Wanamountya (α and β Crucis) were in the river hunting for food when they came across a fish. Naturally they seize their opportunity and quickly speared the fish and took it ashore to eat it. Both set up a campfire (δ and γ Crucis), which are both red in colour) and shared their catch. Their two friends are on the other side of the river (α and β Centauri) and between them are their boomerangs, from which all time is marked, made by the rhythm of the tools hitting the ground.
Another legend was that the stars of the Southern Cross
represents the man Mululu and his four daughters. One day,
when he had grown very old, he called his daughters
together to discuss their future. Having no sons, Mululu
expressed his grave concern that after he was gone there
would be no one to look after and protect them. To avoid
this, he came up with a innovative suggestion to solve
this problem by asking his daughters to join him in the
sky. Mululu explained that a clever medicine man named
Conduk had once explained how this could be achieved.
Eventually Mululu died, so the daughters set off to find
Conduk, who lived to the north. Many days later, they
found the medicine man who directed the four to climb a
rope that lead up into the sky. Naturally, they were
frightened - who wouldn’t be!, but with the
encouragement of Conduk, they climb to the top of the rope
where they found their father waiting for them. Of course,
all four were delighted to be reunited with their father
and these stars shine brightly with their happiness. Today
we can still see them is their celestial home, marked by
the four stars of the Cross, and their father being
represented as Hadar or Beta Centauri.
Others in Australia and South America have seen the stars
as a mark of the footprint of either the Australian
flightless bird, the Emu (Dromaius
novaehollandiae) or the Andes smaller bird, the Rhea
or Ñandû (American Rhea). Another
footprint legend of birds is that the Crux is the talon
marks in the sky amoung the birds of prey like hawks and
eagles. One Aboriginal tribe of Arnham land, the Yirrkula
, says the stars mark as the one of the feet of the
wedge-tail Eagle. The Mapuches, a mid-South
American people of a number of regions with the same
language, occupied the middle latitides across Peru and
Argentina. These people had a rich culture and mythology
and retained stories about the stars in bird, insect and
animal constellations.
In the Peruvian plauteu, the peoples of the Aymeara still
celebrate the day of Chakana, which is a celebration of
the harvest. It is held on 3rd May each year mainly
because the early Peruvians had seen Crux being at it
highest position in the sky at this time.
We have said enough about the Southern Cross and its history, but let us now examine the most the prominent celestial treats it contains. I have selected those visible in small telescope or binoculars. More detailed text may be found of many of these objects elsewhere from this site. If you are familiar with these objects, then think back to your first views of these delights - likely your first deep-sky celestial objects amateurs observed through any optical telescope. For me the first objects were Alpha Centauri, Alpha Crucis and the Jewel Box. These sights I can never forget.
To the eye this thick band of cloudy-white nebulosity that travels through the entire region of the Cross and appears much brighter than many other regions through the circle of the Milky Way. When using telescopes or binoculars, the number of visible stars dramatically increases compared to the average patch of dark sky. Milton sums up this concept very well, in the quote;
”A broad and ample road, whose dust is gold,
as stars appear seen in the Galaxy, that Milky Way.
Which nightly as a circling zone thou seest,
Powdered with stars.“
It is because of this, that many interesting objects will appear, as we are looking into the richness of the rest of the Galaxy. Crux happens to lie in one of the brighter regions of the Milky Way.
Figure 2. Nebulosity of The Coal Sack
|
One of the most obvious feature of the Cross is the dark nebula under Hadar / Beta Centauri and across and underneath Acrux. Unfortunately this object is mainly obscured in city skies because of light pollution, but in country skies, the region is very obvious. The inky-black darken patch looks more like a hole in the firmament but it is a giant cloud of gas obscuring the more distant light of the Milky Way. Closer inspection finds that the Coal Sack edges are rather ill-defined, and by staring at its structure reveals many other structures and bays not often presented in astronomical photographs. Figure 2. clearly shows the outline of the Coal Sack. (marked as a gold line) This the general view of is shape. On the darkest night the main body of this dark cloud slowly diminishes to the south-west, and below Acrux becomes just a small stream of blackness. (Figure 2, shows this as a yellow line) This extends further westwards, looping up towards the bright emission nebula IC 2948 (red) and finishing near the famed Eta Carinae Nebulae NGC 3372 (not shown). Other wisps of dark nebulosity surround the area, and with the naked eye most of major part are shown in the figure. (Brown line) Most prominent is the so-called Dark Doodad Nebula, which is a dark nebula passing through Musca to the South of the Cross. If you eyes are acute enough, you can possibly notice the faint 6.5 magnitude star within the nebulosity. (White dot in Figure 2, above the word "Coal".) Even with binoculars, the internal area in sparsely populated with only a few stars. We can easily assume the Milky Way must be considerable more distant from us than the Coal Sack itself. However, the Coal Sack is currently estimated by astronomers to be 190 parsecs (620 light-years) from the Sun. Gas densities are very low, so much so, that even the best vacuums that can be artificially made on Earth are some five thousand times too dense! |
The Coal Sack’s nebulosity was recorded in 1752 from the Cape of Good Hope by Abbé Nicholas-Louis de la Callé (1713-1792), who we now more simply known as Lacaillé. He wrote in “Mémories de l’Academie Royale des Sciences”, pg.286-296 (1755) about the region as;
“One might again include the phenomenon which strikes the eye of those who observe the Southern sky; a space of almost three degrees extent in all directions which appears as dense blackness in the eastern part of the Southern Cross. This appearance is caused by the intensity of the whiteness of the Milky Way which encloses this space and surrounds it on all sides.”
The name Coal Sack probably derives from drovers or miners in the Australian outback. These men presumably were sitting around the campfire, and saw this region as “black as coal” compared to the surrounding bright Milky Way. Sir John Herschel also states an alternative for the origin of the name, saying it was also used in the early 1800’s by sailors when visiting the southern seas. According to Richard Allen, the Coal Sack is also appropriately known as the Black Magellanic Cloud.
1) ACRUX / Alpha (α) Crucis Double Star
Position: 12h 26.6m -63o 06'Acrux or Alpha Crucis is a brilliant blue-white star being easily found at the lowest base or point of the Cross. It is also closest of Crux’s stars to the South Celestial Pole. Examination using binoculars reveals a fainter 4.9 magnitude companion star is some 89″ away, and this has changed little since Father Fontenay from the Cape of Good Hope discovered the pair in 1685. Telescopes greater than about 7.5 cm reveal the brighter component is again double. Often labelled as α1 Cru / Alpha (1) Crucis, the visual magnitudes of these two stars are 1.6 and 2.1 with both separated by 1.4″. All three stars are coloured blue-white. An additional 12.5 mag field star lies roughly midway and slightly east of the wide pair, making an obvious right-angle triangle. Acrux’s field is sprinkled with many stars. This is still one of the most impressive telescopic sights in the whole sky. The view of it still thrills me every time - even though I must have viewed it several hundred times.
Mag. 1.6 / 2.1 / 5.1 Sep. 1.4″ / 89″2) Kappa (κ) Crucis Cluster / The Jewel Box / NGC 4755 / Δ301
Open Star Cluster
Position: 12h 53.8m -60o 21′Some 1o southwest of Beta Crucis / Mimosa is one of the most magnificent and beautiful of the open star clusters that was first discovered by James Dunlop (Δ301). Known as the Jewel Box or NGC 4755, this small ‘A’ shaped asterism is quite different than most open cluster because of its tiny size and numerous bright stars. Its has the given name of the Jewel Box which was given by Sir John Herschel in the 1830’s, after the large number of different coloured stars found within its small 6'arcmin size. When viewed with a small telescope up to fifty to one hundred stars can be seen in a velvet field. My first look at the cluster was at Sydney Observatory through the eleven-and-a-three-quarter inch refractor. I was immediately stunned with its gorgeous richness and beauty, mainly as the city lights made the coloured backdrop appear as a rich royal blue. A faint twinkling of the occasional star gives the cluster an almost Divine nature.
Brightest star is the true Kappa Crucis which is a bright orange-red supergiant, whilst the other stars are coloured mainly blue, white and yellow. How many colour can you see? Distance is estimated at 2.4 kiloparsecs or 7 700 light-years from the Sun, with its age about ten million years - considerably younger than many other open clusters.Note: A far more detailed article appears in the page The Jewel Box
3) Gamma (γ) Crucis Double Star
Position:A brilliant orange-red star at 1.67 visual magnitude. When viewed with binoculars or a small telescope, the bluish companion is easily seen. Although they are not physically connected, the stars have wonderful colour contrast, especially with smaller instruments. Distance is estimated to be 220 ly.
Mag. 1.6/ 6.7 Sep : 130’4) Mu (μ) Crucis Double Star Position:
Two equally bright stars are easily separated even in a smallish telescope. Both stars are bluish in colour in a rich starry field. This pair proved a good test for binoculars as my 7x50’s can just separate the duo, but when tested during poor seeing, became a bit more difficult. You can try viewing Mu Crucis during moonlight, and find the sky glowing velvet, enhancing the this pair. In reality this dual blue giant system estimated to lie 650ltys. away.
Mag: 4.26/ 5.46 Sep: 34.7″5) NGC 4349 Open Star Cluster Position:
This cluster is found north of Acrux by almost 1O, you will need a telescope greater than 10.5cm. The field contains a collection of stars between magnitude 11th to 15th, and it size covers about four or five times the area of the Jewel Box. I saw that the stars were yellowish in colour.6) S Crucis Variable Star Position:
A yellowish star can be found 1.5 degrees north of the star Lambda Cruis, which is a Cepheid variable. Over several days, the star changes in a period of 4.68 days between magnitude 6.1 to 6.8 and spectral class G7 to F6.7) NGC 4103/ Δ291 Open Star Cluster Position
Twenty-five stars between 11th and 14th magnitude lies just across from Epsilon (ε) Crucis. It is a visible in apertures greater than 75cm., and its size is about the same as the Jewel Box cluster.8) T Crucis Variable Star Position:
T Cru is an orange-red star visible in a starry field 0.5oNE of Acrux This is another Cepheid variable that varies as much as S Crucis. The period is 6.753 days, varying between 6.2 and 6.8, and changing spectrally between G1 and G5.9) Iota (ι) Crucis Double Star Position:
This double star is found just south of Beta Crucis, and is a wide pair that appears as a orangery and yellow stars with about three magnitude in difference. The given magnitudes are 4.68 and 7.8, whose separation is a wide 26.7″. This is an interesting for small apertures, especially while in the region of β Crucis.10) AO Crucis Variable Position:
AO Crucis is a variable that can be found about halfway between α Crucis and β Crucis, and about 1.1oSW of Alpha (α) Crucis itself. The reddish colour is quite prominent, though this is a LC-type variable, with the irregular variations change between 7.5 and about 10.0.11) Beta (β) Crucis / EsB 365 Red Star Position:
This deep-red star lies next to Beta Crucis and is so easy to find. Its key feature is that this star colour is the deepest of crimson, and is similar to the colours seen in X TrA or R Centauri. Approximately 9.5 mag, EsB 365 is 45 arc sec. from Beta Crucis. A 10.5 cm is really required to see it clearly, though some claim seeing EsB 365 in 7.5cm is quite possible. Searching for it is worthwhile, as it is quite an enchanting object.
1) Alpha (α) Centauri Double Star
Position: 14h 39.6m -60o 50'Two hours eastward in Right Ascension are the two stars of Alpha and Beta Centauri. Together, both point somewhat north of the Cross, though the name of the ‘Southern Pointers’ still remain. Alpha Centauri is the brighter of the two, and appears distinctly yellow in colour, and is the third bright of all the stars. Navigators have named it Rigel Kentaurus or just Rigel Kent, but this is a recent designation. Amazingly, it did not have a name until the beginning of the 20th Century.
When view through a small telescope the single star separates into two stars of almost the same brightness, given as 0.33 and 1.7 magnitudes. In 2002 finds the separation about 19 arc seconds, which is slowly diminishing. Words can never describe its beauty.
One of the first discoveries of the pair was its fairly rapid binary motion, orbiting in mutual gravitation once every 79.8 years. Not only does it quickly change its position, but it has been found to slightly ‘jump’ against the background stars over the year. This movement is the parallax, that indicates proximity to the Sun. Astronomers in the beginning of the last century knew it was the closest of all the naked-eye stars, whose value is now quoted as 4.3 light-years - 9.4 thousand billion kilometres away. If this is the closest of stars, then how much farther are the rest of the stars in our Galaxy?1a) Proxima Centauri / α (3) (α3) Centauri Star
Position: 14h 03.8m -60o 22′
(See Detailed Alpha Centauri Article)
2) R Centauri Variable Star
Position:
14h 16.6m -59o 54′R Cen is one of the first variables that a southern observer undertakes in a variable star observing programme. Its position is easy to find, as R Cen is roughly just slightly north of the imaginary line between α and &beta Centauri, but more implicitly is closer to β Centauri - some 1.6º to the NEE (PA 74o). R Cen is contained in a quite starry field. The deep blood red colour of this star is remarkable, reminding me of X TrA or even EsB 365 near β Crucis. Throughout the near predictable 550-odd day period, the star changes colour with magnitude, and this is reflected in the M4e to M8 IIIe emission spectra. AOST2 says that the star;
“..near maximum is a fine red star... while near minimum looks crimson.”R Cen has an uncertain distance. Some references still quote values as low as 123pc. (400ly.), but the Hipparcos satellite data places it at 161±13pc. or 525±43ly.
3) NGC 3918 Planetary Nebula
Position:This smallish blue planetary nebula is found on the western of the Cross. Known as the ”Blue Planetary “ or ”The Southerner“, a telescope is needed to see it as its visual diameter is 13″, though the ”star“ is even visible in binoculars. The best way to be certain of it location is to inspect each star, the nebula is the one that will not focus, while the stars will appear as tiny pinpoints. NGC 3918 is one of the brightest of the planetaries in the southern skies and is a bright 8.5 magnitude.
The beauty of the Southern Cross, and the variety of the objects it holds, make it a worthy constellation to start with for the novice amateur astronomer. On a cold and crisp wintery night, gazing at its beauty and charm, soon makes you forget the cold hands and feet. As an observer, if you have not observe some of these objects, then I recommend you do. I am certain you won't be disappointed!
Clear Skies!
1. Brown, P.L., ”Astronomy in Colour“ (1975)
2. Burnham, R.J., ”Burnham's Celestial Handbook. Vol II.“ (1978)
3. Darwin, C., ”The Voyage of the Beagle.“ (1909)
4. Mountford, C.P., ”The Dreamtime Book“ (1973)
5. Vehrenberg, H. & Blank, D. “Handbook of the Constellations” Edition 2 (1973)
| No | Name | Object | Type |
RA 2000 |
DEC 2000† |
Object Description |
| 1 | Alpha Crucis | α Cru | DS | 12 27 | -63 06 | 1.4/1.9/4.9 4.4"/108″ |
| 2 | Jewel Box | κ Cru / NGC4755 | OSC | 12 54 | -60 30 | *=100 D=2.36 kpc / Type=II 2 p (g) |
| 3 | Gamma Crucis | γ Cru / Δ124 | DS | 12 31 | -57 07 | 1.6/6.7 PA 11o 131″. |
| 4 | Mu Crucis | μ Cru / Δ126 | DS | 12 52 | -56 54 | 4.3/5.6 35.0″. |
| 5 | S Crucis | S Cru | VAR | 12 54 | -58 26 | Cepheid 6.2-6.9 / 4.690 days |
| 6 | Iota Crucis | ι Cru / HJ 4547 | DS | 12 56 | -60 59 | 4.7 / 5.6 27.0″ |
| 7 | EsB 365 | ----- | VAR | 12 50 | -59 41 | Red Star of the N Class |
| 8 | Beta Crucis | β Cru | St | 12 48 | -59 41 | Blue-White Star : Spectrum B0.5 III |
| 9 | Alpha Centauri | α Cen | BS | 14 40 | -60 50 | -0.04 / 1.17 20″. |
| 10 | Proxima Centauri | Innes | VAR | 14 31 | -63 10 | Flare Star: Spect: M5/Mag.=11.3 |
| 11 | Beta Centauri | β Cen | VAR | 14 04 | -60 22 | 0.61-0.66 / 3.9: 0.7″ /Beta Cep. B3 |
| 12 | Gamma Centauri | γ Cen | DS | 12 42 | -48 58 | 2.9/2.9: 1.3″. Reduce PA A0III |
| 13 | Omega Centauri | ω Cen | GSC | 13 27 | -47 29 | Mag.=3.65 / F7 VIII / d=5.2 pc. |
| 14 | The Blue Planetary | NGC 3918 | PN | 11 50 | -57 11 | Mag.=8.4 / Type 2b / d =800 pc. |
| 15 | ------- | IC 2944 | NEB | 11 38 | -63 23 | Mag.= 3.1p /Dia = 75′x50′ /Near λ Cen |
| 16 | ------- | NGC 4945 | GAL | 13 05 | -49 28 | Mag. 9.3v / 13' x 1.5'/ d=10 m.ly |
| 17 | Eta Carinae | η Car / NGC3372 | NEB OSC | 11 09 | -60 06 | Var: 2.1p /Dia 80' / d = 1.1 kpc. |
| 18 | Southern Pleiades | IC 2602 | OSC | 10 43 | -64 24 | *=60 / d=150pc. / Type = II 3 m (c) Mag.=1.6 |
| 19 | Football Cluster | NGC 3532 | OSC | 11 06 | -58 40 | *=150/ d=150pc. / Type=II 1 m (f) Mag.=3.2 |
| 20 | Coal Sack | ----- | NEB | 12 53 | -63 00 | Area = 26.6 square deg. / Irregular Nebula |
| 21 | Musca Globular | NGC 4833 | GSC | 12 59 | -70 53 | Mag.=7.4/ Type VIII / F3/ d=5.5 kpc. |
| 22 | Alpha Musca | α Mus | St | 12 37 | -69 08 | Blue Star 2.7v mag / B2 IV-V |
| 23 | Beta Musca | β Mus / R 207 | BS | 12 47 | -69 06 | 3.7 / 4.0 : 1.3″. B2 V |
| 24 | Q Centauri | Q Cen / Δ141 | DS | 13 42 | -54 34 | 5.3/6.7 : 5.3″. / B9 IV |
| 25 | N Centauri | N Cen / RMK 18 | DS | 13 52 | -52 49 | 5.4 / 7.6 : 18″ B8/A3 |
| 26 | R Centauri | R Cen | VAR | 14 17 | -59 55 | 5.3-11.8: Mira Var.p=546days M4-7e |
| 27 | R Muscae | R Mus | VAR | 12 42 | -69 24 | Cepheid 5.9-6.8 p=7.4767 F7 Ib |
| 28 | Carina Cluster | NGC 3114 | OSC | 10 02 | -60 27 | *=30:d=900 pc. Type=II 3r (e) M=4.2 |
| 29 | ------- | NGC 5617 | OSC | |||
| 30 | V745 Centauri | VAR | ||||
| 31 | W Crucis | VAR |
DS = Double Star : OSC = Open Star Cluster : GSC =
Globular Star Cluster: St = Single Star :
NEB = Nebula : VAR = Variable Star : GAL = Galaxy : PN =
Planetary Nebula
(NO ADS!)