The Crux of the matter
During the next two months, early evening shows the Southern Cross, Crux, at its most spectacular, standing upright on the meridian.
Emblazoned on our and our neighbour’s flags, used and abused in a hundred commercial devices, and exploited mercilessly in colonial verse, or worse, the Cross remains the most striking of the southern constellations. For anyone north of latitude 26° North (which runs through southern Libya and Egypt. Bahrain, the Gangetic plain, northern Burma, Okinawa, Miami and just south of the Canary Islands) the Cross is never seen entire, while from the site of Greenwich Observatory’s 4.2-metre telescope atop La Palma at latitude 29° North just two of the four principal stars can be seen.
At 33° South and beyond, the constellation is circumpolar, which is to say that it is always above the horizon even as it swings inverted below the South Celestial Pole. Since we southerners lack a naked eye star at this pole (and thus a simple marker for geographical south) it is fortunate that the Southern Cross is always, weather permitting, visible, for it is the key to one of the two methods of making a reasonably accurate estimate of the pole’s position- something which many a benighted tramper has been thankful for.
As shown in the diagram. the trick is to imagine a line projected through the length of the Cross from top to bottom, and at the same time another line running between the Pointers and at right angles to a line joining them. Where these two projected lines meet is a very fair approximation to the position of the South Celestial Pole. Because the Pointers and the Cross are so close together there is a good chance that even with quite thick scattered cloud one will be able to see all these stars at once, and so make an estimate. Geographical south is always on the horizon directly below the celestial pole.
Smallest of all the constellations, Crux is far from being visually insignificant, for its definite form and the close grouping of its brightest stars immediately attract attention even though it is almost surrounded by Centaurus. Explorers at the end of the fifteenth century, such as the Portuguese sailing south down the western coast of Africa, were the first Europeans to sight the Southern Cross. But it was not until after 1600 that the stars forming the Cross were separated from the Centaur, where they had originally marked the rear hooves and fetlocks.
Serendipitously, when Crux is at upper culmination, i.e. on the meridian and above the South Celestial Pole, the Milky Way forms an almost symmetrical arch from east to west with the cross as a keystone. (This dramatic moment is illustrated in the New Zealand Geographic chart “The Southern Sky”see inside back cover for details). The symbolic significance is obvious, but one should not read too much into it, for Crux, like all the other constellations, is a chance and temporary conjunction of bright stars which, in a matter of a few millennia, will have so changed their relationship with one another that the present pattern will be distorted out of all recognition.
Writing in the 1890s, astronomer R. H. Allen stated that 54 of the stars in Crux were visible to the naked eye, but within modern terminology there are only 40 stars charted with a magnitude of 6 or brighter. Of these, (all of which should be visible on a dark night under a clear, unpolluted sky) one is hard put from the suburbs of a city like Auckland to sight more than 12. Unskilled observers will generally report seeing only the five brightest stars of the Cross, which suggests that their night sky from the back porch contains only about 300 visible stars-about five per cent of what is on offer.
It is a sobering thought that in this community, which will argue bitterly, and even go to court over a view of Rangitoto or some other scenic feature, the question of protecting the night sky has yet to be raised. In Australia and the United States of America, communities are rallying to the flag of the Dark Sky Association. Founded originally to protect the great professional observatories, in particular the 5metre (200-inch) Hale telescope at Mt Palomar, the movement has now spread to local authorities with no such multimillion dollar investment to protect, simply on the grounds that the night sky is part of our environment. The key to reasonable levels of illumination is to use light which is only as bright as required for the task, and so directed and shielded that it falls only on the target. The payback is in the savings, both in power consumed and in the lesser cost of low intensity fittings.
The lucida, or brightest star in Crux, which lies at the foot of the cross, a Crucis (also called Acrux) appears as a blue-white star with a magnitude of 0.87, which places it fourteenth in the top 20 brightest stars. Examination with even a small telescope reveals that it is visually a double star with one component only slightly brighter than the other, from which it is about four arc seconds distant. Close by, and in fact part of the same gravitationally bound system, is a third fainter star, magnitude 4.9, distant 90 arc seconds.
All three stars are sub-giants, which means they are much more massive than our sun, generate several thousand times its radiation and are due to pay for their profligate consumption of their stock of hydrogen with an early death, when compared to the ten million-year life span calculated for our sun.
The spectra of a Crucis A and B reveal that both of these stars are themselves spectroscopic binaries, stars with orbiting companions too close to be seen directly, but known to be present because of the rhythmic Doppler shift of the spectral lines. Light from an approaching star is shifted towards the blue part of the spectrum, whereas a receding star produces a red shift.
A pair of stars with their orbital plane lying parallel to our line of sight will, when they are seen in line, crossing our line of Sight, show a normal spectrum, for neither has a velocity component either towards or away from us. However, a quarter of a cycle later, when they are seen at maximum separation, one will be directly approaching us while its companion is receding around its orbit. Thus the spectral lines of the first will be blued and of the other reddened, resulting in a doubling of the lines recorded in the spectrograph.
Further complicating the picture, a. Crucis B itself shows blurred spectral lines, indicating that the star is rotating very rapidly. (Rapid rotation is thought to be a characteristic of very young stars.)
So, this naked eye star turns out to be a system of some complexity, and one which promises a spectacular future, for the two brightest members are blue sub-giants with close companions, and thus candidates for eventually becoming supernova. Being only 360 light years away, the display will be dazzling- as bright as the full Moon. But don’t hold your breath: neither star is showing signs of entering its death throes yet.
ß Crucis marks the end of the eastern arm of the Cross, and is also a more complex object than simple observation reveals, for it is a triple system in which the primary or brightest member is variable. ß Cru. A is a ß Canis Majoris-type variable star-a blue sub-giant or giant fluctuating in brightness, in this case less than 0.1 of a magnitude every five hours and forty minutes. The two fainter companions are relatively distant: B, magnitude 11.2 at 44 arc seconds, and C, magnitude 7.1 at 159 arc seconds.
Just east and south of ß lies the faint star K Cru. which at magnitude 6 is just about at the limit of naked eye visibility under any circumstances. How ever, this star lies in a cluster, and the combined light of its members constitutes a hazy patch of magnitude 4.2 which is known as the Jewel Casket, listed as NGC 4755. First examined by John Herschel from the Cape of Good Hope in the early 1830s, he named it and likened it to “a gorgeous piece of fancy jewellery,” a description endorsed by all who have examined it since.
At low magnifications the cluster has a distinctively triangular shape as defined by the class B super-giants which are its major component. However, right in the centre lies a red super-giant in dramatic contrast with its blue neighbours. In all, the cluster is composed of some 80 stars, and lies 7700 light years away. Were it as close as a. Crucis it would be a blazing group which would just fit inside the upper part of the cross.
At the head of the Cross lies y, a yelloworange star whose spectrum places it amongst the red giants-stars entering their dotage, unstable and paradoxically bloated as their hydrogen fuel runs out. They will have a final brief moment of glory as they explode as novae, blowing their outer envelope away and leaving a white dwarf at the centre of a new planetary nebula.
The fourth star of the Cross, its western arm, is I) Crucis, a blue sub-giant with a magnitude of 2.8, which means that it is markedly fainter than the other three stars of the Cross.
No account of Crux can be comp ete without mentioning the Coal Sack, that dark area which looks like a hole in the Milky Way hard against the south-eastern flank of the cross. Such dark patches, which are only visually detectable against the background of the Milky Way, were at first thought to be holes through it, allowing us a view of the depths of space-William Herschel’s “Loch im Himmel.” They are, in fact, clouds of cold matter principally hydrogen, but also containing a wide variety of simple organic molecules, as well as granules composed of soot and metals. This is the stuff from which stars are made, and also planets, and hence life. Who knows what rough beast may slouch out of these inky depths in the distant future?