In Summer the nights are dominated by Orion and his attendant dogs, Canis Major and Minor, together with the head of Taurus, the bull. Being south of the Equator, we see the traditional patterns of the stars inverted, so although the shapes remain distinct, their associations are masked; Orion standing on his head is neither heroic nor likely. To us, the stars forming his belt and sword suggest a cast iron pot. But for the Maori, Orion at setting was one of their great, double-hulled, ocean-going canoes whose mat sails had enabled them to drive south where no one had gone before.
To astronomers, this area lying immediately south of the Milky Way is one of the most interesting and accessible regions, and contains many objects which are bright enough to be easily seen with binoculars or small telescopes.
Obvious even to the unaided eye is the concentration of the brightest stars towards the plane of the galaxy, marked by the Milky Way. That these stars are not scattered uniformly over the celestial sphere was first established in 1847 by Sir John Herschel.
Unexpectedly, in 1870, when John Gould described the composition and anatomy of this band of bright stars (which now bears his name), he found that it is tilted about 16 degrees to the equatorial plane of the galaxy, In 1924 Harlow Shapley and Annie Cannon showed that the tilt of Gould’s belt is the result of the many hot blue stars in it.
Apart from novae and supernovae, the blue giants are the brightest stars at visual wavelengths, and so are conspicuous to the naked eye. However, being massive, they are youngsters in comparison to our sun. Ranging from five to perhaps 60 solar masses, or even more, they burn their nuclear fuels at high speed, and generally are less than 100 million years old when they leave the Main Sequence of stellar life.
Cooling and reddening as they enter their long drawn out death throes, both Betelgeuse in Orion and Aldebaran are such stars, and are perceptibly orange. Compared to such stars, the Sun is a positive Strudlebug, being already about 5000 million years old, and projected to remain on the Main Sequence for another 5000 million years. There are advantages in moderation!
The giant blue stars are galactic markers as well as mariners’ navigational beacons, for they are recent creations of the great pressure waves which circulate around the galaxy. When these waves pass through the hands of gas and dust which we see as dark nebulae in the Milky Way, a swarm of new stars is formed.
The smallest have masses like that of the Sun, and are at first hidden in the depths of the cloud of material from which they condensed. These, the T Tauri stars, are seldom visible during the early stages of their lives, but there is a most famous exception to be found in Messier 42, the Orion nebula. Here the ball of gas and dust has been blown out on the side towards us so that we look into its heart and at the stars of the Trapezium — which are some of the youngest stars which we can observe.
In fact, these stars are but the brightest and core stars of a cluster of some 300 which are estimated to be no more than 300,000 years old — only one five hundred thousandth of the estimated age of the universe.
The giant stars, those with masses six to sixty times that of the sun, are generated deep inside massive gas clouds, and appear to need highly energetic events, such as the explosion of a nearby supernova, to trigger their formation. But once up and running, these stars produce intense ultraviolet radiation which blasts away the parent cloud in short order, leaving these newborn stars to dominate the sky. In the Pleiades we appear to be seeing this process in its last stages, for the entire group is still embedded in the remnants of a cloud which now appears as shreds and wisps of fluorescing gas.
During the heroic days of observational astronomy—the late eighteenth and early nineteenth centuries—this section of Gould’s belt posed more problems than it offered solutions. Having no model of stellar energy production, nor any satisfactory means of categorising the various types of star, the observers of the time worked both literally and metaphorically in the dark, and were often at a loss for acceptable descriptions of the objects under examination. Nebulae, in particular, were troublesome. Were they or were they not multi-stellar? Was or was one not just getting a hint of resolution into stars?
In Taurus lies the planetary nebula N.G.C. 1514, which is a white dwarf star surrounded by a shell of hot gas blown off when the original star became a supernova. In November 1790, after long and careful observation, Sir William Herschel reported: “A most singular phenomenon; a star 8th magnitude, with a faint luminous atmosphere of circular form, about 3′ in diameter. The star is perfectly in the centre, and the atmosphere so diluted, faint, and equal throughout, that there can be no surmise of its consisting of stars, nor can there be a doubt of the evident connection between the atmosphere and the star.”
Nothing in the physics of the eighteenth century could account for emission nebulosity as we know it. Herschel’s conjecture concerning a “luminous fluid” in the depths of space was a courageous step, for he not only conjured a new entity into existence, but also had to scrap one of his most firmly held beliefs, that all that shines is stellar, and this forced him to reconsider the picture of the universe which he had been constructing for the past fifteen years.
On January 16, 1991, New Zealanders will witness a classical wonder when the sky dragon eats the sun and night falls at noon. On that day, just after 12 pm N.Z.S.T., the shadow of the moon will sweep in from the Tasman, crossing the coast at Westport, then on to Blenheim, Wellington and south of Masterton before passing out into the Pacific. Because of the moon’s distance from the earth at this time, this will be an annular solar eclipse — that is, the moon will not completely cover the sun, but at totality, the moment when the observer, the centre of the moon and the centre of the sun all lie on the same line, we will still see the edge of the sun’s disc around that of the moon.
Unlike lunar eclipses, where the moon passes into the shadow of the earth and the event is visible from anywhere on the night side of the earth, solar eclipses can only be seen from within a band averaging about 160km wide. This band, the path of totality, marks the only area from which the moon appears to cover the sun; on either side of it observers will see only part of the sun’s disc covered.
Thus, at 1211 hours, when people in Blenheim can see the moon centered on the sun, Aucklanders will see the sun as a thick crescent, and at Scott Base the edge of the moon will be seen just lapping onto the sun. For this eclipse the path of totality is 140km north and south of the central track from Wanganui to Ross in the west and from Hastings to Cheviot in the east. Within these limits the whole disc of the moon will be seen against, but eccentric to, that of the sun.
To see the discs concentric to one another, observers must be as near as possible to the centre line of the eclipse. Although annular eclipses deny us a view of the solar carona, that great halo of hot gas extending far above the sun’s disc as we can normally see it, this event will be worth seeing, for there will not be another eclipse of the sun visible from New Zealand until 2036, which is too far into the future for most of us to count on being around to see it.
Although the Black Dragon almost succeeds in swallowing the sun, what remains visible of the solar disc is as bright as ever. Because so little of the sun is in view at totality, the day will darken, but if you look at the sun, the little crescent remaining which imprints on your retina will have undiminished brightness per unit area, and be as damaging as ever, producing an irreparable burn and blind area.
Therefore, under no circumstances look directly at the sun without a suitable filter such as welder’s glass, several layers of fully exposed and developed black & white film or two or three layers of aluminised foil such as florists use. Do not use photographic neutral density filters, as they pass infra-red which will burn your retina.
Better by far is to view a projected image using a telescope or binoculars to throw an image on a sheet of white paper. Alternatively, a good, clean pinhole in a sheet of thin card, or better still, aluminium foil, will produce a projected image of the sun —as do the gaps between the leaves of the trees. That dappled light we normally pass by is in reality a myriad pinhole images of the solar disc, which during an eclipse are changed into a like number of crescent slivers.