Towards the centre
One of the joys of winter is the night sky. Escape from the great bowl of light pollution which we place over our towns and cities and the night sky becomes a hemisphere of splendour. Instead of the few dozen stars familiar to urbanites, there are stars in their thousands.
The Milky Way is transformed from the merest smudge into a glowing band of light rising to a crescendo of intensity in the constellation Sagittarius which passes overhead at this season.
To the eye, the stars and the Milky Way appear to lie on the surface of a sphere of large radius centred on the Earth. The illusion is compelling, and, indeed, the Celestial Sphere was very much part of official cosmology from the time of Aristotle, and even survived Copernicus’s demolition of the Earth-centred model of the solar system by likewise becoming centred on the Sun.
The speculations of the Elizabethan Thomas Digges and, later, of Immanuel Kant in the 1700s, urged a three-dimensional structure of the galaxy, but it was Sir William Herschel who subsequently provided the observational evidence which demonstrated that the galaxy was much bigger and deeper than anyone had thought possible.
At that time, there was our solar system, there were the stars, and, beyond that, there was empty space. As one looked along the Milky Way, the frequent and extensive interruptions of its fabric were interpreted as holes or gaps in the sheet of stars. Thus the Coal Sack, up against the Southern Cross, was regarded as a hole which allowed us to see through the Milky Way into the empty depths of space.
Such a picture accorded with both the appearance of the night sky and the physical knowledge of the day. Indeed, to the eye, the irregularities in the Milky Way in the vicinity of Sagittarius strongly suggest tears and holes in a luminous fabric.
Herschel, pinning his hopes on the general similarity of the stars with one another, was in no position to question the nature of these “holes”. He was already flying in the face of the evidence of the star clusters which, by his system, must be interpreted as spikes or long fingers of stars with their axis directed towards the Earth and with the brightest ones closest and the fainter ones deeper in space.
To see dark areas as clouds, rather than holes, would have been to destroy the foundations of his survey, for such dark clouds would at their edges dim the stars unpredictably, but worse, would suggest the possibility of dark fog, an undetectable and irregular dimming agent.
Eighteenth century physics and chemistry did not have the tools, theoretical or practical, to grapple with this problem, nor with that of the differences between the various stars. Still embedded deep in Western consciousness was the belief that our solar system was central, and it was around such a spot that Herschel mapped the results of his measurements, quite unaware that huge drifts of cold gas and dust block our view of the galaxy, particularly towards its centre.
Only in the last 15 years have plots and pictures made with x- and gamma rays, with radio emissions and various wavelengths of infrared, allowed us to “see” through to the centre of the galaxy. Without this information, the visual, commonsense view of our galaxy is that of a disc of stars with the Sun at the centre. It may be that this disc extends beyond the limit of detection, hence Herschel’s interest in the “space-penetrating power” of his instruments, but then we will still appear to be at the centre.
The patchy nature of the Milky Way around Sagittarius is visible to the unaided eye, and quite clear through binoculars or a small telescope, and each of these dark patches is a cloud of cold gas, mainly hydrogen, contaminated with more or less dust, grains of silicates, minute clumps of carbon and simple organic molecules and free radicles. This is the stuff of the next generation of stars, planets and life (if any).
Some of the hydrogen is unaltered since the formation of the galaxy, perhaps 15 billion years ago; all the rest is the ash of long burned out stars which, in their death throes, may explode as novae or supernovae, seeding space with the elements heavier than hydrogen—quaintly known as “metals” to astronomers.
As in other spiral galaxies, the central bulge of our own is composed of old, yellow, metal-deficient stars which were formed at or near the beginning of the galaxy’s life. Sharing this characteristic are the globular clusters, those wonderful aggregations of hundreds of thousands of stars which in a small telescope appear as soft edged balls of light, but when viewed through larger instruments are some of the most majestic objects in the universe.
One of the finest is located in Sagittarius: NGC6656 or M22. The M stands for Messier, the great eighteenth century French astronomer who specialised in comet-hunting, and published a catalogue of 109 fuzzy non-cometary objects so that future hunters might not be misled.
As it turned out, Messier’s list included some of the most intriguing objects available to small telescopes: globular clusters, open clusters, nebulae, planetary nebulae and galaxies. His list has become the amateur astronomers’ vade mecum for general star gazing, and a beginner’s celestial orienteering course.
In or close to Sagittarius are 16 of Messier’s objects, mainly open clusters and globular clusters. For New Zealanders, Sagittarius is almost overhead during winter, but for Messier, working from Paris, it skimmed above the southern horizon, much of its detail lost through the obscuring effects of an oblique line of sight at low altitude. This effect is exaggerated by light pollution, and in large cities even the brightest stars do not “rise”, but rather they gradually become apparent as they climb through the urban murk.
Running just north of the Teapot (the central part of Sagittarius) is the ecliptic, the mean plane of the Earth’s orbit around the Sun, which also coincides closely with that of the other planets. Immediately north of the handle of the Teapot (the star rho Sagittarii) and just south of the close pair nu’ and nu’ Sgr is the planet Uranus (U), which should be just visible to the naked eye under a dark sky.
Discovered by William Herschel with a 7-inch aperture telescope in 1781, this, the seventh planet, was the first to be discovered by modern astronomy. The other six planets—Mercury, Venus, Earth, Mars, Jupiter and Saturn—had been known since the dawn of history. Actually, Galileo saw and recorded the position of Uranus, but failed to recognise its planetary nature, probably because he was concentrating on the moons of Jupiter and the proof that they did indeed rotate about that planet, contra the establishment, which held that the earth was the centre of all celestial orbits.
Close by, and slightly to the east, is Neptune (N)—too faint to be seen without a telescope. Next out from the Sun after Uranus, its discovery by J.G. Galle at the Berlin Observatory in 1846 was a triumph of Newtonian mechanics. Irregularities in the orbital motion of Uranus led Urbain Leverrier in France and John Couch Adams in England to predict the position and mass of a hitherto unknown planet. Acting on this information, Galle picked up Neptune on the first night of his search, some say during the first hour.
Further east along the ecliptic lies Saturn, unmistakable as it is the brightest object thereabouts. The rings can be seen through even a small telescope or binoculars, though already they have narrowed noticeably as the relative positions of Earth and Saturn bring us steadily closer to the plane of that planet’s equator, and so the rings appear progressively narrower. By 1995 the rings will have narrowed to the merest sliver, disappearing completely as the Earth passes through their plane.
Also shown on the chart is M8, the Lagoon Nebula, which surrounds an open cluster of stars. It is visible to the naked eye as a glowing patch which looks similar to a comet. Through a telescope, the apparent size and complexity of the gas cloud is increased, and the stars of the cluster which are irradiating it and causing it to fluoresce can be clearly seen.
Immediately to the north lies M20, the Trifid Nebula, which is the subject of some of the most striking astrophotographs. The trisection of the bright nebula by three lanes of dark rifts lends it a particular visual interest.