Spring haze

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These are the  months during which the night sky seems formless, for bright stars are few and scattered. The familiar Cross is head down on the southern horizon, the Sagittarius star cloud which marks, yet masks, the centre of our galaxy is setting, and the ever familiar pattern of Orion is just emerging in the east. Overhead Achernar, a Eridani, marks the mouth of the river Eridanus, which meanders across the eastern half of the sky from its spring under Orion’s left foot—marked by Rigel, β  Orionis.

With few obvious splendours—for most of its stars are not bright enough to be visible against the light-polluted atmosphere of our cities and larger towns—this season’s night sky contains the three naked-eye galaxies (Andromeda and the Large and Small Magellanic Clouds), together with the second brightest and most striking of the globular clusters, 47 Tucanae.

Step outside the dome of artificial light with which we shroud our cities, and the hemisphere of night darkens from blue-grey to luminous black. The scattering of stars becomes a multitude. To the south, the striking Great Square of Pegasus sits four square above the horizon, and from its lower, right hand or north-eastern corner, a Andromedae or Alpheratz, it is an easy star hop north-easterly to S and p, thence south-westerly to t, and then as far again to the faint, elongated glow of the great Andromeda nebula, M31 or NGC224.

The bright nuclear region is easily visible through binoculars, but this nebula has been called the most disappointing object in the heavens, for no view of it, even with a large telescope, can compare with the long-exposure photographs used to illustrate almost every book on astronomy.

Yet there is always a prickling at the nape of the neck as one looks over the edge of the horizon down 2.2 million light years. There is the amazement that the several packets of photons have retained their coherence for so long and so far. While it is possible, though improbable, that this light can generate of a fuzzy glow on our retinas, it is against any reasonable expectation that this light should yield images of the individual stars, knots of bright nebulosity and great drifts of dust and cold, light-absorbing gas which are revealed in photo­graphs.

Were we in M31, our view of the Milky Way galaxy would be similar—an oblique view of a large spiral galaxy. The Andromeda galaxy is rather larger and 50 per cent heavier than our own, but these two, each with their retinue of smaller galaxies, are the giants of the local group of eighteen galaxies. Two of M31’s companion galaxies, M32 (NGC221) and M110 (NGC205), are telescopic objects, although very much smaller and fainter.

So accustomed are we to the current general view of the universe, with its vast voids and blazing congeries of stars forming galaxies, that it comes as a shock to find that it was not until as recently as the late 1920s that most astronomers accepted that there were galaxies similar to ours elsewhere in space. That the universe extended beyond the boundaries of the Milky Way was not doubted, but the precise nature of the nebulae was not determined until American astronomer Edwin Hubble made photographs of M31 and M33, which is a member of the local group and in Triangulum. These astrographic plates resolved individual stars and enabled him to identify a number of variable stars of the δ Cephei type.

Using formulae which link the brightness and period of fluctuation of variable stars to their distances, Hubble esti­mated the Andromeda galaxy to be rather more than 1,000,000 light years away.Within a few years he had measured the distance of some 125 nebulae, as they were then known. Because of an incorrect calibration of the period/ luminosity relationship through failure to allow for the effects of interstellar dimming (termed the distance modulus), these initial estimates were 50 per cent short, but never­theless they expanded the known universe far, far beyond any earlier models.

South of Achernar and lying east and west of it are two rather irregular cloudy patches first recorded in European astronomy by the explorer Magellan.

Through binoculars, their astronomical rather than meteorological nature is apparent; they are, in fact, small galaxies situated just beyond the boundary of our own system.

A scant 180,000 light years away, they are by far the closest extra-galactic objects available to us—so close, indeed, that the supernova of 1987 was more clearly observable in that position than if it had been embedded within the dust and gas of our own system. Both clouds contain the full range of objects seen within our system: stars of all types, open and globular clusters, bands of gas and dust together with patches of emission nebulosity marking the site of stellar formation, and a due ration of novae and supernovae.

Lying east of Achernar within the Large Magellanic Cloud is the great “Tarantula” emission nebula, NGC2070 or 30 Doradus. Visible to the naked eye, this huge, complex cloud, measuring about 10,000 x 3000 light years, probably hides within itself an extremely dense cluster of massive stars which are producing high intensity ultra-violet radiation and a powerful stellar wind. If this com­plex were as close as the famous Great Nebula in Orion, M42 or NGC1976, which is about 1600 light years away, then the Tarantula would be visible throughout the day and would cast shadows at night.

Westwards, the Small Magellanic Cloud is further from us than its larger partner, and there is some evidence suggesting that what we are seeing is in fact two small irregular galaxies, one being some 30,000 light years behind the other. The LMC and the SMC both lie in a common cloud of cool neutral hydrogen gas which is extended out towards our galaxy and which was probably gravitationally torn out during a close encounter of the two clouds with our galaxy some 1,000,000,000 years ago.

Immediately to the west of the Small Cloud is a hazy patch which is the great globular cluster NGC104 or 47 Tucanae, affectionately known as “47 Took.” Second in size and brilliance only to  ɯ Centauri, this spherical concentration of some millions of stars is just over 200 light years in diameter. Towards the middle, the concentration of stars is so great that it has been impossible to resolve the central glow into indi­vidual stars.

Early in the nineteenth century, John Herschel, working from the Cape of Good Hope, distinguished between the white stars of outer areas and the rose-tinted members of the core.

Although this distinc­tion was later denied by Agnes Clerke in 1905, it is inescapable when using a large instrument, as I discovered using the Loftus 36-inch Newtonian tele­scope from Pokeno. Having driven 50km down the southern motorway (and placed the Bombay Hill between myself and Auckland’s fearsome light pollution) I was rewarded with that cluster of a myriad lights which is 47 Tuc., and could clearly discern a yellow to pale orange tint in the core—something which was altogether lacking in the cold white of the halo stars.

For astronomers, 47 Tuc. is of particular interest, for not only is it reasonably close (at 13,000 light years) but also its position well above the plane of the galaxy means that our view of it is not obscured by interstellar dust, nor confused by numbers of intervening stars. The globular clusters are some of the oldest objects in the galaxy, and are composed of Population II stars.

These stars, which are also found at the core of  spiral galaxies and in the spherical halo of high velocity stars which surround the disc of the galaxy, characteristically contain virtually nothing but hydrogen and helium. All the other elements are lumped together under the name “metals,” and it is their abundance or other­wise which is one of the main clues as to a star’s origin, history and age.

The elements heavier than helium and up to and including iron are pro­duced particularly during the latter stages of a star’s life, but nuclei heavier than iron require energies only generated in the death throes of massive stars, or cataclysmic events such as supernova explosions.

Stars of modest size, such as our Sun, which contain a significant proportion of metals, have clearly been formed from the aggregation of the remains of pre-existing stars which created and distributed these elements during the final stages of their lives. Thus stars in which such material is but  a trace must have formed at a time when this matter had yet to contaminate the primordial mix of hydrogen and helium. Such stars must be very old—virtually coeval with the galaxy itself. Population II stars are examples of such metal-poor objects, and date from the earliest days of the galaxy and, perhaps, from the very young universe.

Although the globular clusters are Population II objects, they show a considerable variation in their metallic content, ranging generally from 1 to 10 per cent of that of the Sun, though there are a few well towards the centre of the galaxy which approach the metallic content of the Sun.

47 Tuc. is one such globular with a particularly high proportion of metals. It is unusual in that it lies well away from the central region. How this came to be is at present not under­stood. Is it indeed a young globular, the stars of which have condensed out of hydrogen already contami­nated with heavy elements, or is it possible that fre­quent collisions amongst the constituent stars have resulted in the formation of the elements beyond iron?

Both theories have difficulties, and only further study of the con­stituent stars of this and the other globular clusters will enable us to untangle its life story and account for the composition of this striking object.

But whatever conclu­sions we draw, 47 Tuc. will always be regarded as one of the wonders of the southern sky, and one which alone almost justi­fies ownership of a tel­escope.