The ancients, rather like Captain Boyle in Juno and the Paycock, would often look at the sky and ask themselves the question: "What is the stars, what is the stars?" A common conclusion was that these celestial lights were tiny luminous objects set permanently into a solid, black, hemispherical dome - and until about 1700 this remained a perfectly respectable opinion.
It is only more recently that stars have come to be universally accepted for what they are - huge concentrations of hot gases, exuding immense quantities of light, heat and other forms of radiation. They are quite different, of course, from the planets of our solar system, which shine only by the Sun's reflected light.
Looking at the night sky with the naked eye, about 6,000 twinkling stars are visible. Their twinkling, however, is not intrinsic; it is caused by variations in the density of our mobile atmosphere, which bends the light from the star, and causes it to seem to tremble.
Moreover, they are not all the same colour. Most of them vary from a sultry red to steely blue, the difference being dictated by their temperature. Those with a reddish tinge are cooler than the blues, and the yellows - like our Sun - lie somewhere in between; only the very hottest of the stars are white.
These heavenly bodies differ greatly in size, density and temperature. Ever since the days of the Greek astronomers 2000 years ago, the stars have been divided into classes according to their magnitude - or their apparent brightness in the sky - which depends not only on their absolute brightness, but also on their distance from the Earth.
Everything else being equal, the further away a star the dimmer it will seem to be - just as in the case of a line of similar lamp-posts on a street, the nearer lights appear brighter than more distant ones.
But the scale by which the brightness of the stars is catalogued is strange.
Like the handicap system used in golf, it is inverted, so the more brilliant performers are assigned the lower values. Thus a star of magnitude 1.1 is brighter than a star of 2.2; those of magnitude 6 are the faintest that can normally be distinguished by the naked eye, but with good binoculars those of magnitude 8 or even 9 are visible.
Unlike golfers' handicaps, however, the magnitude scale extends below zero to accommodate objects that are very bright. Sirius, for example, has a magnitude of about minus 1.5; Venus at its brightest can be minus 4.4, while the Sun shines with a brilliant magnitude of minus 27.