Comets

The Christmas star

The Christmas Star and Angel compete for the most important position above the top beam of the Christmas shed, where the cow and the donkey keep the Holy Family warm. In other Nativity scenes, we see the lovely little star stuck on the crêpe paper the grotto is made of. Be it either the grotto or the shed, the schools of thought differ, but they share the luminous, reassuring presence of the star, that, according to the legend, guided the Three Kings to the new born baby Jesus.

An enormous snow-ball

It seems quite incredible, but the nature itself of the comet, conveys a Christmassy feeling: science has discovered that these tiny celestial objects orbiting around the sun, are made up mostly of ice, mixed with carbon dioxide, methane and dusts. In other words comets are gigantic dirty snowballs. Some comets rotate very close to the sun following their elliptical course (orbit), similar to a squashed circle. When this occurs, the frozen nucleus heats up and there is a consequent loss of materials in the form of vapour: and it is the comet’s tail that makes it visible and so very beautiful. The journey of a comet around the Sun is really long and extends far from the central star. Some comets, like the most famous one, Halley, take less than 200 years to complete their orbit, while others travel so far away from the Sun that they take thousands or even millions of years to reappear on our side . The aphelion of Halley’s Comet (i.e. the point of its orbit that is farthest from the Sun) is just beyond the orbit of Neptune, i.e. the last planet of the Solar System; and it returns near to the Sun (perihelion) every 76 years. There are comets, instead, that pass near to the Sun only once and travel away for good.

The comet’s cradle

Comets were born at the same time as the Earth and the other planets in the great nebula that gave origin to the Solar System. While in the incandescent centre of the nebula the planets were taking shape, in the outer parts, the temperature was sufficiently low to allow the water to freeze and to aggregate in large nuclei of ice, of sizes that varied from a few hundred metres to various kilometres in diameter. When compared with the planets, the nuclei of the comets are very small, however the same cannot be said of their tails that can reach an incredible size, at times comparable to the diameter of the Sun. And for this reason some comets can be seen with a bare eye.

The star with a tail

The spectacular tail also gives origin to its name: in fact in Greek, kométes literally means “with hair”, and we can imagine this flowing hair tied into a long tail. When the comet travels near to the sun, the solar wind clears away the vapours that are freed by the heat and pushes them away from the surface of the nucleus. The solar wind is certainly not like the air that we are accustomed to on the Earth, it is a current of particles, protons and electrons that rapidly abandons the Sun, spreading out in all directions. It also reaches the Earth at a speed that ranges from 200 to 900 kilometres per second, where it interacts with the atmosphere producing the Aurora Borealis. Every second, the Sun loses up to 800,000 tons of material in the form of solar wind: it seems a very large quantity but it is an infinitely small amount for our Star. Because of the solar wind that only strikes the side of the nucleus facing the Sun, the tail always forms on the opposite side of the comet. Furthermore, since the nucleus is not only made up of frozen water, even two tails may form; the first is long and luminous and consists of only vapours and gas, the second, that is shorter and dark, does not stretch very far from the nucleus because it consists of dusts that are heavier.

Like the snow in the sun

Also the comets die, or better, they continue to wear out till they disappear. The orbits near the Sun lead to a loss of the lighter materials from the nucleus, up to when all that is left of the majestic comet is stone, which then joins the long list of normal asteroids. Some comets, instead, come to a spectacular end, diving into the Sun, that captures them with its powerful force of gravity. Only in some very rare cases, fortunately for us, the comets may fall on the planets leaving behind gigantic craters. This also happened to the Earth; in 1908, in the Tunguska forests in Siberia, a small comet, or perhaps an asteroid, provoked a catastrophic event when it crashed on an uninhabited area, destroying 60 million trees, making them look like toothpicks. Also the moon has scars of numerous impacts with celestial bodies, some of which were certainly comets.

Man and comets

Comets have always fascinated us. The fact that they interrupt the slow and regular motion of the stars in the sky, with a faraway silent and elegant flash, has triggered curiosity, terror and passionate feelings of attraction in mankind, that in different times, in different cultures, has seen them represent divine forces, demons, benevolent beings bringing riches, or bad omens of horrible misfortune. In 1910, the Earth crossed the tail of Comet Halley, provoking panic among the people who were ill informed and who thought this could lead to the risk of poisoning and calamities. In order to survive the imaginary killer clouds released by the comet, someone even thought of selling containers full of “pure air” , and became rich. The first document containing information about comets, dates back to the year 239 B.C. and is found in the writings of some Chinese astronomers who described a star with a brush shaped tail. Those ancient astronomers did not know it, but they had seen Halley’s Comet. Comets also appear in numerous works of art. The most ancient pictorial representation that is known, is in Rome in the Catacomb of Priscilla where a fresco of the III-IV century A.D. shows a large star, without a tail, above the Nativity scene. The first tail tapering behind a comet appeared in the Nuremberg Chronicle in the VII century, while the most famous comet with a tail was painted by Giotto in the Scrovegni Chapel in Padua in 1305. And it is thanks to Giotto that today we hang a comet above the shed of the Christmas Nativity scene. In fact the great painter painted the comet, which in this case was Halley’s Comet, just above the shed in Bethlehem.

Was it really a comet?

Science sometimes spoils the picture, and claims it is doubtful that a comet really guided the Three Wise Men. Today astronomers are able to reconstruct the image of the celestial vault, very precisely, at any time of history, and when Jesus was born, i.e. most probably between the year 7 B.C. and 4 B.C. no comet should have been present. Therefore, while historians state that the real Christmas is slightly older than the Traditional one, the astronomers state that the only appearance of a comet in that period dates back to the year 12 A.D., too many years later. And it was the usual Halley’s Comet. If we exclude a comet, what could have appeared in the skies of Bethlehem on Christmas night? According to the astronomers who studied the Chinese records of those times, the main clues lead to a supernova. In fact the ancient texts indicate that in the year 5 B.C. a very luminous star appeared in the sky and was visible there for 70 whole days. A supernova is a dying star that collects what is left of its strength to donate a sensational explosion in the heavenly vault, which is often visible from a distance of light years. Also from the Earth. According to other scholars not even the story of the supernova is plausible, and they believe that it is more probable that the Star of Bethlehem was no more than the planet Jupiter aligned with Saturn. An alignment of planets is an apparent nearness of the celestial bodies seen from our planet. In practice, the intense light of Jupiter together with that of Saturn may have given the illusion of a particularly brilliant celestial body, that disappeared when the planets moved apart. So, for once, let us feel authorized to disobey science and continue to let our Nativity scenes be watched over by the Star of the East - it would be rather difficult to replace it with a plastic supernova or the alignment of papier-mâché planets.