Astronomy is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe.

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 If you gaze at the sky at sunrise or sunset - very unlikely though with our hectic lifestyles - and see a sort of bright & fuzzy object, congratulations, you’ve probably sighted a comet. Now do a quick check with your local observatory to see if a comet was being expected, and if not, then congratulations once again, you just might be able to name it.

But it is not often that you can see comets with an unaided eye. The ones that can be seen with the naked eye are called, what else, but, naked-eye comets. A comet visible to the naked eye appears, on an average, about once in five years. But even then it is helpful to have a pair of binoculars or a telescope.

Comets, normally named after their discoverers, are tiny when compared to our planets (ranging in diameter from a few hundred yards to several miles, although they are seldom more than ten kilometres across). They are irregular in shape and are made up of a mixture of dust, ice (water) and frozen gases. They are therefore also known as ‘dirty snowballs’ or ‘dirty icebergs’. These gases, water and dirt did not for some reason become part of the planets in our solar system and thus it is very likely that studying them will give us a better idea about our own history.

If you observe a comet, it will appear to move very slowly, when in fact it is hurtling through space at several thousands of miles per hour. This is because very few of them come within a few million miles of Earth and so the huge distance makes them appear very slow. Also at times the comet is coming towards us or moving away from so and us might even appear stationary. Comets are generally visible for periods ranging from a few days to several months, and appear to change little in position night-after-night.

Comets have a core or a nucleus, made up of mainly ice and dust, and is frozen solid. In this state they only reflects light and so are generally invisible in the far outreaches of the solar system, it is only when they approach the sun, do they become luminous. As comet nears the sun, the nucleus begins to warm up and some of the ice begins to evaporate, these gases carry dust particles along with them and create the coma, that envelope the nucleus in a cloud of gas and dust. The dust reflects still more light; while the gases absorb ultraviolet rays and begin to glow.

As they get closer to the sun, the comets develop tails of radiant material that stretches into millions of kilometres. The tail always faces away from the sun and is shaped by the solar winds as well as the radiation emitted by the sun. What is really interesting is that comets can sometimes split their tails; they can even have multiple tails and can even lose the tails at times.

Comets orbit around the sun like the other planets, the only difference being that while our orbits are nearly circular, the orbits of comets are highly elliptical and can take them to the furthest corners of the solar system and beyond. Their orbits range from a few years to several thousands, but unless they are thrown out of orbit or crash into an object, many of them return at periodic intervals of times. Most of the comets would be periodic in nature, but since several of them have orbits ranging into tens of thousands of years, it is not possible to gauge the periodicity accurately.

It is assumed that the origin of most of the comets exist outside the solar system in a sort of hollow sphere, known as the ‘Oort Cloud’, named after Dutch astronomer Jan H. Oort, who proposed the idea. Any disturbance to this cloud can cause one or more of the comets to get loose from their orbits and start off their journey into space, and often they end up orbiting around the sun. It is assumed that the Oort cloud possibly has a trillion comets.

The comets end is either a slow death or a spectacular one, complete with fireworks. Assuming that it does not crash into the sun or one of the planets or get thrown out of the solar system, the sun slowly strips these comets of their material each time they pass by, so after many passes, the comet is left without its ice and gases. What is left is a rocky terrain, which might them orbit the sun as an asteroid.

In case you are wondering as to how we know so much about these wonders? It is by using tools like spectroscopy, with which, for instance, we can study the light emitted and reflected by the comet, and using that we can determine the various elements present in the comet because each element has a different chemical signature.

And as to what would happen if a comet were to crash into earth. Probably total annihilation as in the case of the dinosaurs. Of course a lot would depend on the size of the comet as well as whether the impact is direct or glancing and most importantly on what action we take. A crash would completely wipe out the places where it lands, while the earthquakes and dust clouds created would ensure that all forms of life are totally destroyed.

But what if earth only passes through the orbit of a comet? In that case, we would be bombarded with a meteor shower. For e.g. the Comet Halley results in the Orionid shower in October, while the Comet Swift-Tuttle causes the Perseid meteor shower every year between August 9 and 13. Meteors are small rocks, sometimes as fine as grains of sand. When these meteors enter the earth’s atmosphere, due to friction, they get heated up, and make the air around them glow. They last for only a few seconds before completely disintegrating and are commonly known as shooting stars. Some of them might be big enough to reach the earth’s surface and are called meteorites.

It is also assumed that early collisions between earth and comets resulted in the vast amounts of water that now make up 3/4th of the earth’s surface. It is only because of these waters that life on earth was possible, so in that sense we possibly owe all our waters and therefore our lives to these brilliant comets.

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