As the ancients gazed at the night skies, they were occasionally startled to see strange celestial objects intruding upon the familiar pattern of stars, Moon, and planets. These mysterious apparitions looked like fuzzy stars with long trains of light, moving from one constellation to another and cutting across the paths of the planets at every conceivable angle. The trains of light suggested a woman's tresses; hence, the celestial intruders came to be known as "long-haired stars", or kometes, the Greek word for "long-haired".
A bright comet was a terrifying spectacle in antiquity. It was thought to foreshadow some dire catastrophe-plague, famine, war, or perhaps the death of a ruler. Today we realize that comets are simply another member of the solar system, and that their coming is no more portentous than the appearance of the first stars at twilight.
HOW COMET APPEAR
When a particular comet is first discovered, it usually appears as a faint, diffuse body with a dense area near its center. This dense part, which sometimes looks like a tiny star. i known as nucleus. The nebulous, or veil-like, region around it is the coma, Nucleus and coma together from the head of a comet.
In a certain number of cases, however, a spectacular transformation takes place as the comet approaches the Sun. The coma changes from a diffuse, round mass to sharply defined layers, called envelopes. Nebulous matter streams away from the comet's head in the direction opposite to the Sun and forms an immense tail. Most comets of this type have only one tail. A very few have two or more. Some comets occasionally also have forward spikes. As a comet recedes from the Sun, the tail can no longer be seen, the coma becomes diffuse again, and, in the great majority of cases, the comet itself disappears from view.
1 theory, proposed by Dutch astronomer J.H. Oort in 1950, holds thate there is a vast storehouse of comets - as many as 100billion, perhaps-in the icy reaches beyond the farthermost planetary orbit. A given comet would normally remains entirely inactive in the "deep freeze" of space unless the passage of a star disturbed it. The comet then would swing into the sphere of gravitational attraction of a major planet and would revolve around the Sun a few hundred or a few thousand times until it disintegrated.
Photographs by the Hubble Telescope provide the first direct evidence that a huge belt of at least 100million comets is circling the solar system beyond orbit of Neptune. This ring, called the Kuiper belt, was first suggested in 1951 by Dutch astronomer Gerard P. Kuiper, and appears to be the origin of many comets.
The general belief is that the nucleus consists of a vast number of small, solid bodies held together by mutual attraction. The nuclei of a certain comets that have ventured close to Earth have been measured with considerable precision. The tail of a great comet of 1861, for example, stretches across two-thirds of the sky and was bright enough to create shadows on the ground. Yet it had nucleus less than 100 miles in diameter.
As the nucleus of comet approaches the Sun, the solar heat vaporize the material on the outer surface of the nucleus. Escaping gases, carrying fine dust with them, diffuse into the coma. They are the swept away by the force of the Sun's radiation to form tail. The gases and the dust they transport are illuminated partly because they absorb ultraviolet light and re-emit it in the form of visible light.
The tail which flows away from the Sun, increase in breadth as the distance from the head increases. The tail does not form an exact line between the Sun and the comet's head. The greater the distance from the head, The more the gases and dust that make up the tail lag behind. Hence the tail often has the shape of curved horn, with its tip at the comet's head.
When comet turns away from the Sun, the material that formed the tail is swept off into space. In time, comets gradually lose all their substance, unless it can be replenished by dust and by gas molecules swept up in the course of their journeys through space.
ORIGIN AND STRUCTURE
How do comets originate? According to one theory, they represent celestial building block left over after the formation of the planets. According to another, they are remnants of shattered worlds. All this is pure conjuncture, as are the various theories that attempt to explain how comets are launched on their journey around the Sun.1 theory, proposed by Dutch astronomer J.H. Oort in 1950, holds thate there is a vast storehouse of comets - as many as 100billion, perhaps-in the icy reaches beyond the farthermost planetary orbit. A given comet would normally remains entirely inactive in the "deep freeze" of space unless the passage of a star disturbed it. The comet then would swing into the sphere of gravitational attraction of a major planet and would revolve around the Sun a few hundred or a few thousand times until it disintegrated.
Photographs by the Hubble Telescope provide the first direct evidence that a huge belt of at least 100million comets is circling the solar system beyond orbit of Neptune. This ring, called the Kuiper belt, was first suggested in 1951 by Dutch astronomer Gerard P. Kuiper, and appears to be the origin of many comets.
The general belief is that the nucleus consists of a vast number of small, solid bodies held together by mutual attraction. The nuclei of a certain comets that have ventured close to Earth have been measured with considerable precision. The tail of a great comet of 1861, for example, stretches across two-thirds of the sky and was bright enough to create shadows on the ground. Yet it had nucleus less than 100 miles in diameter.
As the nucleus of comet approaches the Sun, the solar heat vaporize the material on the outer surface of the nucleus. Escaping gases, carrying fine dust with them, diffuse into the coma. They are the swept away by the force of the Sun's radiation to form tail. The gases and the dust they transport are illuminated partly because they absorb ultraviolet light and re-emit it in the form of visible light.
The tail which flows away from the Sun, increase in breadth as the distance from the head increases. The tail does not form an exact line between the Sun and the comet's head. The greater the distance from the head, The more the gases and dust that make up the tail lag behind. Hence the tail often has the shape of curved horn, with its tip at the comet's head.
When comet turns away from the Sun, the material that formed the tail is swept off into space. In time, comets gradually lose all their substance, unless it can be replenished by dust and by gas molecules swept up in the course of their journeys through space.
0 comments:
Post a Comment