Competitive Exams: Planetary Science

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  • Astronomy: It is the observational and theoretical study of celestial bodies of the interviewing regions of space and the universe as a whole space. It is near-vaccum existing beyond the atmosphere of all bodies in the universe. The extent of space; i.e.. whether it is finite to infinite is as yet unresolved.
  • Astronomical Distances: A light year is an astronomical unit of measurement of distance; it is the distance that light, moving at 300, 000 km/second, travels in a year. Astronomical unit is the average distance between the earth and the sun. It is approximately one crore km.
  • Universe: It is the entire group of heavenly bodies that exist-all the planets, stars, galaxies and anything else that can be thought of. On the scale of atoms, the universe has over 90 percent of hydrogen. Most of these atoms arc collected into gaseous stars which like the sun, are transforming hydrogen into helium by means of nulcear fusion reactions in their extremely hot interiors. This hydrogen burning is therefore slowly changing the composition of the universe. The observation that the universe is expanding implies that the universe is evolving. According to “Big Bang Theory” all the matter in the universe was concentrated in a very dense and hot primeval fine ball in the beginning. An explosion occurred about 20 billion years ago and since then the matter in the universe is moving away in the form of galaxies. On account of continuous expansion the galaxies will go beyond the boundary and will be lost. Therefore the number of galaxies per unit volume will go on decreasing and ultimately we will have an empty universe. If the total mass of the universe is more than a certain value the expansion may be stopped by its gravitational pull and the universe may contract again. Thus there may be alternate expansions and contractions giving rise to a pulsating universe. The third possibility is that new galaxies are continuously being created out of empty space to fill up the gap caused by the galaxies which leave the observable part of the universe. This is known as steady state theory.
  • Galaxy: Galaxies are giant assemblies of stars, gases and dust into which most of the visible matter in the universe is concentrated. Each galaxy exists as a separate though not always entirely independent system held together and organized largely by the gravitational interactions between its various components. The galaxies can be divided into three broad categories elliptical; spiral; irregular. Our milky way is the giant star to which the sun belongs. The galaxy has a spiral structure and like other spiral galaxies, is highly flattened. It contains around (1011) stars. The solar system is situated in the central plane of the milky way.
  • Stars: Stars are made up of self-luminous mass of hot gases held together by its own gravity. An average star consists of 70 percent hydrogen, 28 percent helium, 15 percent carbon, nitrogen and neon, and 0.5 percent iron group and heavier elements. The stars have the largest fraction of the mass of the universe. Stars are born, produce nuclear energy, evolve and eventually die. At the end of its I life the production of energy in the star is to large and rapid that the star explodes in the form of nova or supernova, throwing out large portion of its envelope into interstellar space. The core that remains behind may end up as one of the following three types of stellar remnants.
  • Dwarf Star: If the original mass of the star is between 2 and 5 solar masses, the back pick of the supernova explosion will compress the core of the star to nuclear densities giving rise to a neutron star. The mass of a neutron is less than 2 solar masses and its radius is about 10 kms.
  • Neutron Star: If the original mass of the star is between 2 and 5 solar masses, the back kick of the supernova explosion will compress the core of the star to nuclear densities giving rise to a neutron star. The mass of neutron star is less than 2 solar masses and its radius is about 10 kms.
  • Black Hole: If the original mass of the star is more than 5 solar masses, the back kick of the supernova explosion becomes so violent that the core continues to contract indefinitely giving rise to a black hole. As the contraction proceeds, the radius decreases continuously and acceleration at the surface goes on increasing due to gravity. Finally a stage comes when the value of the gravitational pull has become so high that even photons cannot escape from the surface of the body. On the contrary any particle or photon approaching it will be immediately swallowed.
  • Constellations: These are the groups of the stars forming special shapes, e. g. Ursa major or the great bear, Ursa minor or the little bear.
  • Satellite (natural): Satellites are the natural bodies that orbit a planet. The nine major planets of the solar system have a total of at least 49 known satellites between them. In addition the numerous small bodies that comprise the rings of the Saturn, Jupiter and Uranus may be regarded as natural satellites.
  • Sun: Sun is the central body of the solar system and is the nearest to earth. It looks brightest because it is only 8 light minutes away from the earth. The outermost part of the sun's atmosphere is called ‘Corona’ ‘Photosphere’ is the visible surface of the sun. The stratum of the sun's atmosphere immediately above the photosphere and below the corona is chromosphere. The temperature at the centre of the sun is about 14 million (14 × 106) Kelvin. The sun is made up of gases. Hydrogen accounts for 70 percent of its mass, and the remainder is made up of 28 percent helium and 2 percent all other heavier eiements. The controlled thermonuclear reactions are responsible for the generation of solar energy.
  • Planet: A planet is a heavenly body. That moves in an orbit around a star, e. g. the sun. The planets appear to be very bright stars that do not twinkle. There are nine planets in our solar system. Mercury, Venus, Mars, Jupiter and Saturn are visible to the naked eye. Venus is the brightest object in the night sky, leaving out the moon. Venus is visible only during the early morning or evening and therefore it is called morning or evening star.
  • Asteroids: They are also called as minor planets which circle in a broad belt between the orbits of mars and Jupiter. They are chunks of rocks covered in frozen gases.
  • Comet: Comet is a part of solar system. A comet travels on a regular path or orbits around the sun. They are made up of solid matter combined with gases. A comet has a head and tail. As a comet approaches the sun a tail usually appears behind it. A few comets appear periodically, e. g, Halley's comet which comes after every 76 years.
  • Meteros or Shooting Stars: These are the r: Lcks that travel through space. Each day many millions of them enter the earth's atmosphere from the outer space. Most of them are destroyed by air friction of the atmosphere. A few are so large that they completely burn away. These strike the earth and are called meteorites.