-Shakil Raiman

 Star  Planet  Moon  The Solar System  Comet  Asteroid  Gravitational Field Strength  Newton’s Gravitational Law  Orbital Speed  Galaxy

 A planet is a non-luminous celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity.  Our Earth (or the Earth) is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System.  Earth is the only place in the universe where we know life exists

 A star is a massive, luminous ball of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter.  The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible from Earth during the night when they are not outshone by the Sun or blocked by atmospheric phenomena.  Historically, the most prominent stars on the celestial sphere were grouped together into constellations and asterisms, and the brightest stars gained proper names. Extensive catalogues of stars have been assembled by astronomers, which provide standardized star designations.

 Moons are non-luminous natural objects that orbit a planet. Moons are also called natural satellite of a planet.  The Earth has just one moon.  It is approximately Km away from earth.  It has a mass and surface gravity just one-sixth that of the Earth.  1 lunar month = 29.5 days.  Some planet have no moon and some have more that one. Mars have two moon, Jupiter has over 60 moons and Uranus has 27 moons.

 The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it.  The four smaller inner planets are Mercury, Venus, Earth and Mars.  The four outer planets are substantially more massive. They are Jupiter, Saturn, Uranus and Neptune.  The Solar System is also home to a number of regions populated by smaller objects  The orbits of the planets are elliptical with the Sun close to the centre.

 A comet is an icy, small Solar System body which, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail.  They are approximately 1-30 Km in diameter and made of dust and ice.  At times they are very close to the sun while at other times they are found at the outer reaches of the solar system.

 Asteroids are minor planets or rocks that orbit the Sun.  There is a belt of asteroids between the orbits of Mars and Jupiter. They vary greatly in size from just a few metres to several hundreds of kilometres across.

 Satellites are objects that orbit a planet. They are held in orbit by gravitational forces.  Moons are examples of natural satellites.  Some objects that orbit the planets are manufactured objects. They are artificial satellites.

 The strength of gravity on a planet or moon is called its gravitational field strength.  The symbol of gravitational field strength is g.  Gravitational field strength on the Earth is approximately 10 N/kg, and on the Moon is approximately 1.6 N/kg.

 Newton suggested that between any two objects there is a force of attraction. This attraction is due to the masses of the objects. He called this force gravitational force.  The size of this force depends upon:  The masses of the two objects  The distance between the masses.

 The speed of satellites vary greatly depending on the task they are performing. For example, communication satellites are put in high orbits and travel at approximately 3 km/s, which those monitoring the whole surface of the Earth are put into low polar orbits with speeds of about 8 km/s

 Speed = distance / (time period)  Distance = 2×  × radius of orbit  Time period = time for one complete orbit (T)  Orbital speed = 2  r/T

 A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas dust, and an important but poorly understood component tentatively dubbed dark matter. The name is from the Greek word galaxias, literally meaning "milky", a reference to the Milky Way galaxy.

 Typical galaxies range from dwarfs with as few as ten million (10 7 ) stars, up to giants with a hundred trillion (10 14 ) stars, all orbiting the galaxy's center of mass. Galaxies may contain many star systems, star clusters, and various interstellar clouds.  The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.

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