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Revision Notes on Physical World

  • Light year

The distance travelled by light in vacuum in one year is called a light year. It is a unit of length used to measure astronomical distances. Its value is 9.46\times1015 m.

  • Astronomical unit (AU)

The average distance between the earth and the sun is called astronomical unit.

  • Astronomy

It is the branch of science which deals with the study of heavenly bodies of universe.

  • Solar system

Solar System(a) The sun and all the objects moving around it taken together is called solar system.

(b) Solar system consists of the sun, the eight planets and other heavenly bodies like asteroids and comets etc.

(c) Nearest star from the earth other than the sun is alpha centauri.

  • Sun

(a) The sun is a typical example of a star.

(b) Seventy percent of sun’s mas is hydrogen, twenty eight percent helium and two percent heavier elements from lithium to uranium.

(c) The sun’s mas is about 2\times1030 kg which is more than 3\times105 times that of the earth and radius 1.4\times106 km.

(d) The source of sun’s energy is the process of nuclear fusion taking place in it.

(e) The sun rotates about its axis and completes one rotation in 25 days.

  • Photosphere

It is the inner part of sun which appears as a bright disc. It is a denser mixture of gases and vapors. Its thickness is about 500 km and its temperature is about 6000 K.

  • Chromosphere

It is the outer part of sun, just above the photosphere. It is a rarer mixture of gases and vapors.

  • Planets

(a) Mercury: Nearest, smallest and hottest planet of the solar system. Life is not possible on mercury.

(b) Venus: Also called morning star and the evening star is brightest amongst all.

(c) Earth: It is the only planet which contains suitable conditions for evolution and survival of life. It has only natural satellite named moon.

(d) Mars: Nearest planet to earth. It is redish. it has traces of O2 but percentage of O2 is not sufficient for evolution and survival of life. Mars is laso called red planet. The temperature of mars varies from 21º C to 27º C. It has two satellites known as phobos and deimos.

(e) Jupiter: Largest planet of the solar system having the maximum number of satellites. Jupiter is the largest planet of the solar system. Its temperature is about 140º C. It has 63 satellites and no life possible on the Jupiter.

(f) Saturn: It has ring around it. Saturun is the second largest planet of the solar system. Its temperature is about 180º C. It has 61 satellites.

(g) Uranus: It is the only planet which rotates from east to west on it axis. Uranus appears green due to the large amount of methane and ammonia clouds in its atmosphere. Its temperature is about 127º C. It has 15 satellites.

(h) Neptune: It has no special characteristic. Neptune is very far away from the sun. It has two satellites.

  • Terrestrial Planets

The four planets nearest the sun, Mercury, Venus, Earth and Mars are called terrestrial planets.

These planets have well defined surface of rocks and soils.

  • Jovian planets

The four planets Jupiter, Satrun, Uranus and Neptune are called jovian planets. These planets do not have a solid surface.

  • The Moon

(a) It is the natural satellite of the earth.

(b) It revolves around the earth once in 27.33 days.

(c) Diameter is 3476 km.

(d) Distance from the earth is 357000 km.

(e) Mass of thec moon is 0.123 times the mass of the earth.

(f) Maximum temperature at day is 117º C and at night is – 171º C.

  • Asteroids (Minor Planets)

(a) The small, rocky, irregularly shaped objects revolving around the sun, are called asteroids or minor planets.

(b) There is a belt of asteroids between the orbits of mars and jupiter.

(c) The largest asteroid is ‘Ceres’.

(d) Asteroids can have moons too. The asteroid Ida has its moon, Dactyl.

  • MeteoroidsMeteoroids

Small pieces of rock travelling through space are called meteroids. These are fragments from an asteroid, a comet, a moon, mars, etc.

  • Meteors

When a meteoroid enter earth’s atmosphere, it heats up due to friction. The hot vapors give off light before cooling down. A streak of light caused by a vaporizing meteoroid is called a meteor or a shooting star.

  • Meteorite

A meteoroid that hits the ground is called a meteorite.

  • Comets?Comets

Comets are objects which move in highly elongated orbits around the sun. They are made up of frozen gases and rocks. They have a long tail which always points away from the sun.

  • Inner structure of the earth

(a) Crust: It is the thinnest and outermost layer. It is about 10 km thick under the oceans and could be up to 45 km thick under the continents.

(b) Mantle: It lies below the crust. It extends up to a depth of about 3000 km. The temperature inside the mantle is about 3000 km. The temperature inside the mantle is about 1000º C.

(c) It is the most inner part of earth. It is made up of iron mostly. The outer part of core is in molter state have a temperature about 4000º C and inner part is in solid state because of very high pressure.

  • Stars

(a) The heavenly objects which shine like the sun due to their own energy are called stars.

(b) The sun is the nearest star to the earth. After the sun, alpha centuri is the next nearest star to the earth.

(c) A person can see nearly 5000 stars with the naked eye on a clear night.

(d) In addition to these stars, many groups of bright and faint stars called constellations can be see in the sky.

(e) The space between stars is almost empty with traces of interstellar gas and dust.

  • Brightness of stars

The brightness of stars is represented through a system of magnitudes.

  • Red giant

With the successive contraction of the core and the expansion of the outer layers, the temperature of the outer layers falls to make the star appear red. At this state, it is called red giant.

  • Nova or super nova

The release of energy in the star is so large and rapid that a violent explosion called nova or super nova occurs and throws out a large portion of outer layers back into outer space leaving behind the core of the star.

  • Death of a star

The decay of the star is called the death of star.

  • Chandra shakher limit

When the original mass of the star ≤ 1.4 solar mass, the core of the star tends to die as white dwarf which just cools off slowly, changing its colour from white through yellow to red and finally becomes black. This mass limit of 1.4 solar mass is called Chandra shakher limit.

  • Black hole

It is a region in space in which the gravitational force is so great that no object can escape from it. The magnitude of a star is the measure of its brightness, when observed from the earth.

  • Galaxy

A huge group of stars and other celestial bodies bound together by gravity is called a galaxy. Our galaxy is called milky way galaxy. Milky way galaxy is a spiral galaxy.

  • Constellation

A group of stars forming a recognizable pattern is called a constellation. Few constellations are Ursa Major (great bear), Leo (lion), Pisces (fish), Taurus (bull), Hercules (a hero), Orion (a hunter), etc.

Ursa Major have seven brightest stars forms a shape of great bear.

  • The big-bang theory

About 15 billion years ago, the whole matter of the universe was concentrated in a highly dense small region. A sudden explosion broke this region into pieces which start to move away from each other. The instant of this explosion is called the big bang. Therefore, the universe is expanding continuously.

  • ?Eclipses


Eclipse occurs when the sun, moon and earth lie in straight line. 

(a) Solar eclipse

When moon passes between the earth and the sun, cutting off the light of the sun from the earth. Solar eclipse may be partial, total or annular.

(b) Lunar eclipse

When the earth comes between the sun and the moon, a full moon during rotation in their respective orbits. A ring shaped lunar eclipse is not possible because the size of the moon is less than that of the earth.