cbse class 8 univers

7/30/2019 cbse class 8 univers

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UNIVERSE The universe is a huge wide-open spacethat holds everything from the smallest

particle to the biggest galaxy.

It contains all solar systems, stars,planets, satellites etc.

The universe extents from the Earth forat least 10 Billion Light Years and itsprobably still expanding.

The Sun is always losing weight in factScientists have worked out that it

loses around 4 Million tonnes everysecond, this is the amount of hydrogengas that the sun turns into energyevery second.

The gravitational field inside a blackhole is so strong that it can swallowanything in the universe, even apassing star and its light.


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CELESTIAL BODIES

The various stars, moons, planets, shooting stars, meteorsetc that are part of the universe are called celestial bodies. The celestial bodies in our Solar System are divided into

different sections: the inner planets of Mercury, Venus,

Earth and Mars separated by the asteroid belt from the

outer planets of Jupiter, Saturn, Uranus and Neptune.

According to the accepted modern theory, the objects in theSolar System are segmented into three distinctclassifications: planets, dwarf planets and other small

bodies.

Celestial bodies in our Solar System are composed of threeclasses of material: ice, gas and rock.


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SOLAR SYSTEM

The Solar System is made up of all the planets thatorbit our Sun. In addition to planets, the Solar System

also consists of moons, comets, asteroids, dwarf

planets, and dust and gas.

Everything in the Solar System orbits or revolvesaround the Sun.

Scientists believe that the Solar System evolved froma giant cloud of dust and gas.

The Sun contains 99.85% of all the matter in the SolarSystem. The planets, which condensed out of the same

disk of material that formed the Sun, contain only

0.135% of the mass of the solar system.


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STARS

Stars are heavenly bodies which continuouslyradiate heat and light and this phenomenondistinguishes them from other celestial bodies.

Another star close to earth after the sun is Alpha Centauri . Red Dwarf stars are smaller than our sun. And since they are smaller, they

also have less mass. Because of their small size, these stars burn their fuel

very slowly, which allows them to live a very long time. This also causes

these stars to not shine as brightly as others. Some red dwarf stars will live

trillions of years before they run out of fuel.

1. The star that is closest to the Earth is the sun. The sun is nearly

150,000,000 km (15 million km) away from earth.

2. A Star is a gigantic, glowing ball of plasma.

3. Stars are first formed as clouds of dust and gas.

4. Stars give off a tremendous amount of energy in the form of lightand heat.

5. Most stars are between 1 billion and 10 billion years old.

6. Stars that have really strong gravity grow smaller over time and

eventually turn into black holes.

7. The hottest stars give off a bluish light while the cooler stars give

off a reddish light.

8. Stars seem to twinkle because their light travels through theearths

atmosphere and the turbulence in the atmosphere affects the way stars

are seen.

9. All the stars that make up the Milky Way Galaxy revolve around the

center of the galaxy once every 200 million years or so.

10. The oldest star that has been discovered is HE 1523-0901, which is

an estimated 13.2 billion years old.


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A super giant star is the exact same thing as a giant star onlymuch bigger. Remember that as a star gets older it begins to run

out of fuel. As the star runs out of fuel, it will start to burn

out.

Light year: One light year is the distance travelled by light in one year.The distance of stars and other heavenly bodies are measured in terms of light

year. The distance of the sun from the earth is about 8 light minutes and that of

Alpha Centauri is about 4.3 light years.

The speed of light=3x100, 000 km/s

Therefore one light year =3x100, 000 km/s x 365 x 24 x 60 x 60 sec

=9,460,000,000,000 km or 9.46 x 1012 km

Pole star(polaris)While other stars seem to move from east to west, there is

one star that remains stationary or fixed. It lies in the north

and close to axis of rotation of the earth. It has been used by

travellers since ancient times to find directions at night.All the stars appear to revolve around pole star. The two end

stars of Ursa Major when extended through a line in north

direction lead to pole star.


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CONSTELLATIONSA constellation is a group of stars that make an imaginary shape in the night sky. They areusually named after mythological characters, people, animals and objects. In different parts ofthe world, people have made up different shapes out of the same groups of bright stars. It is likea game of connecting the dots. In the past creating imaginary images out of stars became usefulfor navigating at night and for keeping track of the seasons.

A constellation does not have only 5 to 10 stars. It has a large number of stars. All the starswhich make up a constellation are not at the same distance. They are just in the same line of sight

in the sky. Some major constellations are Ursa Major, Ursa Minor, Orion and Cassiopeia.

Ursa Major is also known as Big Dipper, the Great Bear, or the Saptarshi. There are seven

prominent stars in this constellation. It looks like a big ladle or question mark. It can be seen in

summers in the early part of night.

Orion can be seen in winters in late evenings. It is also called the Hunter. There are seven or

eight prominent stars in this constellation out of which four are arranged in the form of a

quadrilateral. The three middle stars form the belt of the hunter.

The brightest star Sirius is located close to the Orion. When weimagine a straight line passing

through the middle stars of Orion towards the east, we see the star Sirius.

Cassiopeia is visible during winter in early part of night. It looks like a distorted letter W or M.


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PLANETS Planets are celestial bodies orbiting a star. Besides orbiting, a

planet also rotates on its axis.

The time taken by a planet to complete one rotation on its axis iscalled its period of rotation. There are eight planets in our solar system. They are Mercury,

Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Easy ways to remember the order of the planets are themnemonics: "My Very Excellent Mother Just Sent Us Nuts".


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Planets do not have light of their own. They appear bright becausethey reflect the light of the sun that falls on them.

The astronomers have demoted Pluto so there are nowonly eight planets; Pluto is no longer officially a planet.Now they're calling it a "dwarf planet".

The largest planet is Jupiter. It is followed by Saturn, Uranus,Neptune, Earth, Venus, Mars and Mercury.

The Inner Planets vs. the Outer Planets

The inner planets (those planets that orbit close to the sun) are quite

different from the outer planets (those planets that orbit far from the

sun).

The inner planets are: Mercury, Venus, Earth, and Mars. They arerelatively small, composed mostly of rock, and have few or no moons.

The outer planets include: Jupiter, Saturn, Uranus, Neptune, and Pluto (a

dwarf planet). They are mostly huge, mostly gaseous, ringed, and have

many moons (again, the exception is Pluto, the dwarf planet, which is

small, rocky, and has four moons).

Differences between planets and stars:

Stars PlanetsThey have light of their own. They do not have light of their own.

They appear bright because they reflect

the light of the sun.

They are stationary but seem to move

from east to west.

They move around a star called sun.

They are very big and gaseous in nature. They are comparatively quite small and

rocky. They appear big because of their

closeness to earth.

They are infinite in number. There are eight planets in our solarsystem.

Mercury (budh)It is the smallest planet and closest neighbour of sun .Most of the time

it is hidden due to the glare of sun. It has no satellite. It can be seen


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just before sunrise or just after sunset. It completes one revolution in

88 days. It has no atmosphere.

Venus (shukra)

It is the brightest and hottest planet of our solar system. Carbondioxide present in its atmosphere traps heal molecules, shows

green house effect, and cause excessive heating. Sometimes it is

seen in the western sky after sunset or it can be seen in the eastern

sky before sunrise. So, it is also called morning or evening star. It

does not have any moon. It rotates from east to west. Just like our

moon, Venus also shows phases.

EarthIt is the third planet in terms of distance from the sun.It is the only planet known to have life on it because of thesuitable conditions like:

1. It has the right temperature, it is neither too hot nor toocold.

2.It has oxygen and water to sustain life.3.It has an atmosphere and ozone layer to protect us from

ultraviolet rays.

Earth rotates on its axis from west to east. That is whysun appears to rise in the east and sets in the west.

It completes one rotation in 24 hours. Earth is known as blue-green planet because of reflection of light

from water and landmass on its surface.

The plane of the equator is called equatorial plane. The plane inwhich the earth revolves round the sun is called orbital plane. These

two planes are inclined at an angle of 23.50 and the axis of the earth

is inclined to its orbital plane at 66.50


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Rotation of earth is responsible for the formation of day

and night. The part of earth which receives light of sun

has day and the part that is away from sunlight has night.

Formation of seasons- Seasons are formed due to revolution of earth a

the sun. It takes about 365 days and 6 hours to complete one revolution

Changes in seasons occur due to two reasons:1. Earths axis of rotation is slightly tilted.2. Distance of the earth from sun is not constant

because its orbit is elliptical and not circular.


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Mars(Mangal)It appears as a red object in the sky. So, it is also called redplanet. It is earths nearest neighbour planet. It is

approximately half the size of earth. It has two small

satellites.

Jupiter (Brihaspati) It the largest planet of our solar system.

It is a gaseous body consisting mainly of hydrogen and helium. It has a faint ring around it. Around 1300 earths can be placed in it. It has a large number of satellites. Its mass is around 318 times that of earth. It has a thick atmosphere which reflects most of sunlight

falling on it. So, it is the brightest planet after Venus.

It rotates very rapidly.Saturn (shani) It is a cold planet because of its distance from the sun isconsiderable.

It is yellowish in colour.It has a large number of satellites.


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It is known for its beautiful ring that encircles it. It is least dense among all the planets. It has a density less than

that of water.

Uranus (arun) It is the third largest planet in our solar system. Its atmosphere contains hydrogen and methane. It also rotates from east to west. It has highly tilted rotational axis. Therefore, in its orbital

motion it appears to roll on its side.

Neptune (varun)

Being very far away from sun, it has a very low temperature.It has 8 natural satellites.

SatelliteA natural satellite is a natural object that goes around (orbits) a planet. It is usually called a

moon and is large and ball-shaped. Some planets have many moons, and some have none. Peoplealso use rockets to send machines into orbit around the Earth. These machines are called artificial

(man-made) satellites. They help us to communicate, the telephones and televisions need these

satellites. Their distance, size and mass are lesser than the natural satellites. India has also made

some satellites. Aryabhatta was the first Indian satellite. Some other Indian satellites areEDUSAT,INSAT,IRS,Kalpana-1 etc.

MOONMoon is the only natural satellite of earth. The moon travels around the earth in a

circle called an orbit. It takes the moon 27 days and 8 hours to go around the earth

once. The moon completes one rotation on its axis as it completes one revolution

around the earth.

The moon does not have light of its own. It shines because of sunlight falling on it

gets reflected towards us. So, only that part of moon is visible which is lighted by

the sun and facing towards us.

The day on which the whole disk of moon is visible is called full moon day. Then

it shrinks back to a thin crescent and finally on fifteenth day it is not visible. It is

called new moon day. Then it starts growing again till full moon day.

The changes in the shape of moon are called its phases. When the bright part of

moon is getting bigger, it is called waxing. When it is becoming smaller it is called

waning. When the moon is more than half lit, it is called gibbous moon and when

it is less than half lit it is called crescent moon.


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The time period between one full moon to the next full moon is slightly

longer than 29 days. In many calendars this period is called a month.

Surface of moon

Its surface is rocky and barren. It has craters, plains, mountains and valleys. Some mountains are

as high as that of mountains on earth.

It has no atmosphere and no water. There is no wind and no climatic changes. On July 21, 1969 the American astronaut, Neil Armstrong, landed

on the surface of moon for the first time. He was followed by

Edwin Aldrin.

CometsComets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains

and frozen gases. As comets approach the Sun they develop enormous tails of luminous material

that extend for millions of kilometers from the bright head, away from the Sun. They revolvearound the sun in highly elliptical orbits. Their period of revolution is usually very long. Many

comets appear periodically. Halleys comet appeared in 1986.

AsteroidsAsteroids are rocky and metallic objects that orbit the Sun but are too small to be considered

planets. They are contained within a main beltthat exists between the orbits ofMars and

Jupiter.


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Meteors and meteoritesA meteoroid is a small rock or particle of debris in our solar system. They range in size from dust

to around 10 meters in diameter.A meteoroid that burns up due to friction as it passes through the Earths atmosphere is known asa meteor.

Several "shooting stars" or meteors per hour can usually be seen on any given night. Sometimes

the number of meteors seen increases dramatically: these are termed "meteor showers".A meteoroid that survives falling through the Earths atmosphere and colliding with the Earthssurface is known as a meteorite.

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