CBSE Class 8 Physics – 17. Stars and the Solar System (Notes)

CHAPTER-17         Stars and The Solar System

Celestial Object आकाशीय पिण्ड :- Any naturally occurring object in the universe is called a celestial body or heavenly bodies.                                Ex:-Stars, planets, satellites, comets, etc.

  Astronomy खगोल शास्त्र :- The branch of science that deals with the observation of celestial bodies is known as astronomy

* Telescope is the instrument which is used for observation.

Galaxy आकाश गंगा:- A huge system of billions of stars and clouds of dust and gases is called galaxies.

* Our sun belongs to a galaxy called Milky Way (Akash Ganga in Hindi).

* The nearest galaxy to the Milky Way is Andromeda, which is 2 million light years away.

Stars:- The huge glowing ball of hot gas, mainly hydrogen and helium is called star. The sun is the nearest star from us. It is at a distance of about 150,000,000 (one hundred fifty million) kilometer from us. The next nearest star from us is Alpha Centauri.   This is about 40,000,000,000,000 (forty trillion) km from us.

* They have their own heat and light.

* They twinkle in the sky.

* They are larger than planet.

* They are mostly made up of hot gaseous.

Planets:- The heavenly bodies that move around the sun are called planets.

* They have their own heat and light.

* They do not twinkle in the sky.

* They are smaller than star.

* They are mostly made up of rocks.

Light Year प्रकश वर्ष :- The distance travelled by light in one year is called light year.

Distances of stars are usually expressed in terms of light years.

The velocity of light is 3×10⁵ Km/sec Or  3×10⁸ m/sec

The distance of earth from sun is 1.5×10⁸ Km = 15×10⁷ Km

The sun light reach on earth in 8 minutes 20 Seconds.

1Light year = 9.46×10⁷ Km

Calculation 3×10⁵ Km/sec×3600 sec×24×365=946080×10⁷ Km= 9.46×10¹² Km

Alpha Centauri is 4.3 light years away from us.

Astronomical Unit खगोलीय मात्रक :- The distance between the sun and the earth is called one astronomical unit (AU).

 1AU = 1.5×10⁸ Km = 15×10⁷ Km

This unit of distance makes it easy to express the distance of different planets from the sun.

* Do you know why the stars are not visible during the day? In fact, the sunlight is so bright during the day that we are unable to see the stars. This     is similar to the blinding effect of the headlights of a car. When the headlights of a car fall on your eyes, you are unable to see anything else.

Apparent Movement of Stars, Sun and the Moon

All the celestial bodies appear to move from east to west.

This happens because the earth rotates from west to east.

When you are sitting in a moving train the objects on the Platform appear to be moving in the opposite direction. The same illusion happens to show the apparent movement of celestial bodies.

Pole Star ध्रुव तारा :- The brightest star of Ursa Minor that appears in the northern sky is called pole star/north star.

While other stars appear to move with gradual progress of night, the pole star remains fixed at its position. This happens because the pole star is in line with the axis of the earth.

It is only visible from the northern hemisphere. However, the fixed position of the pole star has helped humans since ages.

Sailors and travellers used the pole star to find way when they had to travel during night.

Constellations (Taramandal):-The group of stars that make a recognizable pattern in the sky is called a constellation.

          Ex:-   Great Bear-Ursa Major, Little Bear-Ursa Minor, Orion the Hunter and Scorpio the Scorpion.

Different constellations may have different names in various cultures. There are twelve signs of zodiac these are: 

Aries, Taurus, Gemini, Cancer, Virgo, Leo, Libra, Scorpio, Capricorn, Sagittarius, Aquarius and Pisces.

1. Ursa Major:
(i)  This constellation consists of seven stars and appears in the northern sky.
(ii)  It looks like a big ladle or a question mark. There are three stars in the handle of the ladle and four in its bowl
(iii)  It is also known as Big Dipper, the Great Bear or the Saptarshi.

2. Orion (mriga)/Hunter: 

(i) This constellation is visible in the southern sky and this can be easily seen during summer months in India.                                                              (ii) It also consists of seven or eight bright stars.  The three middle stars in Orion represent the belt of the hunter and the remaining four bright stars are arranged in the form of a quadrilateral.
(iii) Orion is also called the Hunter.

3. Sirius:
(i) It is the brightest star in the sky.
(ii) To find the location of Sirius, imagine a straight line passing through the belt of three middle stars of Orion.

4. Cassiopeia: Cassiopeia: We can see it in the northern sky during winter in the early half of the night.
(i) This constellation consists of five prominent stars that form the destroyed form of M or W. 

(ii) It is another popular constellation in the northern sky.                                                                                                                                                  (iii) It is visible during winter in the early part of the night. 

Little Bear:  A group of seven stars and the pole star is a part of little bear. It is also known as Ursa Minor or Laghusaptarshi.

Scorpions: This constellation has seven bright stars and other faint stars and together forms the figure of a scorpion.

Solar system सौर परिवार :-The word ‘sol’ means the sun and ‘system’ refers to its family.

The family of sun, eight planets, satellites and some other celestial bodies (as asteroids and meteoroids) is called solar system.

All other members of the solar system keep on revolving around the sun. This is possible because of the gravitational attraction between the sun and these bodies.

The solar system is 4.6 billion years old. Different members of the solar system are as follows.

The Sun: 

The sun is the centre of the solar system.

The sun is a huge store of heat and light energy.

It is mainly composed of hydrogen gas. The hydrogen gas in the sun keeps on changing to helium gas and this process releases huge amount of energy. This is like thousands of atom bombs working together.

Planet: 

A celestial body which revolves around a star is called a planet.

There are eight planets in the solar system they are-

According to distance from sun

Mercury बुध, Venus शुक्र, Earth पृथ्वी, Mars मंगल , Jupiter वृहस्पत, Saturn शनि, Uranus अरुण and Neptune वरुण

 

According to size greatest to smaller

Jupiter वृहस्पत > Saturn शनि > Neptune वरुण > Uranus अरुण > Earth पृथ्वी > Venus शुक्र > Mars मंगल > Mercury बुध

Satellite उपग्रह: 

A celestial body which revolves around a planet or another celestial body is called a satellite.

Mercury and Venus do not have known satellites.

Jupiter has the largest number of known satellites.

Mercury: 

This is the smallest planet in the solar system and is nearest to the sun.

It is at a distance of 0.4 AU from the sun.

This is so close to the sun that it cannot be seen without a powerful telescope.

Venus: 

Venus is at a distance of 0.7 AU from the sun.

This planet is almost of the same size as the earth.

Venus is the hottest planet on the earth.

After moon; this is the brightest object in the night sky.

Venus is visible in early evening during the summer months and in early morning during winters. Due to this, Venus is also called the morning and evening star.

Earth: 

Earth is the only planet where life is known to exist.

According to scientists earth is at a perfect distance from the sun and hence it has the right combination of gases, materials and temperature to support life.

A major portion of the earth’s surface is covered with water and hence it appears blue from the space. Due to this; earth is also called the BLUE PLANET.

Mars: 

Mars is at a distance of 1.5AU from the sun.

Mars appears red because of the presence of iron oxide on its surface. Hence, it is also known as the RED PLANET.

Deimos and Phobos are the two natural satellites of mars.

The four planets before the asteroid belt are called rocky planets or TERRESTRIAL PLANETS because they are made of rocks.

The four remaining planets which are after the asteroid belt are called GASEOUS GIANTS because they are mainly composed of gases.

Jupiter: 

Jupiter is at a distance of 5.2 AU from the sun.

It is the largest planet in the solar system.

It is 1300 times the size of the earth. But because of its lower density it is just 388 times the mass of the earth.

There are 67 known satellites of Jupiter.

Saturn:

Saturn is at a distance of 9.5 AU from the sun.

It is famous for its rings. The rings around this planet are composed of rocks, gases and vapours. These rings can be seen through a telescope.

There are 62 known satellites of Saturn.

The density of Saturn is so low that it can float on water.

Uranus: 

Uranus is at a distance of 19.2 AU from the sun.

Uranus rotates from east to west like Venus.

The axis of Uranus is slightly tilted on its orbit. Due to this, Uranus appears to be rolling on its sides.

There are 27 known satellites of Uranus.

Neptune: 

Neptune is at a distance of 30 AU from the sun.

There are 14 known satellites of Neptune.

This is the farthest planet from the sun.

Moon:–    

The celestial object that revolves around a planet is known as moon or satellite.

The moon is the only natural satellite of the earth.

This is our nearest neighbour in the universe.

The moon is at a distance of about 400,000 km from the earth.

In terms of size; the earth is 81 times bigger than the moon.

There is no atmosphere on the moon.

It is full of craters and mountains. Some of the mountains on the moon are as high as the highest mountains on the earth.

Phases of Moon

The moon does not appear similar on different days rather its shape changes every day. The different shapes of the moon are called the phases of moon.

New Moon:  When the moon is between the sun and the earth, we cannot see the moon at all. It is known as new moon or Amavasya.

Crecent moon: The next day of new moon, only a thin slice of the moon can be seen which is known as crecent moon.

Waxing Crescent: Within a few days of the new moon day; the moon appears like a crescent. This is called the waxing crescent.

1^st Quarter: Within about a week from the new moon day; the moon appears as a semicircle. This is called the 1st quarter moon.

Waxing Gibbous: Within about 10 days from the new moon day; the moon appears as a circle with some portion cut off. This is called the waxing gibbous moon.

Full Moon: Within about 15 days from the new moon day the moon appears as a complete circle. This is called the full moon.

Waning Phase: The waxing phase of the moon is followed by the waning phase. During this phase; the full moon reduces in size and goes through the waning gibbous, 2nd quarter and waning crescent phases. Within about 29 days from the new moon day; the next lunar month begins with the next new moon.

Blue moon – Generally there occurs one New Moon every month. However, at times, there can be two new moons in a single month. The second New Moon in a month is called the Blue Moon.

The phases of the Moon hold an importance in the Indian society and culture as different festivals in India are celebrated depending upon the Phases of the Moon.

Festivals of India	Relation with the phases of the Moon
Diwali	Celebrated on New Moon Day
Guru Nanak birthday	Celebrated on Full Moon Day
Maha Shivratri	Celebrated on the 13th night of Waning Moon
Eid-ul-Fitr	Celebrated on the Day following the Crescent Moon

Reason for Phases of Moon: The periods of rotation and revolution are same in case of the moon. This means that the moon takes the same time in completing one rotation and one revolution. Due to this, only one surface of the moon is visible from the earth.

We also know that the moon does not have its own light rather it reflects the sunlight. So, the sunlit portion of the moon is visible to us. When the moon, the earth and the sun are in a straight line and the earth is between the other two; we are able to see the full moon. On the other hand, when the moon is in between the earth and the sun; we are able to see the new moon, i.e. the whole surface of the moon appears in dark.

The surface of the Moon

The surface of the Moon is barren and all covered with dust.

There are Craters or bowl-shaped cavities present on the surface of the Moon.

There are steep and high mountains present in huge numbers on the Moon.

Some of the mountains are as high as the highest mountains found on the earth.

There is no presence of water and atmosphere on the moon.

Figure- Surface of the Moon

Can we hear sound on the Moon?

There is no atmosphere on the Moon that is, no gases are present on the Moon that could act as a medium for the propagation of sound. Hence, the astronauts of the Moon cannot hear each other.

They generally use signs or microphones to communicate with each other.

Who was the first Astronaut to land on the moon?

Neil Armstrong was the first person to land on the moon followed by another Astronaut Edwin Aldrin. Neil Armstrong landed on the moon on July 21 1969.

What is a lunar eclipse?

The lunar eclipse occurs when the moon, the Earth and the Sun are very closely aligned with each other and the earth is present exactly in the middle of the moon and the sun.

Hence, the shadow of the earth falls upon the moon.

Therefore during the lunar eclipse, the moon does not reflect light of the sun but the light that it receives from the earth.

As a result, the moon appears reddish in colour during a lunar eclipse.

Figure 4 Lunar Eclipse

Asteroid Belt:-The asteroid belt is present between the orbits of Mars and Jupiter. This belt extends between 2.3 and 3.3 AU from the sun. Asteroids are made up of rocks, metallic minerals and some ice. They are remnants of formation of the solar system.

Comet:-A comet is a celestial body which revolves around the sun in a highly elliptical path. When a comet passes from near the earth, it becomes visible to us. Halley’s Comet and Hale Bopp are some of the famous comets. Halley’s Comet appears after every 76 years. It appeared last time in 1986 and is expected to appear again in 2062.

Meteor:-When a small object from the space reaches the atmosphere of the earth it enters at a very high speed. The high speed creates so much friction that the object burns off before reaching the earth. Such objects are called meteors.

They are visible as shooting stars in the night sky.

Meteorites:-An object can be too big to be completely destroyed by burning. Such objects reach the surface of the earth and are called meteorites.

A meteorite can cause huge damage to the earth like destroying a huge population of living beings.

Artificial Satellites:- The man-made satellites are called artificial satellites. These satellites are made for various purposes; like remote sensing, telecommunication, defence, etc. Data from the satellites is utilised by meteorologists to make predictions about weather.  Communication satellites help in working of mobile phones, television, etc. Global Positioning System (GPS) works because of these satellites.