## The Earth and Space

Subject: Science

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#### Overview

The family of the sun that includes the sun, eight planets, satellites, asteroids, etc. is called the solar system. The sun is the major component of the solar system. This note provides us information about planets, stars, and constellations.

#### Solar System

The solar system is made up of the sun and everything that orbits around the sun like planets, moons, comets and meteoroids. Every planet in solar system orbits around the sun in elliptical orbits due to gravitational force. The existence of a solar system in the universe is due to the influence of gravitational force between the members of the solar system. Planets are the heavenly bodies that revolve around the sun. Some of the examples of planets are earth, jupiter, mars, etc. Satellites are the heavenly bodies that revolve around the planets. Some of the examples of satellites are the moon, titan etc. Comets are the small irregular bodies in the solar system that are made up of ice and small amounts of dust and rocks. Asteroids are the small rocks that revolve around the sun.

#### Sun

The sun is the largest object in the solar system. It is a star. It is a ball of fire. Everything in the solar system revolves around the sun. The sun contains about 99.8% of solar system mass. The diameter of the sun is about 13,92,400 km. The mass of sun is about 1.988 $\times$ 1030 kg. The sun is about 109 times as large as Earth and 333,000 times as massive. The sun is about 15 crore km far from the earth. The temperature of the surface of the sun is about 5400oC and the temperature of its core is about 1,50,00,000oC. The sun is about 4.5 billion years old.

#### Planets

Planets are the heavenly bodies that revolve around the sun. There are eight planets in our solar system, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune (from order). Among them, Jupiter is the largest planet and Mercury is the smallest planet and Mercury is the nearest planet and Neptune is the farthest planet from the sun. Venus is the hottest planet whereas Neptune is the coldest planet. Pluto is not the planet but it is classified as a dwarf planet. There is an asteroid belt in the solar system that is located between Mars and Jupiter. It is made up of rocks. Those planets that lie inside the asteroid belt are the inner planet and those planets that lie outside the asteroid belt are the outer planet. Mercury, Venus, Earth and Mars are called inner planets and Jupiter, Saturn, Uranus and Neptune are called outer planets. In this note, we discuss the eight planets of solar system which are given below,

• Mercury: Mercury is the smallest and the closest planet to the sun. It has no moon. It is very hot during daytime. There is no life in mercury as there is no water and atmosphere. It takes 59 days for mercury to revolve around the sun. It revolves faster than the other planets as it is very close to the sun.
• Venus: Venus was once considered as twin planet to the earth until the scientists discovered that its temperature was about 480oC. It is the second closest planet to the sun and also to the Earth. It is the second brightest object in the sky after the moon. It is the second largest inner planet. It is also known as morning star and evening star as it can be seen in the eastern sky in the early morning and in the western sky in the evening. There is no life on this planet as it is very hot and there is no water.
• Earth: Earth is the only planet with life on it. It is the third planet from the sun. It is the largest planet among the inner planet. The earth has favourable climatic condition and environment for the existence of living organisms. It is also known as a green planet. The earth appears blue when viewed from the outer space. This layer contains a protective layer of ozone gas that protects living organisms from the ultra- violet radiation coming from the sun.
• Jupiter: It is the largest planet in our solar system. It is the fifth planet from the sun. It has dozens of the moon. Jupiter looks colourful object when we see from the telescope. It has a faint ring around it. The cloud is formed due to the freezing of methane and carbon dioxide. It has great red spots in its atmosphere due to the atmospheric hurricane.
• Saturn: It is the second largest planet and is known for its fabulous ring system. It is the unique feature of this planet. It is the sixth planet from the sun. It has dozens of moons. Titan is the largest satellite of this planet. Its atmosphere contains mostly hydrogen and helium.
• Uranus: Uranus is the third largest planet. It is the seventh planet from the sun. It was the first planet that was discovered using a telescope. It has dozens of the moon. It is made of water, methane, ammonia, hydrogen and helium. It rotates from east to west.
• Neptune: It is the distant planet in the solar system. It contains water, methane, ammonia, hydrogen and helium. It is the eighth planet from the sun and is an extremely cold planet. It has more than a dozen moons and six rings. Triton is the biggest among all the moons.
The Planets in our Solar System
 Name of the Planet Distance from the sun (in km) Period of revolution(days/years) Period of rotation(hour, days) Average diameter(in km) Mass (kg) Temperature(K range or average) Number of satellites Mercury 5.76 $\times$ 107 87.97 days 58.85 days 4851.2 3.3$\times$ 1023 100- 700K 0 Venus 10.7 $\times$ 107 224.7days 243.02 days 12035.2 4.87$\times$ 1024 726K 0 Earth 14.88 $\times$ 107 365.25days 23 hrs.56 min. 12672 5.98$\times$ 1024 260- 310K 1 Mars 22.56 $\times$ 107 286.98days 24 hrs. 37min. 22 sec 6742.4 6.42$\times$ 1023 150- 310K 2 Jupiter 76.8 $\times$ 107 12 years 9 hrs. 55 min. 30 sec 1139040 1.90$\times$ 1027 120K 63 Saturn 144 $\times$ 107 29.5years 10 hrs. 30 min. 115811.2 5.69$\times$ 1026 88K 60 Neptune 288 $\times$ 107 4years 17 hrs. 14 min. 24 sec 50441.6 8.68$\times$ 1025 59K 27 Uranus 448 $\times$ 107 164years 16 hrs. 6 min 36 sec. 48972.8 1.02$\times$ 1026 48K 13

#### Stars

We see numerous twinkling objects when we see in the clear sky at night. These twinkling objects are known as stars. They are very bright as they have their own sources of light. They are hot and bright luminous objects. Star are the masses of hot gases. There are different shape and sizes of stars. Some are big and some are small. But when we see the clear sky at night, those stars only looks big which are nearer to the earth and those stars looks small that are far away from the earth. The sun is also medium- sized star. It appears big because it is very near to the earth. We get light and heat from the sun. We do not get light and heat from the other star because they are very far from the earth.

Distance of Stars

As we know that, stars are located very far from each other. They are located millions of kilometers far from the earth. So, it is impossible to measure the distance in meter and kilometer. So, the distance between the star and the earth is measured in a light year. The total distance covered by light in one year is called light year. Light travels about 3 lakh kilometers in one second and it covers 9.5 $\times$ 1012 kilometers in one year.

One light year = Speed of light $\times$ One year
= 300000 km/s $\times$ 365 days
= 300000 km/s $\times$ 365 $\times$ 24 hours
=300000 km/s $\times$ 365 $\times$ 24 hours $\times$ 60 minutes
=300000 km/s $\times$ 365 $\times$ 24 hours $\times$ 60 minutes $\times$ 60 seconds
= 9.5 $\times$ 1012km

Brilliance of Stars

When we see the clear sky at night, we can see stars twinkling. Some stars twinkles with very bright light whereas some stars twinkles with faint light. Different stars have different level of brightness. The brightness or brilliance of star depends on its surface temperature, size and distance from the earth. The distant stars have less brightness whereas the nearer stars have less brightness.

Colours of Stars

When we see in the clear sky at night, all the stars looks same. Some stars may look brighter than others but they all look white in colour. But when we see the sky through the telescope, the stars looks in a different colour. They may be red, orange, yellow, white and blue in colour. The colour of a star depends on its surface temperature and indirectly on its size. The stars with less surface temperature look red and stars with high-temperature looks blue.

 Colour of Star Temperature Green\White 36000+oC Blue 25000oC White 11000oC Yellow/white 7500+oC Yellow 6000oC Orange 4000oC Orange/Red 3400+oC Red 3000oC

Structure of Stars

Stars are giant luminous heavenly bodies that are made of hydrogen and helium gases. There are billions of stars in the sky. Stars produce heat and light due to the nuclear fusion that takes place inside the stars. Stars are not permanent because there is continuous change in the composition of stars.

Motion of Stars

Stars do not remain in the same position. One star is in a state of motion with respect to another. Stars change their relative position in the space.

Differences between Planets and Stars

Planets and stars are different from each other. Planets are non- luminous body and stars are the luminous bodies. Planets revolve around the sun whereas stars do not revolve around the sun. Some of the differences between planets and stars are discussed below,

 Planets Stars Planets are the luminous body. Stars are non- luminous body. Planets do not twinkle. Stars twinkle at night. Planets have a very low temperature ranging from -330oC to 865oC. Stars have a very high temperature ranging from 3000oC to 25000oC. They are made of rocks, dust particles, and water vapour. They are made of hydrogen and helium gases. There are only nine planets in the solar system. There are billions of stars in the celestial sphere. Their diameter ranges from 4851km to 1139040 km. Most of the stars are big in size but only a few of them are smaller in size.l

#### Constellations

We see millions of stars in the sky. They are at different positions. Some stars are seen bright and some are seen dim. But when we join or arrange the bright stars, special identifiable shapes are formed. They may be shapes of some animals or some objects. These groups of stars, when arranged, form shapes of objects or some animals are known as constellations. Therefore, the constellation can be defined as the small groups of a star that form the pattern or shapes of some objects or animals. Till now, there are 88 constellations named by our astrologist. Out of them, 12 constellations are called zodiac. Some of the common examples of constellations are the Big dipper, Orion, Virgo, Ursa major, Sagittarius etc. These shapes are given names of gods, animals, zoidiac, virgo, heroes etc. by the astrologists. Some of the information about some famous constellations are discussed below,

• Ursa Major: Ursa Major is also known as 'The Great Bear' is a well- known northern hemisphere constellation that contains seven big stars. This group of stars is called big dipper or great bear. Big dipper includes Dubhe, Merak, Phecda, Megrez, Alioth, Mizar and Alkaid. Dubhe and Merak are the brightest stars in Ursa Major. It is seen in the summer season in the northern sky.
• Ursa Minor: Ursa Minor is also known as 'The Little Bear' is a northern hemisphere constellation. It is also known as a little dipper. It contains seven less bright stars than big dipper out of which four form the 'bowl' of the little dipper and three stars form the handle of the ladle. Polaris is the most famous star in Ursa Minor. Ursa Minor can be seen in the northern hemisphere.
• Orion: This constellation is also known as 'The Hunter' as it contains numbers of stars arranged in the shape of the hunter. Some of the stars present in it are Mintaka, Alnilam, Rigel, Saif, etc. The brightest stars in Orion are Rigel, Betelgeuse, and Bellatrix. This constellation can be seen in the northern hemisphere in the winter season.
• Cassiopeia: This constellation can be seen in the far northern sky. It consists five major stars in the shape of English Alphabet 'W'. They are also known as, 'The Lady of the Chair'. This constellation was named for Queen Cassiopeia, the mother of Andromeda (and the wife of Cephus). The brightest star in Cassiopeia is Schedar (alpha Cas) and second-brightest is Caph (beta Cas).

Identification of Constellations

Since Nepal lies in the northern hemisphere, we can only see the constellations of the northern hemisphere. We cannot see the constellation of the southern hemisphere. The constellation seen in a particular season are not visible in another season as the earth revolves around the sun. For the identification of constellation, at first, we should find the position of two famous constellations of the north pole, Ursa major and Ursa minor. Ursa Major appears like the shape of a question mark. We can also identify other constellation by using sky map or chart of constellations.

##### Things to remember
• Solar system is the family of the sun that includes the sun, eight planets, satellites, asteroids, comets, etc.
• The sun contains about 99.8% of solar system mass.
• There are eight planets in our solar system, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune (from order).
•  Mercury is the smallest and the closest planet to the sun.
• Earth is the only planet with life on it.
• Uranus is the third largest planet.
• The brightness or brilliance of star depends on its surface temperature, size and distance from the earth.
• Stars are giant luminous heavenly bodies that are made of hydrogen and helium gases.
• It includes every relationship which established among the people.
• There can be more than one community in a society. Community smaller than society.
• It is a network of social relationships which cannot see or touched.
• common interests and common objectives are not necessary for society.
##### Questions and Answers

Solar system is the family of the sun that includes the sun, eight planets, satellites, asteroids, comets, etc. The names of inner planets are given below,

• Mercury
• Venus
• Earth
• Mars

Those planets which lies outside the asteroid belts are called outer planets. The names of outer planet are given below,

• Jupiter
• Saturn
• Uranus
• Neptune

Any three differences between planets and stars are given below in table,

 Planets Stars Planets are non- luminous body. Stars are luminous body. Planets do not twinkle. Stars twinkle. Their number is uncountable. There are eight planet in our solar system.

Constellations are the groups of stars which do not change their location relative to each other but form a fixed shape. Any four names of constellations are given below,

• Ursa Major
• Ursa Minor
• Cassiopeia
• Orion

The names of eight planets on the basis of their position or order is given below,

• Mercury
• Venus
• Earth
• Mars
• Jupiter
• Saturn
• Uranus
• Neptune
The distance travelled by light in one year is called light year.
Constellations are the groups of stars, making a particular shape.
Any two examples of constellations are orion and scarpio.
Stars are those heavenly bodies which are very hot and luminous.
Solar system is the family of the sun that includes the sun, eight planets, satellites, asteroids, comets, etc.
The planets enclosed by the asteroid belts are called inner planets.
The planets outside the asteroids belt are outer planets.
##### Quiz

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