ICSE Class 8 Physics Chapter 01 The Universe

The Universe

The heavenly bodies such as the sun, moon, planets and stars have been known to mankind since the existence of life on the earth. In ancient times, people were of the opinion that these heavenly bodies were Gods and spirits. They also believed that the universe consists only of the limited area where they live or consists of the places which were known to them.

However, with the passage of time, man gained knowledge about the universe. The branch of science which deals with the study of heavenly bodies is called astronomy. Astronomers (scientists who study about the heavenly bodies) explored lots of theories about the universe and they are still in the process of this study. The universe is thus defined as the vast surrounding space which includes everything that exists from the earth to the most distant parts of space that one can possibly see. The earth, stars, planets, satellites and all the objects in it are parts of the universe. It is not known till date how big the universe is and whether it has any limit or not. We can imagine the vastness of the universe from the fact that the planet earth on which we live is only a tiny part of this big universe.

Teacher's Note

The universe's immensity helps us appreciate that Earth is just one small world among countless others, much like how a single grain of sand represents an entire beach.

The Stars

We know that during daytime, the sun is the only object visible in the sky. However, as soon as it becomes dark after the sunset, the sky appears dotted with thousands of bright objects. When viewed from the earth, the objects which appear to twinkle are called stars. These stars emit the light of their own.

Most of the stars are bigger than the earth. Some stars are even much bigger than the sun. They appear small because they are very far away from us. Stars are only visible at night but it does not mean that they are not present in the sky during daytime. They cannot be seen during the day because of the bright light of the sun.

There are other star-like bodies in the sky which do not twinkle but appear to shine with a steady light. They are called planets. They do not emit light of their own but reflect the light received from the stars. Planets revolve around the stars while rotating around themselves. The earth is also a planet. It revolves around the sun while rotating on its axis from west to east.

We see another bright body in the sky at night i.e., the moon. The moon revolves around the earth. Such bodies which revolve around a planet are called satellites or moons.

We can see around 3,000 stars through our naked eyes. The other stars can be seen through a telescope. The stars are very far away from each other.

Teacher's Note

The reason stars twinkle at night but planets shine steadily relates to how their light travels through Earth's atmosphere, similar to how heat waves above hot pavement make distant objects shimmer.

The Solar System

All the heavenly bodies like the planets, satellites, asteroids, comets and meteors which constitute the universe move around the sun to form a solar system. The word solar is derived from the Latin word sol which means the sun. We will now study in brief in this chapter about these heavenly bodies.

The Sun

Stars are the celestial bodies that continuously emit heat and light. The sun is also a star located at the centre of the solar system. It is about 4.5 billion years old and comparatively, it is younger than many other stars. The sun appears large as compared to other stars because it is nearer to the earth. Though many stars are bigger than the sun, still they appear to be tiny or dot-like because they are far away from us. Most of the stars are so far away from the earth that even light from them takes millions of years to reach the earth. The distances of stars are, therefore, expressed in terms of light years. One light year is the distance travelled by light in one year at a speed of about 300,000 kilometres per second. Light year is a unit of distance and is equal to 300,000 x 365 x 24 x 60 x 60 km or 9.46 x 10¹² kilometres.

The approximate distance of the sun from the earth is 150,000,000 km, which means that light takes around 8 minutes and 20 seconds to reach from the sun to the earth with a speed of about 3.0 x 10⁸ ms^-1. The diameter of the sun is about 100 times the diameter of the earth and its mass is more than a million times the mass of the earth. At the surface of the sun, the temperature is about 6000°C but near the centre of the sun, the temperature is about 15 million degree celsius.

The sun is full of hydrogen and helium gases, so it is a glowing ball of hot gases. It is the only source of heat and light for all the planets of the solar system. For the existence of life on the earth, heat and light are the essential factors which we receive from the sun. This is an endless process with which we are blessed.

The sun is considered to have four layers although there is no clear boundary between these layers.

Corona: It is the outermost layer of the sun's atmosphere. It is full of gas particles although the density of gas particles is very low. It has a temperature of about 6,000°C.

Chromosphere: The layer which is below the corona is called the chromosphere. It is several thousand kilometres thick (about 2,500 km), but sometimes, this can expand even up to 16,000 km because gases can flow and spread. The average temperature of the chromosphere is about 28,000°C.

Photosphere: It is a layer below the chromosphere. It is about 550 km thick and we usually refer to it as the surface of the sun.

Core: Main mass of the sun is at its centre. This is called the core of the sun. At the edge of the core, the temperature is about 1 million degrees centigrade, but near the central region, it is believed to be about 15 million degrees centigrade. The fusion of hydrogen into helium produces very high energy. This energy produced increases the temperature.

Teacher's Note

The sun's incredible temperature and energy production can be compared to a massive nuclear reactor in space that powers all life on Earth through the light and warmth it provides daily.

Planets

At night if you look carefully at the sky, you will observe that some objects do not twinkle like stars. Our ancestors noticed that they appear to change their positions with respect to the stars. They called them planets which means wanderers. In Hindi, planets are called Grah.

The planets also receive most of the heat from the sun.

Planets which are nearer to the sun travel at a faster speed than those which are far away. The earth is the only planet which is known to sustain life in the planetary system.

Planets revolve around the sun in large elliptical paths called orbits. Every planet has its own orbit and each planet traverses through its own orbit while revolving around the sun. In ancient times, people called them wandering stars. Gradually they were named as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto (Fig. 1.3).

According to the latest theory there are now only eight classical planets while Pluto which was the ninth planet is known as a dwarf planet.

A classical planet is a celestial body that:

1. orbits around the sun.

2. has sufficient mass for its self-gravity to pull it into a nearly spherical shape.

3. has a clear neighbourhood around its orbit.

Why Pluto is not a classical planet:

It fails to meet the third condition. Its oblong orbit overlaps the orbit of Neptune. Other dwarf planets are Eris, Haumea, Makemake and Ceres.

Thus, a new definition of classical planet and dwarf planet has been given by International Astronomical Union (IAU) after a meeting in prague with 2500 scientists.

So as a result we now say that there are only eight planets revolving around the sun.

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