Glossary

Introduction

In Unit 1 and 2, you learnt about the Earth and its satellite, the Moon. Apart from the Earth, there are other objects that revolve around the Sun, such as planets, satellites revolving around some planets, minor planets, asteroids and so on. All these objects, together with the Sun, are called the ‘Solar System’. Let us learn about some of the important components of the Solar System.

The Sun

The Sun is a star, which means it produces energy. There are many many stars in the Universe. The Sun is only one of them! It is neither too big nor too small. It is located quite far from the centre of the galaxy (we will learn a little more about the galaxy in Lesson 4). It is not special in any way! As any other star, the Sun has a huge mass, and hence, it exerts gravitational force on all the planets and other smaller objects in the Solar System. That is why all the objects in the Solar System revolve around it.

Even if it is an ordinary star, the Sun produces a great amount of energy. We get light, heat and other forms of energy only due to the Sun. How does the sun produce such a large amount of energy? The Sun is largely made up of Hydrogen (₁H,), which is the lightest element. When the nuclei of four hydrogen atoms combine, Helium (₂He), the second lightest element, is produced. The mass of one Helium nucleus is a little less than the mass of four Hydrogen nuclei. This is because a small bit of the mass gets converted into a large amount of energy. This process is called ‘nuclear fusion’¹. This process is continuously going on in the Sun, resulting in continuous explosions on the Sun. As a result of this, the Sun is very hot (temperature at the core is around 1.57×10⁷ K, temperature on the surface is 5,772 K), and it emits energy in different forms (radio waves, microwaves, infrared, visible light of all colours, ultraviolet rays and xrays).

When seen using special instruments², a sudden flash of brightness is sometimes seen on the surface of the Sun. This is known as a solar flare. We also see dark spots (called sunspots) on the surface of the Sun. These parts are comparatively cooler than the rest of the surface. See Figure 1 to take a closer look at the Sun.
 

Figure 1: Closer look at the Sun

Figure 1a: Surface of the Sun with a solar flare (captured on 9 June 2002)
(Credit:  By NASA - https://solarsystem.nasa.gov/galleries/a-handle-on-the-sun)
 

Figure 1b: The sunspots (captured in September 2011)
(Credit: By NASA - http://www.dailymail.co.uk/sciencetech/article-2042428/Best-auroras-seen-Britain-thanks-huge-solar-flares.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=16800815)

¹The amount of energy that will be produced from a certain mass is given by Einstein’s famous equation: E=mc² (where ‘E’ is the energy produced from ‘m’ amount of mass and ‘c’ is the speed of light). If you want to know how this equation was derived and how it works, you will have to learn some advanced physics!

²Never look at the Sun with your naked eyes or telescope. That will hurt your eyes and you may even lose your eyesight! We need special instruments to observe the Sun. Do it only when a knowledgeable person is around.