The billions of stars which we see in the sky every night, are made up of hot gases. Even the Sun is a star. However, there are stars that are a million times brighter than the Sun, but they do not appear so because of their huge distance from the Earth. Have you ever wondered just how far these stars are from the Earth? This distance is measured in units of light years. One light year is the distance travelled by light in one year, (its velocity being 3x105 km or 3 hundred thousand kilometres per second). The star nearest to the Earth (after the Sun) is Proxima Centauri. Its distance from the Earth is 4.28 light years. This star is visible only in the Southern Hemisphere. The nearest star visible in the Northern hemisphere is Sirius, which is also called the Dog Star. Its distance from the Earth is 8.8 light years. In addition to this, there is another neighbouring star called Alpha Centauri, which is 4.37 light years away from us. The farthest star from the Earth, which is visible with our naked eye, is more than eight million light years away from us. If we use a powerful telescope, we can even see stars that are 1000 times more distant. Did you know that there are some stars which are so far away from the Earth that light from them takes more than 1,000 million years to reach us? Right from the ancient times, people have been inventing devices to reveal more secrets of the stars. Scientists have developed various kinds of optical and radio telescopes to study stars. They have gained considerable knowledge relating to the heavenly bodies with the help of these telescopes.