A black hole is a place with immense gravitational pull. The pull is so huge that not even a light ray can escape it. The reason behind such a strong pull is that a lot of matter has been squeezed into a small space. Black holes are observed when a star is in its last stage of life. These are invisible and only specified space telescopes can detect them. Black holes can be big or small but even the tiniest black hole has the mass of a large mountain. Some black holes are 20 times bigger than the Sun. These are named stellar black holes. However, the largest black holes have masses more than 1 million suns put together. They are called super-massive black holes. One such super massive hole ‘the Sagittarius’ is present at the centre of the Milky Way galaxy.