Stars are the most fascinating objects in the sky. They are gigantic balls of gas that people have been interested throughout history. Although they are so big they look small because they are so far away. Humans have always told stories about stars gave them names and saw patterns in them.
The most important star is the sun. Planets revolve around the centre of the solar system. Our sun is a medium sun, about the same size as most of the other stars in the universe. It consists of hydrogen and helium, which combine to produce energy. This reaction, that makes stars shine so brightly, is called fusion. The temperature on the surface of the sun is about 10,000° C, while the core is thousands of times hotter.
Stars are organized in clusters called galaxies. Our sun, together with the solar system is a part of our galaxy, the Milky Way. It is only one of billions of other galaxies in the universe.
Stars often look so small because they are so far away. The nearest star is Alpha Centauri. It takes light from this star about 4.5 years to reach the earth.
The billions of stars in our universe have different sizes and colors. Some shine yellow like the sun, others are red, white or blue. The color of a star tells us how hot it is. Blue stars are the hottest and red ones the coldest ones. As for size, astronomers speak of giants and dwarfs.
The dimmest stars in the universe are the red dwarfs. They are very small and only have a surface temperature about 3,000° C. Proxima Centauri is such a red dwarf. Although it is as far away as Alpha Centauri we can only see it through a telescope.
The biggest stars in our universe are the blue supergiants. They shine a million times brighter than our sun and have a surface temperature of up to 60,000° C. Because they shine so brightly we can see those that are very far away.
Stars begin as clouds of dust and hydrogen, called nebulas. When they get hot enough they start burning hydrogen and produce energy. This process, called fusion, can last billions of years. When a star runs out of hydrogen the fusion process stops and it starts cooling down.
Most stars become red giants after they have burned away all their hydrogen. The core gets smaller but the temperature increases. The area around the core expands because of the high temperature. When a star gets very big it sometimes explodes. Such an explosion is called a supernova and lasts only for a few days. During this phase stars burn billions of times brighter than they normally do. Sometimes the material of a supernova collapses and turns into a very dense ball of matter called a neutron star. It sends out strong radio waves called pulsars.
Sometimes supernovas have enough energy and mass to collapse inward. They become a black hole. Nothing can escape its gravity and it pulls everything into it. Scientists cannot see a black hole because it has no light. They probably exist in the middle of galaxies.