Stars are massive celestial bodies made primarily of hydrogen and helium that produce light and heat through nuclear fusion in their cores. They vary in size, temperature, and brightness, and are classified into different types based on these characteristics. The closest star to Earth is the Sun, which is essential for life on our planet. In the night sky, stars appear as points of light, but they are often located light-years away from Earth. Astronomers use telescopes to study stars and their properties, including their age, composition, and distance. Some well-known stars include Sirius, the brightest star in the night sky, and Betelgeuse, a red supergiant in the constellation Orion.