Stars are giant balls of hot gas, mostly hydrogen and helium, held together by their own gravity. Deep in their cores, atoms smash together in a process called nuclear fusion and release the heat and light that make a star shine. Our Sun is a star, and on a dark clear night you can see thousands more glittering above you. The Milky Way alone holds somewhere between 100 and 400 billion stars.
- Stars in the Milky Wayapprox. 100 to 400 billionMost are smaller and cooler than the Sun
- Nearest star to EarthProxima Centauri4.24 light years away
- Brightest in our night skySirius8.6 light years away
- Largest known starStephenson 2-18About 2,150 times the radius of the Sun
- Sun's surface temperatureapprox. 5,500 °CThe hottest stars top 40,000 °C
- Lifespan of an average starapprox. 10 billion yearsTiny red dwarfs may last a trillion years
What are stars made of?
Almost every star is mostly hydrogen (around 70%) and helium (about 28%), with a small amount of heavier elements mixed in. The star's own gravity squeezes its core so tightly that hydrogen atoms fuse together to make helium, and that fusion releases enormous amounts of energy as heat and light. Every breath you take, every plant on Earth, every drop of food you eat: it all began as energy made in the heart of a star.
Why do stars have different colours?
A star's colour tells you how hot its surface is. Cool stars glow red or orange (think of the embers of a fire). Stars like our Sun are yellow-white at around 5,500 degrees C. The hottest stars are blue-white and can be over 40,000 degrees C. Astronomers use these colours to sort stars into types, from cool red dwarfs to enormous blue supergiants.
How do stars form?
Stars are born inside huge clouds of gas and dust called nebulae. Gravity slowly pulls the densest patches of the cloud together. As the gas falls inwards, it heats up. After tens or hundreds of thousands of years, the centre of the clump gets so hot and dense that hydrogen atoms start to fuse together into helium. That moment is the birth of a new star. The leftover gas and dust around it sometimes flattens into a spinning disc, and over time that disc can become a family of planets.
The Orion Nebula, just below Orion's Belt, is one of the closest stellar nurseries to Earth. Even with a small telescope you can see new stars being born in it right now.
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