What is a star?
A star is an enormous sphere of hot plasma — mostly hydrogen and helium — held together by its own gravity and powered by nuclear fusion in its core. Our Sun converts about 600 million tonnes of hydrogen into helium every second, releasing the energy that lights the entire solar system.
How a star ignites
Stars are born inside cold clouds of gas and dust called nebulae. When a region of the cloud collapses under gravity, the core heats up; at around 10 million °C, hydrogen nuclei begin fusing into helium and the object officially becomes a star. Below roughly 8% of the Sun's mass, fusion never starts — those 'failed stars' are brown dwarfs.
Why stars don't explode (or collapse)
A star spends its life in a balance called hydrostatic equilibrium: gravity pulls everything inward while the pressure of fusion pushes outward. The Sun has held this balance for 4.6 billion years and will keep it for roughly 5 billion more.
How stars die
Mass decides everything. Sun-like stars swell into red giants and shed their outer layers, leaving a white dwarf. Stars above ~8 solar masses die as supernovae, leaving neutron stars or black holes. The heavy atoms in your body — iron, calcium, iodine — were made inside dying stars.
You can watch the nearest star live in Cosmik's 3D solar system — the Sun at the centre, rendered with real physics-inspired lighting.
Watch: Stars — Crash Course Astronomy #26
Frequently asked questions
What is a star made of?
Mostly hydrogen (~73%) and helium (~25%) in a plasma state — atoms stripped of their electrons by heat. Heavier elements make up under 2%, yet that sliver is where planets and people come from.
How long do stars live?
It depends on mass, and backwards from intuition: the biggest stars burn out in a few million years, while red dwarfs — the smallest — will smolder for trillions. Our mid-sized Sun gets about 10 billion years, and it's halfway through.
What's the closest star to Earth?
The Sun, 150 million km away. The next closest is Proxima Centauri at 4.24 light-years — over 268,000 times farther. Every other star you see at night is farther still.
Keep exploring
- Explore the Sun live in 3D
- Total solar eclipse — August 12, 2026
- What is an orbit?
- Brightest objects in tonight's sky
- All space explainers
See it yourself tonight
Theory is better with a sky. Get the next ISS pass times over New York, London, Paris, Tokyo, Berlin or Sydney — or any of 550+ cities. Aurora hunter? Check tonight's forecast for your city.
