What is a star?
A star is a sphere of plasma — ionised gas — large enough that gravity compresses its core to temperatures and pressures where nuclear fusion ignites. The fusion reactions release energy that creates outward pressure, balancing the inward pull of gravity. This equilibrium, called hydrostatic balance, can last for billions of years until the fuel runs out.
The Sun is our closest star: 1.4 million km in diameter, 333,000 times Earth's mass, about 4.6 billion years old with roughly 5 billion years of hydrogen fuel remaining. It is entirely typical among stars — a G-type main-sequence dwarf, unremarkable except for being our parent star. Most stars in the Milky Way are smaller, cooler, and far longer-lived red dwarfs.
How many stars are there?
The Milky Way alone contains an estimated 100–400 billion stars. The observable universe contains roughly 2 trillion galaxies, most with billions to trillions of stars each — giving a total of around 10²⁴ stars. That is more than the number of grains of sand on all Earth's beaches, by a large margin.
Stars are born in clouds of gas and dust called nebulae, live on the main sequence fusing hydrogen, and die in ways determined by their mass — from quiet puffs (white dwarfs) to spectacular explosions (supernovae). The full sequence is covered in our stellar life cycle guide.