New stars form in large clouds of gas and dust. A giant molecular cloud where many stars form is called a stellar nursery. A star is a giant ball of gas. In a star's core, hydrogen nuclei join to make helium in a process called nuclear fusion, and hydrogen is by far the most abundant element in most stars. The Sun is the closest star to Earth, and our Sun is an average-sized star.

Stars come in different sizes, and there are bigger stars and smaller stars than the Sun. Stars also come in different colors, such as red, orange, yellow, white and blue. A star's color tells about its temperature. Blue-white stars are much hotter than the Sun, and red stars are cooler. When a star runs out of hydrogen in its core, it swells into a red giant. A star's mass decides how fast it uses its fuel: massive stars burn out in millions of years while low-mass stars shine much longer.

Astronomers call a star that is fusing hydrogen to helium in its core a main sequence star. Such stars make up around 90% of the universe's stars, and they range from a tenth to 200 times the Sun's mass. Red dwarfs make up around 75% of the Milky Way's stars, because they are born in great numbers and live so long. Red dwarfs churn their material, so they can burn through all their hydrogen over trillions of years. Some low-mass red dwarfs may live up to about 14 trillion years, longer than the current age of the universe. A red dwarf is too faint to see with the unaided eye.

A protostar is a baby star: a hot core that forms as a clump in a cloud collapses, before fusion begins. Fusion releases energy that heats a star and keeps it from collapsing under gravity. Brown dwarfs are not technically stars. They have between 13 and 80 times the mass of Jupiter, more than planets but not enough for the fusion of a main sequence star.

Words to know

• hydrogen: star fuel
• fusion: joining nuclei
• dwarfs: small stars
• mass: amount of matter