What is a star, exactly?

What is a star?

Photo of the Sun.   EIT - SOHO Consortium, ESA, NASA

Photo of the Sun.
EIT - SOHO Consortium, ESA, NASA

We're all pretty familiar with the stars. We see them on most clear nights as tiny, twinkling pinpricks of light in the sky. Stars are the topic of countless poems, stories, and nursery rhymes alike. But just what is a star, exactly?

A star is a luminous ball of gas, mostly hydrogen and helium, held together by its own gravity. Nuclear fusion reactions in its core support the star against gravity and produce photons and heat, as well as small amounts of heavier elements. The Sun is the closest star to Earth.

Where do stars come from?

According to current star formation theory, stars are born as clumps within gigantic gas clouds that collapse in on themselves. The cloud’s material heats up as it falls inward under the force of its own gravity.

When the gas reaches about 10 million K (18 million degrees F), hydrogen nuclei begin to fuse into helium nuclei, and the star is born. Energy from nuclear fusion radiates outward from the center of the burgeoning star, and gradually halts the gas cloud’s collapse.

Types of Stars

Image credit: NASA

Image credit: NASA

A star’s color relies on its temperature: hotter stars emit bluer light and cooler stars emit redder light.  Temperature is also correlated to mass. Red dwarf stars have as little as 0.075 solar masses and a visible surface temperature less than 4,000 K. The most massive star known is R136a1, a Wolf-Rayet star 265 times the Sun’s mass — its visible surface temperature hovers at a searing 50,000 K.

The most massive (and hottest) stars exhaust their energy supply within a few million years, while tiny and cool red dwarf stars can keep on burning for many billions of years.


From the coolest dwarf to the hottest, most massive giant, find any star magnitude 8.5 and brighter with the classic Sky Atlas 2000.0 (Deluxe Laminated).