What a star is

Every second the Sun turns about 4.26 million tonnes of its mass into light and heat.

A ball of gas that holds itself together

Look up on a clear night and every point of light that does not wander is a star, a sun like ours but much farther away. NASA describes stars as giant balls of hot gas, mostly hydrogen with some helium and small amounts of other elements. The Sun is the one we can study up close. Its visible surface is made of about 91% hydrogen and 9% helium by number of atoms, with oxygen, carbon, neon, nitrogen, iron and a few others in parts per million.

A star is a tug of war that never ends. Gravity pulls every bit of gas toward the centre. Pressure from the hot gas pushes back outward. When the two balance, the star keeps a steady size for millions or billions of years. The Sun is big enough that this balance needs a crushing load at the centre: NASA's model gives a central pressure of about 250 billion times the air pressure at sea level and a central temperature of about 15.7 million kelvin.

Where the energy comes from

At that temperature hydrogen nuclei, which are single protons, hit each other hard enough to merge. In a chain of steps four protons end up as one helium nucleus. The helium weighs slightly less than the four protons did, and the missing mass leaves as energy, following Einstein's E = mc². The Sun's fact sheet lists a mass conversion rate of 4,260 million kg per second, which is 4.26 million tonnes. That sounds like a lot, but the Sun holds 1.9884 × 10³⁰ kg, so it can keep going for billions of years.

The energy does not rush out. NASA says light from the core bounces around in the radiative zone, and then rising bubbles of hot plasma in the convection zone carry the energy upward. The trip from the core to the top of the convection zone takes about 170,000 years. From the surface, sunlight needs only about 8.3 minutes to cross the 149.6 million km to Earth. The total output, the Sun's luminosity, is 3.828 × 10²⁶ watts. Astronomers use this as a unit: a star with a luminosity of 10 gives off ten Suns' worth of energy.

An average star

The Sun is 1.39 million km across, 109 times Earth's width, and it would take 1.3 million Earths to fill it. Yet NASA calls it a medium-sized star. In the model, move the Zoom slider to the right: Arcturus, an orange giant, is about 25 times wider than the Sun, and Betelgeuse about 700 times. Move it left and the red dwarf Proxima Centauri, the nearest star after the Sun, is only about one seventh of the Sun's width. NASA estimates that the Milky Way alone holds more than 100 billion stars and the universe could contain up to one septillion, a one followed by 24 zeros.

The Sun is labelled G2 V. The letter G and the number 2 describe its surface temperature, 5,772 K, and the Roman numeral V says it is a dwarf, a star on the main sequence that is still fusing hydrogen in its core. NASA puts the Sun at 4.6 billion years old and a little less than halfway through its life, with about 5 billion years to go before it swells into a red giant. Later units explain both labels.