How big is the Sun?

The Sun is a star, the only one in our solar system. It is a huge ball of hot gas, mostly hydrogen and helium, held together by its own gravity. In its core, hydrogen fuses into helium, and the energy of that fusion makes it shine.

It has no solid surface. What we see as its edge is the photosphere, the layer where its light finally escapes into space. Measured across that glowing ball, the Sun is 1,391,400 kilometres wide. A trip around its equator would be 4,371,000 kilometres. Even the International Space Station, at its orbital speed, would need 6.6 days to go round it once.

Beside the planets it is a different kind of thing. Earth is a small ball of rock. Jupiter, the largest planet, is a giant of gas. The Sun is about 9.7 times as wide as Jupiter, and about 109 times as wide as Earth.

Width grows into volume fast. Make a ball twice as wide and it holds eight times as much. So a star about 109 times as wide as Earth has room for about 1.3 million Earths.

Even Jupiter is small beside it. About 921 Jupiters would fit inside the Sun.

Mass tells a different story from volume. The Sun is made mostly of hydrogen and helium, the lightest elements, so on average it is far less dense than rocky Earth. That is why it holds about 330,000 times Earth's mass, not 1.3 million times.

Real planets would not pack together neatly, of course. Round things leave gaps between them. The number is a measure of space, not a recipe. It is still the clearest way we know to feel how much of the Sun there is.

Nobody can put a star on a scale. Astronomers weigh the Sun by watching what circles it. How fast Earth goes round, and how far away it is, tell us how strongly the Sun pulls, and so how much it holds.

The answer is about 330,000 times the mass of Earth, and about 1,000 times the mass of Jupiter. All eight planets, the moons, the asteroids and the comets together are only a thin remainder. The Sun holds 99.87 per cent of the solar system's mass. That is why everything else goes round it.

The Sun is also getting lighter. Every second it loses about 5.5 billion kilograms: most of it turned into light, the rest blown away in its wind. Since you opened this page, that adds up to 0 kilograms. It sounds enormous, but over its whole life so far the Sun has lost only a very small part of itself.

In our sky the Sun looks no bigger than the full Moon. (Never look straight at it.) That is a coincidence of size and distance. The Sun is about 400 times as wide as the Moon. Right now it is also about 386 times as far away as the Moon is on average. The two almost cancel, so the two discs look nearly the same size.

That is why total eclipses of the Sun can happen. When the Moon passes exactly in front of the Sun, it only just covers the bright disc. For a few minutes, the Sun's faint outer atmosphere, the corona, shines around the black Moon.

The fit is not perfect. The Moon's distance changes through its orbit. When it is farther away than usual, it looks a little too small, and a ring of sunlight stays around it: an annular eclipse. The Moon is also drifting slowly away from us, so total eclipses will end one day, very far in the future.

For all its size, the Sun is not unusual among stars. Astronomers class it as a yellow dwarf, a middle-sized star in the long, steady middle of its life. NASA calls it an average star by size. Many stars are much larger.

Its nearest neighbour is a different kind. Proxima Centauri, about 4.2 light-years away, is a small, cool red dwarf. And the Sun is only one of between 100 billion and 400 billion stars in our galaxy, the Milky Way. Nobody has counted them, so the number is a wide estimate.

Ordinary is a gift here. Because the Sun is a typical star, studying it close up teaches us how stars everywhere work. Because it is steady, Earth has had a very long time of stable light. The Sun has shone for about 4.6 billion years, and has enough hydrogen in its core for about 5 billion years more.