How big is Sun? | Scale of the Universe

The Sun is the star at the center of the Solar System. It is about 1.39 million kilometers (865,000 miles) across, 109 times the width of Earth. That figure is the width of the photosphere, the glowing layer that looks like the Sun's surface.

Size

The Sun is a huge ball of hot gas, mostly hydrogen and helium, so it has no solid surface. Its edge is taken to be the photosphere, the layer that gives off most of the Sun's visible light. NASA gives the Sun's mean radius, the distance from its center to the photosphere, as 695,700 kilometers. Twice that is its width: 1,391,400 kilometers.

A 2008 study led by Margit Haberreiter put the radius at 695,658 kilometers, give or take 140 kilometers. In 2015 the International Astronomical Union (IAU) rounded that figure to 695,700 kilometers and adopted it as the nominal solar radius. It is now a standard unit that astronomers use to give the sizes of other stars.

The Sun is also very nearly a perfect sphere. NASA gives its flattening as 0.00005, which means its width through the poles is within about 70 kilometers of its width at the equator.

Size comparisons

The Sun is about 109 times as wide as Earth and about 10 times as wide as Jupiter, the largest planet.
About 1.3 million Earths would fit inside the Sun, by volume.
The Sun is about 400 times as wide as the Moon and about 390 times as far from Earth as the Moon is. So the two look almost the same size in the sky, a little over half a degree across, and the Moon can cover the Sun's bright disk in a total solar eclipse.
Alpha Centauri A, the next larger star in the explorer, is about 1.2 times as wide as the Sun. Its partner Alpha Centauri B, the next smaller, is about 86% of the Sun's width.
If the Sun were shrunk to the size of a basketball, Earth would be a 2-millimeter bead 26 meters away. The nearest other star, Proxima Centauri, would be about 7,000 kilometers away, more than half the width of the real Earth.

How the Sun shines

The Sun's core reaches about 15 million degrees Celsius (27 million degrees Fahrenheit). That is hot enough for nuclear fusion, which joins hydrogen into helium and releases energy. This energy powers the Sun's heat and light. It takes about 170,000 years to travel from the core to the top of the convection zone, the layer directly beneath the photosphere.

The same energy explains the Sun's size. Gravity pulls the Sun's gas inward, while the heat from fusion creates pressure that pushes outward and keeps the star from collapsing. The photosphere, where the light escapes into space, is far cooler than the core, at about 5,500 degrees Celsius (10,000 degrees Fahrenheit).

The Sun's past and future

The Sun formed about 4.6 billion years ago from a spinning cloud of gas and dust called the solar nebula. Most of the cloud's material fell to the center to make the Sun, which holds 99.8% of the Solar System's mass. Much of the rest formed the planets.

Scientists predict that the Sun is a little less than halfway through its life. As it starts to run out of energy, it will swell into a red giant, becoming so large that it will swallow Mercury and Venus, and possibly Earth. About 5 billion years from now it will become a white dwarf, a dim, dense star like Sirius B, which packs about the Sun's mass into a ball a little smaller than Earth.