The sun: Facts about the bright star at the center of the solar system

The sun is a ball of gas and plasma made mostly of hydrogen. The sun uses these vast stores of hydrogen to generate the heat and light that sustain our planet. It does this through a process called nuclear fusion, in which two hydrogen atoms combine to create a different element, helium. The sun is about three-quarters hydrogen and one-quarter helium, with tiny amounts of metals.

The larger a star is, the more rapidly it burns through its hydrogen. Some of the largest known stars  —  such as those with masses 40 times that of the sun  —  will live just 1 million years. By contrast, the sun will have a lifetime of around 10 billion years.

Different parts of the sun reach different temperatures. The sun's core gets as hot as 27 million F (15 million C). The part of the sun we can see from Earth is called the photosphere, which is the "surface" of the huge ball of plasma. The temperature of the photosphere is about 9,900 F (5,500 C).

Above the photosphere is the loose outer atmosphere of the sun, known as the corona. We can't see the corona from Earth under ordinary conditions, though it can be photographed during a total solar eclipse.

The sun formed around 4.6 billion years ago. At that time, the area of the Milky Way galaxy that would become the solar system was a dense cloud of gas  —  the leftovers of an earlier generation of stars. The densest region of this cloud collapsed and created a seed, called a protostar, that would become the sun. As this young protostar grew, planets, moons and asteroids formed from the remaining raw material, and then began circling around the growing sun as they were sucked into orbit by the star's powerful gravity.

At the heart of the sun, this same force sparked nuclear fusion. The heat and light from this nuclear reaction allowed life on Earth to evolve and prosper. However, this reaction will eventually lead to the sun's death when it runs out of nuclear fuel.

The sun is around halfway through its lifetime. Our star is locked in a constant battle as outward pressure from nuclear fusion fights the inward pull of gravity. When the sun runs out of hydrogen in about 5 billion years, the inward force of gravity will win.

The center of the sun will collapse, compressing into a dense core. Helium will start fusing into even denser elements, like carbon, nitrogen and oxygen. While this happens, the heat generated by the fusing of these elements will push the sun's outer shell to swell. This will be bad news for the inner planets of the solar system — including Earth.

As the sun becomes a type of star called a red giant, its outer shell will expand to the orbit of Mars, gobbling up Mercury, Venus, Earth and Mars. But the red-giant phase is not when the sun will die.

The outer layers that swell during the red-giant phase will become a shell of gas called a surrounding planetary nebula. This shell will be shed after approximately 1 billion years. This will expose the star's smoldering core, which, by this point, will be a dense ball called a white dwarf.

As a white dwarf, the sun will dim. The material from the planetary nebula will spread out into the galaxy and form the building blocks of the next generation of stars and planets.