First look
The idea, in plain words
The Sun is a star. It looks different from the others only because it is so close: the next nearest is about 270,000 times further away.
It is a ball of glowing gas with no solid ground anywhere. What looks like a sharp edge is simply the depth at which the gas becomes too thick to see through.
It is enormous. Its width is 109 Earths side by side, and 1.3 million Earths would fit inside. It holds 99.86% of all the mass in the Solar System. The planets, moons, asteroids and comets together are the other 0.14%.
It shines because of what goes on in the middle. There the gas is so hot and so tightly squeezed that hydrogen is forced together to make helium. Each time, a little mass disappears and comes out as energy. That energy, after a very long journey to the surface, is sunlight.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Star
- A ball of hot gas held together by its own gravity and shining by nuclear fusion in its core.
- Plasma
- Gas so hot that its atoms have lost electrons, so it carries electric current and is steered by magnetic fields.
- Nuclear fusion
- The joining of light atomic nuclei into heavier ones, which releases energy.
- Core (of the Sun)
- The inner quarter of the Sun's radius, the only part hot and dense enough for fusion.
- Radiative zone
- The thick layer round the Sun's core where energy works its way outward as light, absorbed and given out again countless times.
- Convective zone
- The outer third of the Sun, where heat is carried up by rising and sinking gas, like water at a rolling boil.
- Photosphere
- The layer of the Sun that we see, where the gas turns clear enough for light to get out.
Worth knowing
The Sun turns about 4 million tonnes of its mass into light every second, and has done for 4.6 billion years. It has barely made a dent.
Worked out from measurements. NASA NSSDC Sun fact sheet
The centre of the Sun is at about 15.7 million K. Nobody has measured it directly: it comes from models that are checked against the Sun's vibrations and the neutrinos it gives off.
From a model, not a direct measurement. NASA NSSDC Sun fact sheet
Where this comes from
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.