First look
The idea, in plain words
Every star in the night sky is a sun. Ours looks different only because it is close.
A star is a huge ball of gas. It is so heavy that its own weight squeezes the middle until it is millions of degrees hot. At that heat the gas starts to burn in a special way, called fusion, and the star shines.
That is the whole difference between a star and a planet. A planet shines by borrowed light. A star makes its own.
Stars come in many sizes. The smallest are dim, red and not much wider than Jupiter. The biggest give out more light than a million Suns.
The Sun is somewhere in the middle, and brighter than most of its neighbours.
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.
- Nuclear fusion
- The joining of light atomic nuclei into heavier ones, which releases energy.
- Plasma
- Gas so hot that its atoms have lost electrons, so it carries electric current and is steered by magnetic fields.
- Luminosity
- The total power a star gives out, in every direction and at every wavelength.
- Brown dwarf
- A ball of gas too heavy to be a planet but too light to keep hydrogen fusion going, so it never becomes a true star.
- Red dwarf
- A small, cool, faint star. Red dwarfs are by far the commonest kind of star.
- Main sequence
- The long, steady part of a star's life while it fuses hydrogen in its core.
Worth knowing
Of the 108 stars this site's catalogue holds within 20 light years, 85 give out less than a thirtieth of the Sun's visible light. Only six outshine the Sun. The typical star is small, red and far too faint to see by eye.
Worked out from measurements. HYG star database v4.4, David Nash
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.