The numbers
- What it is
- A ball of plasma that shines by itself
- How it makes light
- With nuclear fusionIn its own core
- Where the Sun fits
- Among the most common starsOn the main sequence
How it happens, step by step
A ball of gas
It all starts with a ball of gas, made mostly of hydrogen, held together by its own gravity.
The center is squeezed
The weight of the outer layers squeezes the heart of the star, which gets denser and hotter all the time.
The nuclei join
At those temperatures the lightest nuclei crash into each other and fuse, releasing energy every time.
The light gets out
The energy climbs slowly up to the surface and from there sets off into space as light and as heat.
The balance holds
The push outward balances the gravity that squeezes. As long as the fuel lasts, the star stays steady.
To get an idea
A star is a tug of war between gravity, which squeezes, and energy, which puffs it up.
The color of a star tells its temperature: the blue ones are the hottest, the red ones the coolest.
Fun facts
These are not in the book: there was no room on paper.
Stars are also sorted by brightness, from hypergiants down to white dwarfs.
Besides light and heat, a star gives off a flow of particles called the stellar wind.
Most of the chemical elements that exist today were made inside the stars.
The bigger and brighter a star is, the shorter it lives: it uses up its fuel much faster.
The energy made in the heart takes a very long time to reach the surface.
The stars we can see with the naked eye are only a few thousand, all very close to us.
Test yourself
The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.
Question 1 of 5★ is the one from your book
What is the difference between a star and a planet?
Question 2 of 5
How does a star make energy?
Question 3 of 5
What keeps a star steady?
Question 4 of 5
What does the color of a star tell you?
Question 5 of 5
What kind of star is the Sun?
Riddles
Inside me two forces have been battling for billions of years: one pushes everything toward the center, the other outward. As long as neither one wins, I stay as I am. What am I?
My color is not a decoration but a thermometer: if you see me blue I am scorching, if you see me red I am the coolest of the bunch. What am I?
Try it yourself
Compare a star with a visible planet: the star twinkles, the planet does not. The air makes the light of points shake, not of small disks.
Turn on a gas stove and look at the colors of the flame: the blue part is the hottest. Stars work the same way.
The words
- plasma
- A gas so hot that its atoms have broken into charged pieces.
Sources: Wikidata (Q523), Wikipedia (Stella).