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Why and How Do Stars Shine?

Twinkle, twinkle, little star…

I’ve got a lot of questions for you!

Like, what are you, anyway?

How do stars shine?

Why do you “twinkle”?

Hmm, I guess we should start with…

What is a star?

A star is a bright ball of super hot gases, or plasma, held together by gravity.

That’s right, the same force that keeps you on Earth also holds stars together!

Our sun is an example of a yellow dwarf star. Actually, every star you see in the sky is a sun. Just very, very far away!

sun is a star

So how do stars form?

Stars form from cold clouds of dust and gas in space called nebulas.

Because these clouds are very cold (much colder than anywhere on Earth), the atoms bind together and the gases form tight clumps called protostars.

As gravity pulls more gases closer together, the protostars begin to spin.

Just like your feet get hot when you rub them on the carpet, spinning the protostar causes the gas atoms to rub against each other. This creates friction or heat energy.


The spinning also causes heavier elements to move to the center of the protostar, making it more dense.

Eventually, the protostars get so hot and so dense that nuclear reactions can happen.

Thus, a star is born.

But how and why do stars shine? 

Stars have intense gravity constantly pushing inwards.

This gravity causes the dense core of the star to heat up and sets the stage for nuclear fusion to take place.

Nuclear fusion is when two or more smaller atoms come together to make a larger one. In stars, nuclear fusion usually turns hydrogen atoms into helium ones.

This fusion process creates huge amounts of radiation, light, and energy in the core of the star.

lightning bolt

Over time, electromagnetic energy particles called photons make their way to the surface and launch into space as solar wind.

Eventually, that light reaches your eyes in the form of sunlight or shining stars.

How long it takes depends on distance.

While it only takes sunlight about 8 minutes and 20 seconds to get here, light from stars in the far off Eagle Nebula can take 7,000 years for us to see with the Hubble Telescope!

Experiment: why do stars twinkle?

So we know what stars are, how they form, and how they shine.

But what’s with that twinkling? Let’s find out!



For this experiment, you’ll need:

  • A glass bowl
  • Water
  • A flashlight
  • Aluminum foil
  • A black piece of paper
  • Scissors


  1. Fill the glass bowl with water (about 2/3 full is fine) and set it aside.
  2. Cut small pieces of aluminum foil and shape them into stars. (It’s OK if they’re clumpy-looking, most stars are!)
  3. Place your black piece of paper in front of you. Place your stars on the paper, then place the bowl of water on top of your stars.
  4. Turn off the lights and shine the flashlight into the bowl to illuminate your stars.
  5. Tap the bowl and observe what happens to the light as the water moves.


Your flashlight in the still water was bright and constant, but as the water moved, your stars appeared to twinkle.

So what’s going on here?

As star light reaches Earth, each light beam is refracted–or slightly bent–by our planet’s atmosphere. Just like the moving water bent the flashlight beams in your mini night sky.

earth light refraction

Stars appear to twinkle because the various temperatures and densities of our atmosphere make the light zigzag before it can reach us on the planet’s surface.

Basically, it’s an illusion!

Now that you’re such an expert on the big questions, like, “How do stars shine?”, check out these other cool facts about planets and test your knowledge about the moon!