From a cold cloud
How Does a Star Get Started?
A star begins as a cold cloud of gas that falls in on itself until the middle gets hot enough to switch on.
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Step 1 of 5
It starts colder than anything on Earth.
The gas between the stars is thin and desperately cold — colder than any freezer, colder than the Antarctic. Nothing about it looks like the beginning of a star.
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Step 2 of 5
Gravity gives it nowhere to go but inward.
Every part of the cloud pulls on every other part. With nothing to hold it apart, the cloud slowly falls towards its own middle, and the middle gets crowded.
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Step 3 of 5
Squeezing gas makes it hot.
This is the same effect as a bicycle pump warming under your hand. Crushing gas into a smaller space raises its temperature, and the cloud's own weight is doing the crushing.
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Step 4 of 5
At about ten million degrees, it switches on.
That is the temperature where hydrogen atoms start fusing together and releasing energy. The outward push of that energy finally balances the inward pull, and the collapse stops.
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Step 5 of 5
Stars are almost never born alone.
One cloud is enough material for hundreds or thousands of stars, so they light up in batches. The clusters you can see in the sky are those batches, still drifting apart.
The Sun was born in such a group. Its siblings scattered long ago and we do not know which stars they are.
The short version
A star forms when a cold gas cloud falls into itself until the squeezed centre reaches ten million degrees and fusion begins.
Try it yourself
Point binoculars at the middle of Orion's sword. The fuzzy patch is a nursery with new stars lighting up inside it.