Marcus Chown explains, in tweets, one of science's great miracles
In the beginning was a cold (-260°C), dark interstellar cloud of gas and dust, an inky blot against the background stars.
The cloud may have hung there forever, doing nothing, if not for a kick, perhaps from the blast wave of an exploding star (supernova).
About 4.55 billion years ago, the cloud began shrinking under self-gravity, its gas becoming squeezed together ever more tightly.
When a gas is squeezed, it gets hot. The outward force exerted by warming gas should therefore have stopped the shrinking gas in its tracks.
But molecular hydrogen, carbon monoxide etc shed heat as light (microwaves), which escaped cloud, robbing it of ability to oppose gravity.
Initially, the cloud spun slowly (since Milky Way is spinning slowly). But, as it shrank, it spun faster, like ice skater pulling in arms.
Cloud shrank quicker between poles than around waist, where outward ‘centrifugal’ force opposed gravity. Became a flat, spinning pancake.
Tweeting the Universe by Marcus Chown is out on 3 November from Faber.