Tweeting the Universe by Marcus Chown & Govert Schilling

Author:Marcus Chown & Govert Schilling [Chown, Marcus & Schilling, Govert]
Language: eng
Format: epub
ISBN: 9780571278442
Published: 2011-11-02T16:00:00+00:00

71. How do stars work?

A star is a giant ball of gas. It forms when an interstellar cloud of mostly hydrogen and helium begins shrinking under its own gravity.

The shrinkage continues until the core becomes so squeezed and hot it triggers ‘nuclear fusion’ of hydrogen into helium. Byproduct: heat.

Hot gas, pushing outward, stops gravity in its tracks. The ball shrinks no further. No longer a gas ball but a glowing gas ball: a star.

Key fact: nuclear fusion is hugely sensitive to temperature. If temperature goes up, it is boosted; if goes down, it is throttled back.

So, if heat generation drops, core shrinks/heats, boosting fusion; if heat generation climbs, core expands/cools, throttling back fusion.

Consequently, a star has a natural in-built thermostat. Keeps it perpetually balanced between shrinking and expanding.

Nothing is forever. Fusion reactions change hydrogen into helium, which, being heavier, sinks to centre, squeezing & heating up the core.

The internal structure of a star therefore changes gradually. The star evolves. And, sooner or later, its stable balance is upset.

A low-mass star like Sun evolves into a profligate red giant as it runs out of hydrogen. Then it dies a slow death as a fading white dwarf.

A high-mass star evolves ever more extreme conditions, triggering new fusion reactions, which achieve a series of new stable balances.

But gravity never goes away. Each new balance is short-lived. A star may win a few battles against gravity. But it can never win the war.

Eventually, gravity crushes the core to a black hole or to a ball of neutrons. This triggers a catastrophic explosion – a ‘supernova’.

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