7.17P Explain how the balance between thermal expansion and gravity…

The idea

A star is held in a tug-of-war. Gravity pulls every layer inward, while the hot gas pushes outward because its thermal energy raises its pressure. While the two forces balance, the star stays stable on the main sequence and its size hardly changes. When the balance shifts, the star's life changes stage. If fusion in the core runs out and the outward push falls, gravity wins and the core contracts and heats. If the core becomes hot enough for a new fusion stage, the extra thermal expansion can push the outer layers outward, so the star swells into a giant or supergiant. The same balance explains why a more massive star, with stronger gravity, needs a hotter core and burns through its fuel faster.

What earns the marks

• States that gravity pulls the star's material inward while thermal pressure from hot gas pushes outward.
• Explains that a stable main-sequence star keeps a constant size because the inward and outward effects balance.
• Describes that when core fusion declines the outward push weakens, so gravity makes the core contract and heat up.
• Explains that renewed heating and fusion increase thermal expansion, so the outer layers swell into a giant or supergiant.
• Links the strength of gravity to mass, so a more massive star has a hotter core and a shorter life cycle.

Common mistakes and how to put them right

• Claims that gravity alone holds a star in a fixed size.

  Put it right by stating that thermal pressure balances gravity.

• Says the outward push comes from the star spinning.

  Put it right by identifying thermal pressure from hot gas as the outward effect.

• Assumes a star shrinks steadily for its whole life.

  Put it right by explaining that the balance shifts between stages.

• Treats expansion and gravity as acting at different times only.

  Put it right by showing they act together and set the star's size.

Exam tips

• Name both opposing effects, gravity inward and thermal pressure outward, before describing any change.
• Use the phrase 'balance' to explain a stable star, then say which effect wins when the balance shifts.
• Link each stage change to a cause, such as fuel running out, rather than listing stages in order.
• Check that any statement about mass explains why gravity and core temperature differ.