Both when a star first forms and when it becomes a red giant, gravity and gas pressure are in a kind of tug‑of‑war that shapes the star.

Quick Scoop

One similarity

  • In both the star’s birth and its red giant phase, gravity pulls the gas inward while pressure from very hot gas and nuclear fusion pushes outward, creating a new stable structure.
  • At formation, this balance forms a main‑sequence star; when it becomes a red giant, a new balance is set up with fusion in a shell around the core and expanded outer layers.

One difference

  • When the star first forms, nuclear fusion starts by burning hydrogen in the core, and the star is relatively small, hot, and compact on the main sequence.
  • When it turns into a red giant, the core has run out of hydrogen, the inner core contracts while fusion continues in a surrounding shell, and the star’s outer layers swell to many times their original size and cool, turning red.

TL;DR: Similarity – gravity versus pressure sets up a new stable star in both cases. Difference – first time it’s a small, hot, hydrogen‑burning core star; later it’s a huge, cool red giant with a shrunken core and fusion in a shell.

Information gathered from public forums or data available on the internet and portrayed here.