Stars and the Universe
The Sun as a star, nuclear fusion, the life cycle of stars, galaxies, redshift and the Big Bang.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 6.2.1Describe the Sun as a star that releases energy by nuclear fusion of hydrogen into helium.
- 6.2.2Describe the life cycle of a star, including the difference between low-mass and high-mass stars.Supplement
- 6.2.3Describe redshift as evidence that the Universe is expanding.Supplement
8 minute read
Stars and the Universe
The Sun as a star
The Sun releases energy by nuclear fusion, in which hydrogen nuclei join together (fuse) to form helium nuclei, releasing enormous amounts of energy. This is the same basic process that powers every star.
The life cycle of a star
Stars form from clouds of dust and gas (a nebula) pulled together by gravity, and their eventual fate depends on their mass:
Low-mass stars (like the Sun): main sequence star → red giant → the outer layers drift away, leaving a white dwarf, which slowly cools.
High-mass stars: main sequence star → red supergiant → supernova (a huge explosion) → the remnant becomes either a neutron star or, for the most massive stars, a black hole.
Galaxies and redshift
A galaxy is an enormous collection of billions of stars, held together by gravity. Our Sun is one star within the Milky Way galaxy.
Light from distant galaxies is shifted towards the red (longer-wavelength) end of the spectrum — this is redshift, and it shows that those galaxies are moving away from us. Crucially, the further away a galaxy is, the greater its redshift and the faster it is moving away. This distance-dependent pattern is the key evidence that the Universe is expanding in all directions, and it supports the Big Bang theory — that the Universe began from an extremely hot, dense point and has been expanding and cooling ever since.
Think of it like this
The expanding Universe is often pictured as spots painted on the surface of a balloon: as the balloon is inflated, every spot moves away from every other spot, and spots that started further apart move apart faster — exactly the pattern astronomers see in galaxy redshifts, with no special "centre" to the expansion.
Common misconceptions
- Thinking a star "burns" like a fire, in a chemical reaction. Stars release energy through nuclear fusion, an entirely different (and vastly more powerful) process happening in their extremely hot, dense cores.
- Believing every star ends its life the same way. The outcome — white dwarf versus supernova and neutron star/black hole — depends critically on the star's mass.
- Assuming redshift just shows that some galaxies happen to be moving away. The specific pattern — that recession speed increases with distance — is what points to universal expansion rather than random galaxy motion.
In the exam
- Life-cycle questions often ask for the correct order of stages — learn the two sequences (low-mass and high-mass) as complete chains, since a step out of order loses the mark even if the words used are right.
- For redshift, the key phrase examiners want is that more distant galaxies show greater redshift — stating only "galaxies are redshifted" without the distance relationship is an incomplete answer.