Stars, red shift and the expanding universe
Physics · GCSE · The School
A star is a balance between two forces
Gravity pulls a star inward; the pressure from fusion in the core pushes outward. While they balance, the star is stable — our Sun has been in this state for about 5 billion years. Everything that happens at the end happens because the fuel runs out and that balance fails.
Mass decides the ending
A star like the Sun swells to a red giant, sheds its outer layers as a planetary nebula, and leaves a white dwarf that cools. A star much more massive becomes a red supergiant, explodes as a supernova, and leaves a neutron star or, if massive enough, a black hole. The route is set by mass, not by age.
Red shift: light stretched by expansion
Light from distant galaxies has its wavelength stretched towards the red end, and the more distant the galaxy, the greater the shift. That means distant galaxies are moving away faster. Crucially this happens in every direction, which does not put us at the centre — space itself is expanding, so every observer anywhere would see the same thing.
The evidence, and what it does not settle
Red shift plus the cosmic microwave background radiation — a faint glow at the same temperature in every direction — is the evidence for the Big Bang. It is a well-supported model of how the universe developed. It does not describe what caused it or what preceded it, and physics is honest about that; a theory that explains the observations is not the same as a theory that explains everything.
Why the light stretches: the Doppler effect
A source moving away from you stretches out the waves it sends towards you, lowering their frequency \u2014 an ambulance siren drops in pitch as it passes, for the same reason. Light works the same way: a galaxy moving away stretches its light towards the red end of the spectrum. It is this everyday effect, not a special rule for space, that lets a wavelength shift be read as a recession speed.
An unexplained twist: the expansion is speeding up
Distant supernovae are dimmer than a steady expansion rate predicts, which means the expansion itself is accelerating rather than slowing under gravity as once expected. Physicists call whatever is causing that DARK ENERGY \u2014 a name for the effect, not yet an explanation of it. Citing it shows the model is trusted precisely because it is honest about what it cannot yet account for.