Stars, Galaxies and the Big Bang
Our Solar System and Beyond
The solar system consists of the Sun and all the objects gravitationally bound to it — the eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune), their moons, and countless smaller bodies such as asteroids and comets. The planets orbit the Sun in nearly circular paths, all in the same direction. Gravity provides the centripetal force that keeps them in orbit — the gravitational pull continuously changes the direction of each planet's velocity, keeping it in a circular path without changing its speed.
Our solar system sits within the galaxy called the Milky Way. The universe contains billions of galaxies, each containing hundreds of millions to hundreds of billions of stars. On this scale, distances are so vast that light — travelling at roughly 300 000 km per second — takes years, millennia, or even billions of years to cross them.
Planets orbit the Sun in (almost) circular paths because gravity provides a centripetal force. An object in a circular orbit moves at constant speed but changing velocity — its direction changes continuously, so velocity changes even though speed does not. For a stable orbit, if the orbital speed changes, the radius of the orbit must also change. The further a planet is from the Sun, the slower its orbital speed and the longer its year.
The Life Cycle of a Star
Stars form from vast clouds of gas (mainly hydrogen) and dust. Gravity pulls the cloud inward: as the gas is compressed, work is done on it, and the particle model predicts the temperature rises (compressed gas gets hotter because the particles collide more forcefully as they are pushed closer together). When the core temperature rises sufficiently, hydrogen nuclei gain enough kinetic energy to overcome their electrostatic repulsion and undergo nuclear fusion — fusing into helium nuclei and releasing enormous energy.
This energy release creates an outward radiation pressure that balances the inward gravitational collapse. A star in this stable phase — like our Sun — is called a main sequence star. Our Sun has been on the main sequence for roughly 4.6 billion years and is expected to remain there for a similar period.
Scope note: red giants, red supergiants, white dwarfs, supernovae, neutron stars and black holes are not on this specification. They are here as background. What is assessed is how the Sun formed and how the energy from fusion balances gravitational collapse.
When a star’s hydrogen fuel runs low, fusion can no longer balance gravity. The core contracts and heats further, causing the outer layers to expand dramatically. The star becomes a red giant (for a star like the Sun) or a red supergiant (for a much more massive star).
A star like the Sun eventually sheds its outer layers as a planetary nebula, leaving behind a dense, hot remnant called a white dwarf, which slowly cools over billions of years.
A much more massive star ends its life more violently. Fusion in a massive star’s core builds up progressively heavier elements until iron is reached — iron cannot release energy by fusion. The core then collapses catastrophically and the outer layers are expelled in a supernova explosion. The remnant may become a neutron star or a black hole.
Red-Shift and the Expanding Universe
When astronomers examine light from distant galaxies, they find that the dark absorption lines in the spectrum are shifted towards longer, redder wavelengths compared to the same lines measured in a laboratory. This phenomenon is called red-shift. It arises because the galaxy is moving away from us: as light travels through expanding space, its wavelength is stretched.
A key observation is that more distant galaxies show a greater red-shift — they are moving away from us faster. This pattern holds in every direction, which means the universe as a whole is expanding: space itself is growing, carrying galaxies apart with it. Scientists use this relationship between distance and recession speed as evidence for the expanding universe model.
Red-shift: light from receding galaxies is stretched to longer (redder) wavelengths. More distant galaxies have greater red-shift, meaning they are moving away faster. This is evidence that the universe is expanding.
The Big Bang Model (IaS3 — Scientific Models)
If the universe is currently expanding, then tracing back in time suggests it was once much smaller, hotter and denser. The scientific explanation for the origin of the universe is the Big Bang model. According to this model, the universe began roughly 14 billion years ago in a state of extreme temperature and density, and has been expanding and cooling ever since.
The acceptance of the Big Bang model is a good example of how scientific models develop as evidence accumulates (IaS3). When Edwin Hubble published his observations of galactic red-shifts in 1929, showing that galaxies were receding at speeds proportional to their distance, it provided the first strong observational support for an expanding universe. The model became far more secure in 1965 when Arno Penzias and Robert Wilson detected a faint microwave glow coming uniformly from all directions in the sky — the cosmic microwave background (CMB). This radiation is interpreted as the cooled remnant of the intense heat of the early universe.
Over the last century, telescope technology has improved dramatically. Ground-based observatories can detect visible light from billions of light-years away. Space-based telescopes can observe in wavelength ranges — infrared, ultraviolet, X-ray, microwave — blocked by Earth’s atmosphere. Removing the atmosphere also eliminates atmospheric blurring, allowing much finer detail to be resolved.
Each new generation of observational data has refined our picture of the early universe, the large-scale structure of galaxies, and the expansion rate. These improved data have consistently supported and strengthened the Big Bang model, rather than overturning it — illustrating how accumulating evidence increases scientific confidence in a model (IaS3).
Two key pieces of evidence for the Big Bang: (1) Hubble’s 1929 observations showing that galactic recession speed increases with distance — red-shift evidence for an expanding universe. (2) Penzias and Wilson’s 1965 detection of the cosmic microwave background — the cooled remnant of radiation from the early universe.