Space Physics and the Universe: GCSE Physics Guide

Predict first: if every galaxy is moving away from us — and the further away, the faster it recedes — what does that imply about the universe's past? Red-shifted light points to a single beginning of enormous density and temperature. That is the Big Bang, and GCSE space physics builds the observational case for it.

What is the scale of the universe?

The universe is almost incomprehensibly large. Comparing objects to familiar scales helps make sense of the distances involved:

Object | Approximate size / distance | Scale comparison
Earth | Diameter ~12,700 km | A marble
Moon | ~384,000 km from Earth | A tennis ball 7 m away
Sun | Diameter ~1.4 million km | A basketball 300 m from the marble
Solar System (to Neptune) | ~4.5 billion km radius | The basketball is 30 km away
Nearest star (Proxima Centauri) | ~4.24 light-years | An estimated 80,000 km from the basketball
Milky Way galaxy | ~100,000 light-years across | Contains ~200–400 billion stars
Observable universe | ~93 billion light-years in diameter | Contains an estimated 2 trillion galaxies

One light-year is the distance light travels in one year — approximately 9.46 × 10¹⁵ metres (about 9.5 trillion km). Light from the Sun takes ~8 minutes to reach Earth; from Proxima Centauri, ~4.24 years.

How do stars form and what determines their fate?

Stars form from nebulae — vast clouds of hydrogen gas and dust. Gravity pulls the gas inwards; gravitational potential energy converts to kinetic (thermal) energy, heating the core. When the temperature reaches approximately 10 million °C, nuclear fusion of hydrogen to helium begins, releasing enormous amounts of energy.

Once hydrogen is exhausted in the core, the star's fate depends almost entirely on its initial mass:

Low to medium mass stars (like our Sun): expand into a red giant, then shed their outer layers, leaving a white dwarf

High mass stars (much heavier than the Sun): expand into a red supergiant, explode in a supernova, and leave either a neutron star or a black hole

Mass is the single most important variable in stellar evolution.

What is the life cycle of a star like our Sun?

Nebula — a cloud of hydrogen gas and dust begins to contract under gravity

Protostar — the contracting gas heats up as gravitational potential energy converts to thermal energy; fusion has not yet begun

Main sequence star — nuclear fusion of hydrogen to helium begins; outward radiation pressure balances inward gravity; this stable phase lasts billions of years (our Sun has been here ~4.6 billion years and has ~5 billion years remaining)

Red giant — as hydrogen runs low in the core, the outer layers expand and cool, turning red; helium fusion begins in the core

Planetary nebula — the outer layers are expelled as a glowing shell of gas

White dwarf — the remaining dense core (mainly carbon and oxygen) cools slowly; no fusion occurs; supported by electron degeneracy pressure

Black dwarf (eventual fate) — a cold, dark remnant; the universe is not yet old enough for any white dwarf to have reached this stage

What is the Big Bang theory?

The Big Bang theory is the current scientific model for the origin and evolution of the universe. It proposes that:

Approximately 13.8 billion years ago, all matter, energy, space, and time originated from a single point of nearly infinite density and temperature

The universe has been expanding ever since — not matter exploding into existing space, but space itself expanding

As the universe cooled, subatomic particles formed, then hydrogen and helium atoms (within the first few hundred thousand years)

Gravity eventually pulled hydrogen and helium into the first stars and galaxies

The Big Bang theory is supported by three major lines of evidence:

Red-shift — all distant galaxies are moving away from us; the further they are, the faster they recede

Cosmic Microwave Background (CMB) radiation — the faint thermal afterglow of the hot early universe, detected uniformly in all directions

Abundance of hydrogen and helium — the observed ratio (~75% H : ~25% He) matches predictions from Big Bang nucleosynthesis models

What is red-shift and how does it support the Big Bang?

Red-shift is the increase in wavelength (shift towards the red end of the spectrum) of light from a source that is moving away from the observer. It is the electromagnetic equivalent of the Doppler effect for sound.

When astronomers observe distant galaxies:

Their absorption spectra are shifted to longer (redder) wavelengths

The further away the galaxy, the greater the red-shift

This means all distant galaxies are moving away from us, and more distant ones recede faster

This is exactly what you would expect if the universe began as a single point and has been expanding ever since — every point in space is moving away from every other point, like dots on an inflating balloon.

Hubble's Law quantifies this: recession speed ∝ distance (v = H₀d, where H₀ is the Hubble constant). Measuring red-shift lets astronomers calculate how fast a galaxy is receding and therefore how far away it is.

What is dark matter and dark energy?

Despite the success of the Big Bang theory, two deep mysteries remain:

Dark matter (~27% of the universe): Galaxies rotate faster at their edges than the visible mass within them can explain — the gravitational pull of visible stars and gas is insufficient to hold them together at observed speeds. Scientists infer the presence of dark matter — matter that does not emit, absorb, or reflect light but has gravitational effects. Its nature is unknown; proposed candidates include WIMPs (Weakly Interacting Massive Particles) and axions.

Dark energy (~68% of the universe): Since the late 1990s, observations of distant supernovae have shown that the expansion of the universe is accelerating — distant supernovae are dimmer (further away) than expected if gravity were simply slowing the expansion. This accelerating expansion is attributed to dark energy — a form of energy permeating all of space that acts against gravity.

In total, ordinary visible matter makes up only about 5% of the universe. The remaining 95% is dark matter and dark energy — both currently undetectable by direct observation.

Frequently asked questions

What is the Big Bang theory in simple terms?

The Big Bang theory states that the universe began approximately 13.8 billion years ago from a single point of extreme density and temperature — not an explosion in existing space, but the origin of space and time themselves. Since then, the universe has been expanding and cooling, allowing matter to form into stars, galaxies, and planets. The evidence includes the red-shift of all distant galaxies, cosmic microwave background radiation (the afterglow of the early hot universe), and the observed abundance of hydrogen and helium, all of which match the theory's predictions.

What is red-shift and what does it tell us about the universe?

Red-shift is the increase in wavelength (shift towards the red end of the electromagnetic spectrum) of light from a source moving away from the observer. When astronomers observe distant galaxies, their light is red-shifted — and the further away a galaxy is, the greater its red-shift. This tells us that all distant galaxies are moving away from us, and more distant ones recede faster. This pattern is exactly what would be expected if the universe began as a single point and has been expanding ever since — making red-shift the strongest observational evidence for the Big Bang.

What is the life cycle of a star like our Sun?

Our Sun began as a cloud of hydrogen gas (nebula) that collapsed under gravity to form a protostar, then ignited nuclear fusion to become a stable main sequence star. After ~10 billion years on the main sequence (it is about halfway through), it will expand into a red giant as core hydrogen runs low. The outer layers will then be expelled as a planetary nebula, leaving the dense core as a white dwarf that slowly cools over billions of years. Massive stars follow a different path — they end in a supernova explosion, leaving a neutron star or black hole.

What is the difference between a neutron star and a black hole?

Both form from the collapsed cores of massive stars following a supernova explosion. A neutron star forms when the remaining core mass is roughly 1.4–3 times the mass of our Sun — electrons and protons are crushed together into neutrons, creating an incredibly dense object perhaps 20 km across with more mass than the Sun. If the core mass exceeds about 3 solar masses, even neutron degeneracy pressure cannot prevent further collapse, and a black hole forms — a region where gravity is so extreme that not even light can escape beyond the event horizon.

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