Space Physics

Summary: Covers the Earth, Moon, and Solar System, planetary orbits, the Sun as a star, the lifecycle of stars (from nebula to black dwarf/black hole), galaxies, the Big Bang theory, red shift, Hubble’s Law, and the Cosmic Microwave Background Radiation (CMBR). This topic places Earth in a cosmic context and explains observational evidence for the origin of the Universe. Tags: igcse physics space astronomy cosmology stars big-bang Created: 2026-07-16 Last Updated: 2026-07-16

The Earth

The Earth is a rocky planet, the third from the Sun.

Rotation (spin on its own axis):

• The Earth rotates from west to east once every 24 hours
• This causes day and night — the side facing the Sun is in daylight; the opposite side is in darkness
• The rotation axis is tilted at approximately 23.5° from the vertical — this tilt causes seasons

Orbit around the Sun:

• The Earth orbits the Sun once every 365.25 days (one year). The 0.25 is why we have a leap year every 4 years
• The orbit is elliptical (slightly oval), not a perfect circle
• The Earth’s orbital speed is approximately 30 km/s (107,000 km/h)
• Seasons are caused by the tilt of the Earth’s axis, not by the distance from the Sun (in fact, the Northern Hemisphere is tilted away from the Sun in January, when Earth is closest to the Sun)

The Moon

The Moon is Earth’s only natural satellite. It orbits the Earth approximately every 28 days.

Phases of the Moon: As the Moon orbits the Earth, we see different amounts of its sunlit side from Earth:

• New Moon: Moon is between Earth and Sun — the unlit side faces Earth
• Full Moon: Earth is between Moon and Sun — the fully lit side faces Earth
• First quarter / Last quarter: half of the visible side is lit

Eclipses:

• Solar eclipse: Moon passes between Earth and Sun — Moon’s shadow falls on part of Earth. The Moon blocks the Sun’s light (can be total or partial depending on alignment)
• Lunar eclipse: Earth passes between Sun and Moon — Earth’s shadow falls on the Moon. The Moon appears red/orange (Earth’s atmosphere refracts red light onto the Moon — often called a “blood moon”)

The Solar System

The Solar System consists of the Sun, 8 planets, dwarf planets (e.g., Pluto), moons, asteroids, and comets.

The 8 planets in order from the Sun:

Other Solar System bodies:

• Asteroid belt: between Mars and Jupiter — a region containing many rocky bodies (asteroids). The largest is Ceres (classified as a dwarf planet)
• Comets: small bodies of ice, dust, and rock that orbit the Sun in highly elliptical orbits. As a comet approaches the Sun, it heats up — ice vaporises → forms a visible coma (atmosphere) and a tail (always pointing away from the Sun, driven by solar wind)
• Moons (natural satellites): most planets have moons. Earth has 1; Jupiter and Saturn have dozens

The Sun

The Sun is a star — a massive ball of hot plasma (ionised gas), mostly hydrogen and helium.

Key facts:

• Diameter: ~1.4 million km (about 109 times Earth’s diameter)
• Surface temperature: ~5,500°C
• Core temperature: ~15 million °C
• Energy source: nuclear fusion — hydrogen nuclei fuse to form helium, releasing vast amounts of energy (E = mc²)
• The Sun has been shining for ~4.6 billion years and has enough fuel for another ~5 billion years

Orbits and Gravity

Planets, moons, comets, and artificial satellites all orbit larger bodies due to gravity.

Gravity provides the centripetal force needed for circular/orbital motion. The gravitational attraction between two objects (e.g., Sun and Earth) pulls the smaller body towards the larger one. The smaller body also has tangential velocity — the combination of this velocity and the inward pull of gravity results in a curved (orbital) path.

Key relationships for orbits:

Why planets stay in orbit: If a planet’s orbital speed were to suddenly drop, it would spiral into the Sun. If it were to suddenly increase, it would fly off into space. The balance between gravitational force and tangential speed keeps planets in stable orbits.

The Lifecycle of Stars

Stars form, evolve, and eventually die. The path depends mainly on the star’s mass.

Formation (all stars follow this initial path):

1. Nebula: A vast cloud of gas (mainly hydrogen) and dust in space
2. Gravitational collapse: Gravity pulls the gas and dust together. As the cloud collapses:
   • Gravitational potential energy is converted to thermal energy → temperature rises
   • The collapsing cloud spins faster (conservation of angular momentum)
3. Protostar: A hot, dense ball of gas forms at the centre. Temperature and pressure continue to rise as more mass falls in
4. Main sequence star: When the core temperature reaches ~15 million K, nuclear fusion of hydrogen into helium begins. The outward pressure from fusion balances the inward pull of gravity (hydrostatic equilibrium). The star becomes stable and enters the main sequence — where it spends most of its life

For stars of similar mass to the Sun (low/average mass stars):

5. Red giant: When hydrogen in the core runs out, fusion stops in the core. The core contracts and heats up → causes hydrogen fusion in a shell around the core. The outer layers expand and cool → star becomes a red giant (enormous, relatively cool surface)
6. Planetary nebula: The outer layers of the red giant are gently expelled into space, forming a glowing shell of gas
7. White dwarf: The hot, dense core remains — a white dwarf. It is extremely dense (a teaspoon of white dwarf matter would weigh several tonnes). It no longer undergoes fusion — it simply cools and fades
8. Black dwarf: After billions of years, the white dwarf has radiated all its heat → becomes a cold, dark black dwarf (theoretically — the Universe is not old enough for any black dwarfs to exist yet)

For stars much more massive than the Sun:

5. Red supergiant: After the main sequence, massive stars swell into red supergiants. Fusion creates elements up to iron in the core
6. Supernova: Eventually the core (now iron) can fuse no further (iron fusion absorbs energy, it doesn’t release it). The core collapses catastrophically under gravity, then rebounds in a colossal explosion — a supernova. Supernovae briefly outshine entire galaxies and scatter heavy elements (everything heavier than iron) into space — these elements become the raw material for new stars, planets, and eventually life
7. After the supernova, two outcomes are possible depending on the remaining core mass:
   • Neutron star: If the core is less than ~3 solar masses, it collapses into an incredibly dense ball of neutrons. A teaspoon of neutron star material would weigh billions of tonnes. Pulsars are rapidly rotating neutron stars emitting beams of radiation
   • Black hole: If the core is more than ~3 solar masses, gravity is so strong that nothing — not even light — can escape. The boundary of the black hole is the event horizon

Summary diagram:

Nebula → Protostar → Main Sequence Star →
    ├─ (Sun-like) → Red Giant → Planetary Nebula → White Dwarf → Black Dwarf
    └─ (Massive) → Red Supergiant → Supernova →
            ├─ Neutron Star
            └─ Black Hole

The Universe and Galaxies

A galaxy is a vast collection of stars, gas, dust, and dark matter, held together by gravity. Our Solar System is in the Milky Way galaxy, which is a spiral galaxy containing an estimated 100-400 billion stars.

Types of galaxies:

• Spiral: flat disk with spiral arms and a central bulge (e.g., Milky Way, Andromeda)
• Elliptical: oval-shaped, smooth, featureless light profile — mostly old stars with little gas/dust
• Irregular: no distinct shape, often chaotic — rich in gas and dust with active star formation

The Universe contains billions of galaxies, each containing billions of stars.

Red Shift and the Big Bang Theory

Red Shift

When we observe light from distant galaxies, the spectral lines (characteristic patterns of absorption/emission) are shifted towards the red end of the visible spectrum compared to what we observe from similar elements on Earth.

What red shift tells us:

• The wavelength of the light has been stretched/elongated
• This means the source (galaxy) is moving away from us (analogous to how a siren’s pitch drops as it moves away — the Doppler effect, but for light)
• More distant galaxies show greater red shift → they are moving away faster

Interpretation:

• Almost all galaxies show red shift → almost all galaxies are moving away from us
• The Universe as a whole is expanding — galaxies are not moving through space; space itself is expanding, carrying galaxies apart

Hubble’s Law

Edwin Hubble discovered that the speed at which a galaxy recedes is proportional to its distance from Earth:

recession speed ∝ distance

Or more precisely: v = H₀d (where H₀ is the Hubble constant)

A galaxy twice as far away recedes at roughly twice the speed. This is exactly what would be expected if the Universe is expanding uniformly.

The Big Bang Theory

If the Universe is expanding now, then in the past it must have been smaller. Extrapolating backwards, all matter and energy in the Universe was once concentrated at a single point (a singularity). The Universe began with a colossal expansion — the Big Bang — approximately 13.8 billion years ago.

Evidence for the Big Bang:

The Big Bang was NOT an explosion in space — it was the origin of space and time themselves. There was no “before” the Big Bang, just as there is no “north of the North Pole”.

Dark Matter and Dark Energy

Modern cosmology recognises that ordinary matter (atoms) makes up only about 5% of the Universe:

• Dark matter (~27%): Unseen matter that does not emit, absorb, or reflect light. Its existence is inferred from its gravitational effects on galaxies (galaxies rotate faster than they should based on visible matter alone; gravitational lensing)
• Dark energy (~68%): A mysterious force driving the accelerating expansion of the Universe (discovered in the late 1990s). Its nature is unknown

Sources

• BBC Bitesize GCSE Physics — Space physics guide, BBC (free educational resource)
• OpenStax College Physics — Astronomy chapter, Rice University (free, CC BY 4.0)
• Cambridge IGCSE Physics 0625 — Space physics section, Cambridge Assessment International Education
• CK-12 Physics for High School — Astronomy chapter, CK-12 Foundation (free, CC BY-NC 3.0)

Related Notes

• Forces and Motion — Gravity, centripetal force, circular motion, Newton’s Laws
• Nuclear Physics — Nuclear fusion in stars, fusion as the energy source of the Sun
• Waves — The electromagnetic spectrum (radio through gamma) — how the Universe is observed across all wavelengths; red shift as change in wavelength
• IGCSE-Phys-Index — Full IGCSE Physics index

Keywords

Solar System, planet, orbit, gravity, centripetal force, nebula, protostar, main sequence, red giant, red supergiant, planetary nebula, white dwarf, black dwarf, supernova, neutron star, black hole, galaxy, Milky Way, red shift, Big Bang, Hubble's Law, CMBR, cosmic microwave background radiation, dark matter, dark energy, stellar evolution, nuclear fusion

Common Misconceptions