Beyond Earth: Solar System, Stars and the Universe
Our Solar System
The Sun is a star — a common misconception is to treat the Sun as something fundamentally different from other stars. It is an ordinary medium-sized star, sitting roughly two-thirds of the way from the centre of the Milky Way galaxy. Our solar system consists of the Sun, eight planets, their moons, dwarf planets such as Pluto, asteroids and comets. All objects in the solar system are held in their orbits by the gravitational attraction of the Sun.
The eight planets divide into two groups. The four inner planets — Mercury, Venus, Earth and Mars — are small and rocky. The four outer planets — Jupiter, Saturn, Uranus and Neptune — are much larger gas or ice giants. The outer planets orbit further from the Sun and take much longer to complete one orbit: Neptune’s orbital period is about 165 Earth years.
The solar system has eight planets: Mercury, Venus, Earth, Mars (rocky inner planets) and Jupiter, Saturn, Uranus, Neptune (gas/ice outer planets). The Sun is a star — the same type of object as the billions of other stars in the Milky Way galaxy.
Satellite Orbits
Satellites orbit planets because of the gravitational force between the satellite and the planet. The orbit is a balance between the satellite’s tendency to move in a straight line and the inward gravitational pull curving its path. Artificial satellites serve many purposes and are placed in different types of orbit depending on their use.
Geostationary orbit sits approximately 36,000 km above the equator. A satellite here completes one orbit every 24 hours, matching Earth’s rotation, so it always remains above the same point on the surface. This makes geostationary satellites ideal for communications (television, telephone) and weather monitoring because a ground station can maintain a constant link without needing to track the satellite across the sky.
Polar (low Earth) orbit sits much closer to Earth, typically 200–2000 km above the surface. The satellite passes over both poles as Earth rotates beneath it, eventually scanning the entire surface. This type of orbit is used for mapping, environmental monitoring and military reconnaissance. Because the orbit is lower, the satellite moves faster and has a shorter orbital period (around 90–100 minutes).
The Moon is Earth’s natural satellite. Gravity keeps it in orbit in the same way it keeps artificial satellites in place. For a circular orbit, gravity provides the centripetal force needed to continuously change the satellite’s direction without changing its speed. Objects in orbit are in a continuous state of free fall towards the planet, but their forward velocity means they never reach the surface — they ‘fall around’ it instead.
The closer a satellite orbits, the stronger the gravitational field it experiences and the faster it must travel to maintain orbit. The ISS orbits at roughly 400 km altitude and travels at approximately 7.7 km/s. Geostationary satellites at 36,000 km travel at a much lower orbital speed of around 3 km/s.
The Life Cycle of Stars
Stars form from nebulae — vast clouds of gas (mainly hydrogen) and dust. Gravity pulls the material inward. As the cloud contracts, it heats up. When the core reaches a high enough temperature, nuclear fusion begins: hydrogen nuclei fuse to form helium, releasing enormous amounts of energy. This is the birth of a star, which then enters the main sequence — a long stable phase where the outward pressure from fusion exactly balances the inward pull of gravity. Our Sun has been in its main sequence for around 4.5 billion years and has roughly 5 billion years remaining.
When a star exhausts its hydrogen fuel, its fate depends on its mass:
Sun-sized stars: the core contracts and the outer layers expand, forming a red giant. The outer layers are then gently expelled as a glowing shell of gas called a planetary nebula, leaving the dense core as a white dwarf. The white dwarf has no fusion reactions and gradually cools over billions of years.
Massive stars (many times more massive than the Sun): they expand into a red supergiant and then undergo a violent supernova explosion — one of the most energetic events in the universe. The explosion scatters heavy elements (carbon, oxygen, iron) into space; these elements become part of new nebulae and eventually new stars and planets. The core that remains collapses to form either a neutron star or, if the remnant is massive enough, a black hole.
Sun-sized star: nebula → protostar → main sequence → red giant → white dwarf. Massive star: nebula → protostar → main sequence → red supergiant → supernova → neutron star or black hole.
Red-Shift and the Expanding Universe
When astronomers observe distant galaxies, they find that the light from almost all of them is shifted towards the red end of the spectrum. This is called red-shift. It happens because the galaxies are moving away from us: as the source of light recedes, the waves are stretched, increasing their wavelength and shifting them towards red.
Two crucial observations link red-shift to the history of the universe. First, all distant galaxies show red-shift — every direction we look, galaxies are moving away. Second, the further away a galaxy is, the greater its red-shift, meaning it is moving away faster. This is exactly what we would expect if the universe has been expanding from a single point in the past.
The Big Bang theory proposes that the universe began from an incredibly hot, dense point and has been expanding ever since. Red-shift is key evidence: if galaxies are all moving apart, running time backwards implies they were once all in the same place.
A second line of evidence is the cosmic microwave background radiation (CMBR) — a faint microwave signal detected uniformly from all directions in space. This is the afterglow of the immense heat of the early universe, now cooled to microwave wavelengths as the universe has expanded and cooled. The CMBR was first detected in the 1960s and is regarded as strong confirmation of the Big Bang model.
A Doppler ball is a simple way to model red-shift: a ball with a built-in oscillator produces a sound of higher pitch when moving towards you and lower pitch when moving away, analogous to light being blue-shifted (approaching) or red-shifted (receding) galaxies.