Astronomy: Solar System, Stars and the Big Bang
Our Solar System
The Solar System consists of the Sun (our star), eight planets and their natural satellites (moons), dwarf planets, asteroids and comets. The eight planets in order of distance from the Sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A helpful mnemonic: My Very Excited Magical Jelly Seems Uniform Now.
Planets, moons and artificial satellites all orbit in ellipses (roughly circular paths). Asteroids are rocky bodies orbiting the Sun, mostly concentrated between Mars and Jupiter. Comets follow highly elliptical orbits, moving much faster when close to the Sun than when far away. Artificial satellites orbit the Earth at heights that depend on their purpose — low Earth orbit for imaging and the International Space Station, much higher for communication satellites.
The Solar System contains: the Sun, eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune), natural moons, dwarf planets, asteroids and comets. The Sun’s gravity keeps all these bodies in orbit.
Geocentric and Heliocentric Models
For most of human history, the geocentric model (Earth at the centre) was accepted. Observations of the night sky seemed consistent with the Sun, Moon and planets orbiting the Earth. This view was challenged in 1543 when Nicolaus Copernicus published a heliocentric model.
The heliocentric model (Sun at the centre) was supported and extended by Galileo Galilei in the early seventeenth century. In 1610, Galileo observed four moons orbiting Jupiter through his telescope — this was crucial evidence that not everything in the Universe orbited the Earth, directly challenging the geocentric model. Galileo also observed the phases of Venus, which are only explicable if Venus orbits the Sun.
Copernicus proposed the heliocentric model in 1543. Galileo’s 1610 observations of Jupiter’s four largest moons proved that not all bodies orbit the Earth, providing direct observational evidence against the geocentric model.
Gravity and Orbits
For a circular orbit, gravity provides the centripetal force that causes the planet’s direction to change continuously while its speed remains constant. Because gravity acts towards the centre (the Sun), the planet accelerates — but this acceleration changes the direction of travel, not the speed. Hence the planet’s velocity changes (direction changes) while its speed stays the same.
For a stable circular orbit at a given speed, there is one specific orbital radius. If the orbital speed increases, the orbit must have a smaller radius to maintain balance; a lower speed requires a larger radius. This is why communication satellites at high altitude (geostationary orbit, ~35,786 km) orbit more slowly than the International Space Station at low altitude (~400 km).
The value of gravitational field strength \(g\) and therefore weight \(W = mg\) differ on different bodies. On the Moon, \(g \approx 1.6\) N/kg compared to \(g \approx 10\) N/kg on Earth’s surface. An astronaut with a mass of 80 kg weighs 800 N on Earth but only 128 N on the Moon. Mass does not change with location; weight does.
The Life Cycle of Stars
Stars are born in clouds of gas and dust called nebulae. The life cycle a star follows depends on its mass.
1. Nebula: gravity pulls gas and dust together. As it contracts, it heats up to form a protostar.
2. Main sequence star: when the core is hot enough, hydrogen nuclei fuse to form helium, releasing huge amounts of energy. The outward pressure from fusion balances the inward pull of gravity — the star is stable. The Sun has been in this stage for approximately 4.6 billion years and has several billion years left.
3. Red giant: when the hydrogen fuel runs low, the core contracts and heats further while the outer layers expand and cool, turning the star into a red giant. Helium and heavier elements may then fuse in the core.
4. White dwarf: the outer layers drift away as a planetary nebula, leaving behind the dense, hot core — a white dwarf. It gradually cools over billions of years.
A star much more massive than the Sun also passes through a nebula stage and becomes a main sequence star, but burns through its fuel far faster. When the fuel runs out, the outer layers collapse suddenly and then rebound in a violent supernova explosion. The supernova scatters heavy elements forged in the star’s interior (including elements heavier than iron) across space — this is how the heavy elements in our bodies were made.
After the supernova, the remnant core collapses to become either a neutron star or, for the most massive stars, a black hole. Both have densities far beyond anything we encounter on Earth.
Sun-like star: nebula → protostar → main sequence → red giant → planetary nebula → white dwarf. High-mass star: nebula → protostar → main sequence → red supergiant → supernova → neutron star or black hole. The balance between gravity (inward) and radiation pressure from fusion (outward) determines a star’s stability.
Red Shift and the Expanding Universe
When a wave source moves away from an observer, the observed wavelength is longer and the observed frequency is lower than the emitted values. For light, this is called red shift. When astronomers study light from distant galaxies, they find that spectral lines are shifted towards the red end of the spectrum compared with the same lines produced in a laboratory on Earth. This means those galaxies are moving away from us.
Crucially, the further away a galaxy is, the greater its red shift — meaning it is moving away faster. Edwin Hubble gathered evidence for this relationship in 1929. This systematic red shift of all distant galaxies is strong evidence that the Universe itself is expanding: the space between galaxies is stretching.
The Big Bang and the Steady State Theory
Two competing theories were proposed to explain an expanding Universe:
The Big Bang theory proposes that the Universe originated from an extremely hot, dense point approximately 13.8 billion years ago and has been expanding ever since. If the Universe is expanding, running the expansion backwards suggests everything was once in one place. The Big Bang accounts for the red shift of all distant galaxies.
The Steady State theory proposed that the Universe has always existed in roughly the same form, with new matter created continuously to fill the gaps as galaxies move apart. It also accounts for the observed red shift but makes different predictions about the early Universe.
The discovery of the cosmic microwave background (CMB) radiation by Arno Penzias and Robert Wilson in 1964-65 was decisive evidence in favour of the Big Bang. The CMB is a uniform glow of microwave radiation detected in all directions in space, consistent with the cooling of an initially extremely hot, dense Universe. The Steady State theory cannot satisfactorily account for the CMB, which is why the Big Bang theory became the currently accepted model.
Red shift of galaxies: all distant galaxies show red shift; more distant = greater red shift. This shows the Universe is expanding. The CMB (discovered 1964–65) is the remnant heat from the Big Bang, filling all of space uniformly — the Steady State theory cannot explain it, so the Big Bang became the accepted model.
How Astronomical Observations Have Changed
Early astronomy relied on the naked eye, then optical (visible light) telescopes from the seventeenth century onwards. Modern astronomy uses the full electromagnetic spectrum: radio telescopes detect signals from pulsars and distant galaxies; X-ray and gamma-ray telescopes (space-based) study violent cosmic events. Many telescopes are placed outside the Earth’s atmosphere to avoid absorption, distortion and light pollution. The Hubble Space Telescope and the James Webb Space Telescope are major examples of space-based observatories that have transformed our understanding of the Universe.