Space
How Stars Form and Key Features
How stars form
Stars begin as nebulae → Huge clouds of gas (mostly hydrogen) and dust
Gravity pulls the gas and dust together, causing the cloud to contract and collapse
As material falls inward, it heats up → The core becomes hotter and denser
When the core reaches about 10 million °C, nuclear fusion begins → Hydrogen atoms fuse to form helium, releasing huge amounts of energy
This outward energy pressure balances the inward pull of gravity, and the star becomes stable → It is now a main-sequence star
Key features of stars
Brightness (luminosity) → How much light energy a star gives out per second
Temperature/colour → Hotter stars appear blue/white; cooler stars appear red/orange
Blue = hottest, red = coolest
Size → Stars range from tiny neutron stars to supergiants (see below)
Mass → Determines a star's temperature, size, brightness, and lifespan
Composition → Mostly hydrogen and helium, with small amounts of heavier elements
Using Solar Mass to Compare Star Sizes
Solar mass (M☉) → The unit used to measure a star's mass, equal to the mass of our Sun (1 solar mass = 1 Sun)
Small stars → Less than about 0.5 solar masses (e.g. red dwarfs) → They are small, cool, dim, and live for a very long time
Medium stars → About 0.5 – 8 solar masses (e.g. our Sun, which is 1 solar mass) → They are average in size and brightness, and live for billions of years
Large/massive stars – more than about 8 solar masses (e.g. blue supergiants, which can be tens or even hundreds of solar masses) → They are huge, hot, bright, and burn out quickly (live only millions of years)
Key idea → The more massive the star, the hotter, brighter, and shorter-lived it is. Smaller stars are cooler, dimmer, and much longer-lived
Galaxies
What is a Galaxy?
A galaxy is a huge collection of stars, gas, dust, and dark matter, all held together by gravity
Galaxies contain billions to trillions of stars, plus nebulae, planets, black holes, and other objects
Our galaxy is called the Milky Way
Major Features of Galaxies
Stars → Billions of them, arranged in different patterns depending on galaxy type
Nebulae → Clouds of gas and dust where new stars form
Dark matter → Invisible matter that doesn't emit light, but its gravity helps hold the galaxy together → (Detected indirectly through its gravitational effects)
Supermassive black holes → Found at the centre of most galaxies, including ours → (Sagittarius A* in the Milky Way) → They have enormous gravity
Spiral arms → In spiral galaxies, regions of gas, dust, and young stars where star formation is active
Three Main Types of Galaxies
Spiral galaxies → Flattened disk with a central bulge and curved spiral arms. Contain lots of gas, dust, and young stars → E.g. the Milky Way, Andromeda
Elliptical galaxies → Rounded/oval-shaped, smooth, with no spiral arms. Contain mostly older stars, and very little gas or dust, so little new star formation → E.g. M87
Irregular galaxies → No regular shape, often chaotic-looking. Usually small, with lots of gas and dust, and active star formation → E.g. the Large Magellanic Cloud
Light-Years and Measuring Distances
A light-year is the distance light travels in one year, NOT a unit of time
Light travels at about 300,000 km/s, so one light-year = 9.5 trillion km
Within a galaxy → Distances are measured in light-years → E.g. our Sun is about 26,000 light-years from the centre of the Milky Way; the Milky Way is about 100,000 light-years across
Between galaxies → Distances are so huge they're measured in millions or billions of light-years → E.g. the Andromeda Galaxy is about 2.5 million light-years away
Major Features of the Solar System
The Sun → A medium-sized star at the centre, holding everything in orbit via gravity → Makes up about 99.8% of the solar system's mass
Eight planets → Divided into two groups → Terrestrial (rocky) planets (Mercury, Venus, Earth, Mars) and gas giants (Jupiter, Saturn, Uranus, Neptune)
Dwarf planets → E.g. Pluto, Ceres, Eris → Smaller than planets, often in the Kuiper Belt
Moons → Natural satellites orbiting planets → E.g. Earth's Moon, Jupiter's Ganymede
Asteroid belt → A region of rocky objects between Mars and Jupiter
Kuiper Belt and Oort Cloud → Icy bodies beyond Neptune, source of many comets
Comets → Icy bodies that develop tails as they approach the Sun
Units for Sizes and Distances in the Solar System
Sizes of planets
Measured in kilometres (km) for diameter
Examples:
Mercury = 4,900 km (smallest)
Earth = 12,742 km
Jupiter = 139,820 km (largest → about 11× Earth's diameter)
Distances between planets
Measured in astronomical units (AU) → 1 AU = the average distance from Earth to the Sun (≈ 150 million km)
Examples:
Mercury = 0.39 AU from the Sun
Earth = 1 AU
Jupiter = 5.2 AU
Neptune = 30 AU → The most distant planet
How the Solar System Formed
About 4.6 billion years ago, the solar system formed from a giant cloud of gas and dust called a solar nebula
The cloud began to collapse under gravity, spinning faster and flattening into a disk (like a spinning pizza dough)
At the centre, material clumped together to form the Sun, which ignited via nuclear fusion
In the disk around the Sun, small particles of dust and rock collided and stuck together (accretion), gradually building into larger bodies → planetesimals → planets
Rocky planets formed close to the Sun (where it was too hot for gases to condense); gas giants formed further out, where it was cooler, and they could capture lots of gas
Optical, Radio and Space Telescopes
Optical telescopes
Collect visible light using lenses or mirrors to form images of distant objects
Used for → Observing planets, stars, galaxies, nebulae → E.g. Hubble Space Telescope's images, discovering moons of planets, mapping star clusters
Radio telescopes
Collect radio waves (invisible to our eyes) using large dish antennas
Can operate day and night, and through clouds
Used for → Discovering pulsars (rotating neutron stars), quasars, cosmic microwave background radiation, mapping hydrogen gas in galaxies
Space telescopes
Placed above Earth's atmosphere to avoid distortion and blockage of light by the atmosphere
E.g. Hubble, James Webb
Used for → Capturing sharp images of distant galaxies, studying exoplanet atmospheres, observing infrared light from the early universe
Advantages of Space Telescopes and Their Discoveries
Advantages:
No atmospheric distortion — images are much sharper than ground-based telescopes
No light pollution — no interference from city lights or Earth's glow
Can see wavelengths blocked by the atmosphere → E.g. ultraviolet, infrared, X-rays, which don't reach the ground → This reveals objects invisible from Earth
Discoveries made with space telescopes:
Hubble: measuring the rate of expansion of the universe, discovering that the universe is accelerating, capturing deep-field images showing thousands of distant galaxies, studying exoplanets
James Webb: imaging the earliest galaxies formed after the Big Bang, studying atmospheres of exoplanets, observing star formation in detail.
The Big Bang Theory
The Big Bang is the leading theory for how the universe began
About 13.8 billion years ago, all matter and energy in the universe was compressed into an extremely hot, dense point (a singularity)
This point expanded rapidly → Not an explosion in space, but the expansion of space itself → Space, time, matter, and energy all came into being at this moment
As the universe expanded, it cooled down, allowing subatomic particles to form, then simple atoms (hydrogen, helium), then stars, galaxies, and everything else we see today
The universe is still expanding today
Three Pieces of Evidence Supporting the Big Bang
Cosmic Microwave Background (CMB) radiation → A faint, uniform glow of microwave radiation filling all of space → Predicted as leftover heat from the Big Bang and discovered in 1965 → It is the "afterglow" of the universe's birth
Redshift of galaxies → Galaxies are moving away from us, and the further away they are, the faster they recede → This shows the universe is expanding, meaning it must have been smaller and denser in the past → Consistent with a beginning
Abundance of light elements → The universe contains a specific ratio of hydrogen and helium (about 75% hydrogen, 25% helium, matching what the Big Bang should have produced in its first few minutes Stars alone can't explain these amounts
Evidence Used to Calculate the Age of the Universe
Measuring the expansion rate (Hubble's Law) → By measuring how fast galaxies are moving apart, scientists can run the expansion backwards to work out when everything was together → Giving an age of about 13.8 billion years
Cosmic Microwave Background → Studying the temperature and patterns of the CMB lets scientists measure the universe's composition and expansion history, helping confirm the age
Oldest stars and globular clusters → The oldest known stars are about 13 billion years old, so the universe must be at least that old → Consistent with the 13.8 billion-year estimate (nothing can be older than the universe)