Key Terms in Detail & Fun Q&A
Earth
Earth is the only planet currently known to host life, with more than 70% of its surface covered by ocean. It has temperatures suitable for liquid water plus a life-protecting atmosphere and magnetic field — the starting point for understanding the whole universe.
Is Earth the largest rocky planet in the Solar System? Yes! Among the four terrestrial planets, Earth has the largest diameter and mass.
What colour is Earth from space? Mostly blue, because about 70% of its surface is covered by ocean — which is why it's nicknamed the 'Blue Marble'.
Moon
The Moon is Earth's only natural satellite, about 3,474 km across and an average of roughly 380,000 km away. Its gravity is the main driver of Earth's tides and helps keep Earth's rotation axis stable.
Is the Moon moving away from or toward Earth? Moving away, by about 3.8 cm a year — roughly the speed your fingernails grow.
Could you jump higher on the Moon than on Earth? Yes! The Moon's gravity is only about one-sixth of Earth's, so you could jump roughly six times as high.
Tides
Tides are the periodic rise and fall of sea level, caused mainly by differences in the Moon's (and the Sun's) gravitational pull across different parts of Earth. The sides facing toward and away from the Moon bulge at the same time, so most coasts get about two high tides a day.
How many high tides are there usually in a day? Most coasts get about two high tides and two low tides each day.
Does the Sun cause tides too? Yes, but the Sun's tidal pull is about half the Moon's; when the Sun, Moon and Earth line up you get especially large 'spring tides'.
Tidal locking
Tidal locking is when a body's rotation period equals its orbital period, so it always shows the same face to another body. Over billions of years, Earth's gravity 'locked' the Moon, which is why we only ever see one side of it.
Which side of the Moon can we never see? The far side (often called the 'dark side of the Moon'), which can only be photographed by spacecraft.
Is the 'dark side of the Moon' always dark? No — it gets just as much sunlight, it simply always faces away from Earth, so we never see it.
Magnetic field
Earth's magnetic field is generated by flows in the molten metal outer core and extends into space like a giant shield. It deflects the charged solar wind and cosmic radiation, protecting surface life and funnelling particles toward the poles to create auroras.
Why does a compass point north? Because Earth itself is a giant magnet, and a compass needle lines up with Earth's magnetic field.
Do other planets have magnetic fields? Yes — Jupiter's is the strongest of any planet in the Solar System, while Mars has only weak, patchy local fields left.
Aurora
Auroras are the light emitted when charged particles from the solar wind follow Earth's magnetic field into the upper atmosphere and strike gas molecules. They appear mostly at high latitudes near the magnetic poles, often as dancing green and red curtains.
Are auroras only green? No — there are also reds, purples and blues, depending on which gas the particles hit and at what altitude.
Do other planets have auroras? Yes! Jupiter and Saturn both have auroras even more spectacular than Earth's.
Sun
The Sun is the star at the centre of the Solar System — a hot ball of plasma that shines continuously through nuclear fusion. It holds about 99.8% of the Solar System's mass and provides the energy for almost all life on Earth.
How long does sunlight take to reach Earth? About 8 minutes 20 seconds, so the sunlight you see actually left the Sun about 8 minutes ago.
What colour is the Sun? Actually white (it contains light of all colours); it only looks yellow or orange from Earth because the atmosphere scatters its light.
Nuclear fusion
Nuclear fusion is the process where light atomic nuclei (such as hydrogen) combine into heavier ones (such as helium) under extreme heat and pressure, releasing enormous energy. It is the power source that makes the Sun and all stars shine.
How much hydrogen does the Sun burn each second? About 600 million tonnes of hydrogen fuse into helium, yet it still has enough fuel for roughly 5 billion more years.
Can we do nuclear fusion on Earth? Scientists are working on an 'artificial sun,' hoping to use fusion to provide clean energy.
Terrestrial planet
Terrestrial planets are planets made mostly of rock and metal with a solid surface, including Mercury, Venus, Earth and Mars. Compared with the gas giants they are smaller, denser and have fewer moons.
Which terrestrial planet is the hottest? Venus, not Mercury (the closest to the Sun), because Venus's thick atmosphere causes a runaway greenhouse effect, with surface temperatures around 465°C.
Do terrestrial planets have rings? Generally no obvious rings; the magnificent rings are mainly found on the gas and ice giants.
Mars
Mars is the fourth planet from the Sun and looks red because its surface is rich in iron oxide. It hosts Olympus Mons, the tallest volcano in the Solar System, and the vast Valles Marineris canyon — a focus for the search for past life and for planning future crewed exploration.
How tall is the highest mountain on Mars? Olympus Mons is about 22 km high — nearly three times the height of Mount Everest.
How long is a day on Mars? About 24.6 hours, very close to a day on Earth.
Asteroid belt
The asteroid belt is a ring-shaped region between the orbits of Mars and Jupiter holding a great many rocky bodies. They are leftover material from the early Solar System that never combined into a planet, kept from merging by Jupiter's strong gravity.
Is the asteroid belt crammed with rocks like in the movies? Actually it's very empty — asteroids are on average hundreds of thousands to over a million kilometres apart.
What's the largest object in the asteroid belt? The dwarf planet Ceres, which holds about a third of the belt's total mass.
AU
The astronomical unit (AU) is a distance unit commonly used in astronomy, equal to the average distance from Earth to the Sun, about 150 million km. It is far more convenient than kilometres for measuring distances within the Solar System.
How many AU is Neptune from the Sun? About 30 astronomical units — 30 times the Earth–Sun distance.
How many AU are in one light-year? About 63,000 astronomical units, so a light-year is far larger than an AU.
Jupiter
Jupiter is the largest and most massive planet in the Solar System, a gas giant made mostly of hydrogen and helium. It has the famous Great Red Spot storm and dozens of moons, and its strong gravity shields the inner Solar System from many comet impacts.
What is Jupiter's Great Red Spot? A giant storm larger than Earth that has raged for at least several hundred years.
How many moons does Jupiter have? More than 90 are known, and one of them, Europa, may hide an ocean beneath its icy crust.
Saturn
Saturn is the second-largest planet in the Solar System, famous for its magnificent ring system made of ice and rock fragments. It is also a gas giant, with a density so low it could in theory float on water.
Could Saturn really float on water? In theory yes, because its average density is less than water's — assuming you could find a big enough pool.
What are Saturn's rings made of? Countless chunks of ice and rock, some as small as grains of sand and others as big as houses.
Ice giant
Ice giants are planets like Uranus and Neptune that, beyond hydrogen and helium, contain large amounts of 'ices' such as water, ammonia and methane. They are smaller than the gas giants and look bluish because methane absorbs red light.
What's so strange about Uranus? It rotates almost 'on its side,' with an axial tilt of about 98 degrees, as if rolling along its orbit.
Why are Uranus and Neptune blue? Because methane in their atmospheres absorbs red light and reflects blue light.
Kuiper Belt
The Kuiper Belt is a ring-shaped region beyond Neptune's orbit, dotted with small bodies made of frozen volatiles. It is similar to the asteroid belt but far larger, and Pluto is one of its members.
Where does Pluto live? Right in the Kuiper Belt — it's one of the most famous objects in that region.
Has any spacecraft visited the Kuiper Belt? Yes — New Horizons flew past Pluto in 2015.
Halley
Halley's Comet (1P/Halley) is the most famous periodic comet, returning near Earth about every 76 years. As it nears the Sun, its ices sublimate and jet out gas and dust, forming a long tail that always points away from the Sun.
When can we next see Halley's Comet? Around 2061 — its last return was in 1986.
Why does a comet's tail always point away from the Sun? Because the solar wind and radiation pressure blow the gas and dust in the direction opposite the Sun.
Dwarf planet
A dwarf planet orbits the Sun and is massive enough to be round, but has not 'cleared' the other objects near its orbit. Pluto was reclassified as a dwarf planet in 2006 precisely because of this condition.
How many dwarf planets are in the Solar System? Five are officially recognised so far: Pluto, Ceres, Eris, Haumea and Makemake.
Why was Pluto 'demoted'? Because it hasn't cleared the other objects near its orbit, so from 2006 it was reclassified as a dwarf planet.
Solar wind
The solar wind is a constant stream of charged particles (mainly protons and electrons) the Sun blows out in all directions. It fills the entire Solar System, pushes comet tails away from the Sun, and creates auroras when guided by Earth's magnetic field.
How fast is the solar wind? About 400 to 800 km per second.
Does the solar wind affect Earth? Strong solar storms can disrupt satellites, power grids and radio communications, and they also cause auroras.
Heliosphere
The heliosphere is a vast bubble-shaped region blown out by the solar wind that wraps around the whole Solar System. Its outer edge (the heliopause) marks the boundary between the Sun's influence and interstellar space; the Voyager probes have crossed it into interstellar space.
Where is the 'edge' of the Solar System? One definition is the heliopause — the point where the solar wind can no longer push back the interstellar medium.
Has anything flown beyond the heliosphere? Voyager 1 became the first human-made object to enter interstellar space, in 2012.
Voyager
Voyager 1 and 2 are space probes launched in 1977 and are the most distant objects humans have sent. They have flown beyond the heliosphere into interstellar space, each carrying a 'Golden Record' of Earth's sounds and images.
What's on the Golden Record? Natural sounds of Earth, greetings in many languages and music — a message for any alien civilisation.
Are the Voyager probes still working? Yes — over 40 years after launch they still occasionally send back faint signals, though their power is slowly running out.
Cosmic ray
Cosmic rays are high-energy particles travelling at nearly the speed of light, coming from the Sun, supernovae and even beyond the Milky Way. Earth's magnetic field and atmosphere shield us from most of their harmful effects.
Can cosmic rays harm astronauts? Yes — it's one of the health risks that long missions (such as a trip to Mars) must solve.
Do cosmic rays hit us on the ground? Yes, but the atmosphere blocks most of them, and what's left has had its energy greatly reduced.
Oort Cloud
The Oort Cloud is a hypothesised giant spherical shell of countless icy planetesimals surrounding the outermost Solar System, reaching as far as 100,000 AU. It is thought to be the source of long-period comets.
How far away is the Oort Cloud? Its outer edge may reach about 100,000 AU — roughly halfway to the nearest star.
Have we ever seen the Oort Cloud? Never directly; its existence is inferred from the orbits of long-period comets.
Light-year
A light-year is the distance light travels in a vacuum in one year, about 9.5 trillion km. When distances grow too large for the AU (for example, between stars), we switch to light-years instead.
Is a light-year a unit of time or distance? Distance! It's how far light travels in a year, about 9.5 trillion km.
Is looking at the stars like looking into the past? Yes — starlight takes many years to reach Earth, so you see a star as it was long ago.
Spectroscopy
Spectroscopy is the method of splitting starlight into its colours (a spectrum) to analyse it. Each element leaves unique 'fingerprint' lines in the spectrum, so astronomers can learn a star's chemical composition and temperature without ever touching it.
How can we know a star's composition without touching it? By analysing the spectral lines in its light — every element has a unique 'barcode' fingerprint.
What else can a spectrum tell us? A star's temperature, its speed of motion, and even whether planets orbit it.
Doppler effect
The Doppler effect is the compression or stretching of wavelengths when a source and observer move relative to each other, like the changing pitch of an ambulance approaching and receding. Redshift and blueshift of starlight tell us whether a star is moving toward or away from us.
Where do we meet the Doppler effect in daily life? In the change of pitch as an ambulance or train approaches and then moves away.
What does redshift mean? That the light source is moving away from us; the redshift of distant galaxies is evidence that the universe is expanding.
O B A F G K M
Astronomers sort stars by surface temperature into seven main classes — O, B, A, F, G, K, M — from the hottest, bluest O stars to the coolest, reddest M stars. Our Sun is a G-type yellow star.
How do you remember the order O B A F G K M? The classic mnemonic is 'Oh Be A Fine Girl/Guy, Kiss Me.'
Which class does the Sun belong to? Type G — a medium-temperature yellow star.
Supernova
A supernova is the violent explosion of a massive star at the end of its life, briefly outshining an entire galaxy. The blast flings the heavy elements forged inside the star into space, becoming material for new stars, planets and even life.
Is our body connected to supernovae? Yes! Heavy elements in your body, like calcium and iron, were mostly forged in ancient supernova explosions.
How bright is a supernova? At its peak it can briefly outshine the combined light of the billions of stars in its entire galaxy.
Pulsar
A pulsar is a rapidly spinning, highly magnetised neutron star that beams two cones of electromagnetic radiation from its magnetic poles. When a beam sweeps across Earth like a lighthouse, we receive an extremely regular pulse of signals.
Why are pulsars called 'cosmic lighthouses'? The electromagnetic beams they emit sweep across Earth as they spin, producing extremely regular pulses of signal.
How fast does a pulsar spin? Some rotate hundreds of times per second — faster than a kitchen blender.
Local Bubble
The Local Bubble is a low-density cavity of hot gas about 300 light-years across that surrounds the Solar System. It is thought to have been 'blown' out by multiple supernova explosions millions of years ago, and our Sun is currently passing through it.
Do we live inside a 'bubble'? Yes — the Solar System is passing through a low-density cavity of hot gas blown out by ancient supernovae.
How big is the Local Bubble? About 300 light-years across.
Milky Way
The Milky Way is the barred spiral galaxy we live in, with roughly 100 to 400 billion stars. At its centre is a supermassive black hole called Sagittarius A*, around which the whole galaxy rotates.
How many stars are in the Milky Way? An estimated 100 to 400 billion.
How long does the Sun take to orbit the galaxy once? About 230 million years, called a 'galactic year'; the last time the Sun was here, dinosaurs roamed Earth.
Spiral arm
A spiral arm is a bright band of stars, gas and dust in a spiral galaxy, and one of the most active sites of star birth. The Sun sits on a minor arm of the Milky Way — the Orion Spur.
Where is the Sun in the Milky Way? On a minor arm called the Orion Spur, about 26,000 light-years from the galactic centre.
Why are spiral arms especially bright? Because bright young stars are born there in great numbers.
Black hole
A black hole is an object whose gravity is so strong that not even light can escape, usually formed when a massive star collapses. We cannot see a black hole directly, but we can infer it from its effect on nearby matter and stars.
Will a black hole suck in the whole universe? No — it only affects things close enough; from far away its gravity is just like that of any ordinary object of the same mass.
How far is the nearest black hole to Earth? The nearest known one, Gaia BH1, is about 1,560 light-years away — quite safe.
Accretion disk
An accretion disk is the rotating, disk-shaped structure that forms as matter spirals into a black hole or other massive body. Friction heats the matter to extreme temperatures and releases huge amounts of light and energy, making accretion disks among the brightest objects in the universe.
A black hole is black, so why call it bright? It isn't the black hole itself that glows, but the surrounding accretion disk, spinning fast and heated white-hot.
How hot is an accretion disk? It can reach millions of degrees and emit intense X-rays.
Sgr A*
Sagittarius A* (Sgr A*) is the supermassive black hole at the centre of the Milky Way, with a mass about 4 million times that of the Sun. All the stars of the Milky Way orbit around it.
What's at the centre of the Milky Way? A supermassive black hole about 4 million times the Sun's mass — Sagittarius A*.
Have we photographed it? Yes! In 2022 the Event Horizon Telescope released its first image.
Event horizon
The event horizon is a black hole's 'point of no return' boundary — once crossed, not even light can escape again. It is not a solid surface, but marks the region completely dominated by the black hole's gravity.
What happens if you cross the event horizon? Not even light can turn back, and no signal can ever reach the outside again.
Is the event horizon a solid surface? No — it's just a 'no return' boundary, not anything you could touch.
Spiral galaxy
A spiral galaxy is a galaxy with a flat disk and spiral arms, plus a bright central bulge. The Milky Way and the Andromeda Galaxy are both spiral galaxies, and the arms are active regions of star formation.
Is our Milky Way a spiral galaxy? Yes — and specifically a 'barred spiral galaxy' with a bar-shaped structure at its centre.
What's special about a spiral galaxy's arms? They are where stars are born in great numbers, which is why they look especially bright.
Andromeda · M31
The Andromeda Galaxy (M31) is the nearest large spiral galaxy to the Milky Way, about 2.5 million light-years away, and one of the most distant objects visible to the naked eye. It is approaching us and will collide and merge with the Milky Way in about 4 billion years.
When will Andromeda collide with the Milky Way? In about 4 billion years, when the two will merge into a single larger galaxy.
Can you see Andromeda with the naked eye? Yes, under a dark sky free of light pollution — it's one of the most distant objects visible to the naked eye.
Local Group
The Local Group is the galaxy group that contains the Milky Way, about 3 megaparsecs across with more than 54 member galaxies. It is dominated by two large spiral galaxies — Andromeda and the Milky Way.
How many galaxies are in the Local Group? More than 54, though most are small dwarf galaxies.
Who are the 'big bosses' of the Local Group? Andromeda and the Milky Way — its two largest members.
Interacting galaxies
Interacting galaxies are galaxies that pull on, distort and even collide and merge with one another through gravity. Such interactions trigger large-scale star formation and are a key driver of galaxy evolution.
When galaxies collide, do the stars crash together? Almost never — the space between stars is so vast that galaxies mostly 'pass through' each other.
What happens when galaxies collide? Gravity distorts their shapes and triggers a burst of new star formation.
Galaxy group
A galaxy group is a collection of dozens of galaxies bound together by gravity, the basic building block of the universe's large-scale structure. Larger gatherings are called galaxy clusters and superclusters.
What's the difference between a galaxy group and a cluster? Mainly scale — a group has dozens of galaxies, while a cluster has hundreds to thousands.
What holds a galaxy group together? Gravity, with dark matter providing most of the unseen mass.
Supercluster
A supercluster is one of the largest known structures in the universe, made of many galaxy clusters and groups gathered together. Together with the huge voids between them, they form the filamentary 'cosmic web.'
What's the largest structure in the universe? The 'cosmic web' woven from superclusters and the voids between them — the largest known structure.
How big is a supercluster? It can span hundreds of millions of light-years and contain tens of thousands of galaxies.
Laniakea
Laniakea is the supercluster the Milky Way belongs to, containing about 100,000 galaxies and spanning some 520 million light-years. Its name means 'immeasurable heaven' in Hawaiian.
What does 'Laniakea' mean? It means 'immeasurable heaven' in Hawaiian.
How big is it? It spans about 520 million light-years and contains roughly 100,000 galaxies, with the Milky Way just a tiny member.
Dark matter
Dark matter is a mysterious substance that gives off and absorbs no light and can only be detected through gravity. It makes up about 85% of the universe's matter, acting like an invisible scaffold that holds galaxies and clusters together.
Can we see dark matter? No — it gives off and absorbs no light, and can only be detected through its gravitational effects.
How common is dark matter? It makes up about 85% of the universe's matter — far more than the ordinary matter we know.
Dark energy
Dark energy is an unknown form of energy that drives the accelerating expansion of the universe. It is estimated to make up about 68% of the universe's total energy and is the main factor deciding the universe's ultimate fate.
What does dark energy do? It drives the accelerating expansion of the universe and is one of its most mysterious components.
How much of the universe is dark energy? About 68%; together with dark matter, the ordinary matter we know makes up only about 5%.
Cosmic expansion
Cosmic expansion is the stretching of space itself between galaxies, carrying distant galaxies away from one another. Observations show this expansion is accelerating, with dark energy thought to be the driver behind it.
Are galaxies flying apart, or is space expanding? Space itself is stretching, carrying galaxies away from one another — they aren't flying through space.
Does cosmic expansion have a centre? No — from any galaxy, all the others appear to recede, just like dots on the surface of a balloon.
Observable universe
The observable universe is the region centred on Earth from which light has had time to reach us since the universe began, with a radius of about 46.5 billion light-years. Light from beyond this boundary has not yet arrived, so we cannot observe it.
How big is the observable universe? Its radius is about 46.5 billion light-years — larger than the age of the universe times light speed, because space has kept expanding.
What's beyond the boundary? Very likely much more universe; the light from there simply hasn't had time to reach us, so we can't observe it.