~5,500°C (corona: 1–3 million °C — hotter than surface, still unexplained)
Distance from Earth
~150 million km = 1 AU. Light travel time: 8 min 20 sec.
Composition
~74% hydrogen, ~25% helium, 1% heavier elements
Age / remaining life
4.6 billion years old. ~5 billion years of hydrogen fuel remaining.
The Sun contains 99.86% of all mass in the solar system. Its gravity holds everything from Mercury to the Oort Cloud (~2 light-years away). The Sun is a G-type main-sequence star — perfectly average by stellar standards, which is fortunate: massive stars burn fast and go supernova.
The Sun's internal layers
Core0–25% of radius · ~15 million °C
Nuclear fusion occurs here: 4 hydrogen nuclei fuse into 1 helium nucleus, releasing enormous energy as gamma rays. The core fuses ~620 million tonnes of hydrogen per second. Density is ~150 g/cm³ — 150× water.
Radiative zone25–70% of radius · ~7 million °C
Energy from the core travels outward as photons, but the plasma is so dense that photons are constantly absorbed and re-emitted — a single photon takes 100,000–170,000 years to cross this zone. This is why sunlight you see today was "created" at the Sun's core over 100,000 years ago.
Convective zone70–100% of radius · ~2 million → 5,500°C
Heat is transported by convection — hot plasma rises, cools at the surface, sinks back down in giant cells. Granules (convection cells ~1,000 km wide) are visible at the surface, each lasting ~10 minutes. The Sun's surface (photosphere) is the visible "surface" we see from Earth.
Photosphere500 km thick · ~5,500°C
The visible surface. Sunspots are cooler regions (~3,800°C) where intense magnetic fields suppress convection — they appear dark only by contrast. Sunspot activity follows an 11-year cycle that affects space weather on Earth.
Chromosphere & CoronaAbove surface · up to 3 million °C
The chromosphere (pink during solar eclipses) extends ~2,000 km above the photosphere. The corona — the Sun's wispy outer atmosphere — extends millions of km and is inexplicably hotter than the surface. The solar wind originates here: a stream of charged particles that fills the solar system and drives auroras on Earth.
The Sun in context — stellar classification (Morgan–Keenan)
O-type — Blue supergiant (e.g. Rigel)
50× Sun's mass, 300,000× luminosity. Burns fuel in just millions of years. Ends as a supernova.
30,000–50,000 K
B-type — Blue-white (e.g. Spica)
2–16× Sun's mass. Very bright, short-lived. Responsible for much of a galaxy's UV output.
10,000–30,000 K
A-type — White (e.g. Sirius, Vega)
1.4–2× Sun's mass. Sirius is the brightest star in our night sky. Too short-lived for complex life to evolve.
7,500–10,000 K
F-type — Yellow-white (e.g. Procyon)
~1.2× Sun's mass. Slightly hotter and shorter-lived than the Sun. Some are in habitable zone candidates.
6,000–7,500 K
G-type — Yellow dwarf ← The Sun (also: Alpha Centauri A)
~1× solar mass. Long-lived (~10 billion years). Stable output. Our Sun is 4.6 Gyr old, halfway through its life. Ideal for complex life.
5,200–6,000 K
K-type — Orange dwarf (e.g. Epsilon Eridani)
0.5–0.8× Sun's mass. Dimmer but very long-lived. Increasingly considered top candidates for life-hosting systems.
3,700–5,200 K
M-type — Red dwarf (e.g. Proxima Centauri)
Most common star type (~75% of all stars). 0.08–0.5× solar mass. Extremely long-lived (trillions of years). Prone to intense flares that may sterilise nearby planets.
2,400–3,700 K
Part B · the planets — interactive explorer
Scale visualisation — relative planet sizes (not to orbital distance scale)
Mer
Ven
Ear
Mar
belt
Jupiter
Saturn (rings not shown)
Uranus
Neptune
Inner planets (Mercury–Mars) shown at 10px per Earth diameter. Gas giants scaled at ~1/11th for fit. If Jupiter were truly to scale relative to Earth above, it would be 1,120px wide.
Planetary comparison — diameter relative to Earth
Bar width proportional to diameter. Earth = 12,742 km baseline.
Part C · beyond the planets — the outer solar system
Asteroid belt
Between Mars and Jupiter (~2.2–3.2 AU)
Millions of rocky objects, but total mass only ~4% of Earth's Moon. Despite depictions in films, space between asteroids averages hundreds of thousands of km — the Voyager probes passed through without incident. Ceres (diameter 940 km) is the largest and is classified as a dwarf planet.
Dwarf planets
Pluto, Eris, Ceres, Makemake, Haumea
Pluto (reclassified in 2006) orbits at ~39 AU, takes 248 years per orbit, has 5 moons including Charon (half its size). Eris is slightly larger than Pluto — its discovery triggered the dwarf planet debate. IAU definition: orbits the Sun, roughly spherical, but has NOT cleared its orbital neighbourhood.
Kuiper Belt
30–50 AU — beyond Neptune
A disc of icy objects, the remnants of solar system formation. Contains Pluto, Eris, and thousands of other objects. Short-period comets originate here (those with orbital periods under 200 years). 20× wider than the asteroid belt but 20–200× more massive.
Oort Cloud
~2,000–100,000 AU — the edge of the solar system
A vast spherical shell of icy bodies surrounding the solar system. Long-period comets (orbital periods thousands to millions of years) originate here. Never been directly observed — inferred from comet trajectories. Its outer edge (~100,000 AU = ~1.6 light-years) is roughly halfway to the nearest star.
Part D · Earth's Moon — everything worth knowing
Distance from Earth
384,400 km
Light: 1.3 sec. Varies: 356,500 (perigee) to 406,700 km (apogee)
Diameter
3,474 km
27% of Earth's. Largest moon relative to its planet (except Charon/Pluto)
Orbital period
27.3 days
Sidereal (relative to stars). Lunar month (new moon to new moon): 29.5 days
Rotation period
27.3 days
Same as orbital period — tidally locked. We always see the same face.
Tidal locking — why one face only
Earth's gravity created tidal bulges in the Moon's crust. These bulges created a drag that gradually slowed the Moon's rotation over billions of years until rotation period exactly matched orbital period. Now perfectly synchronised — the far side ("dark side" is a misnomer — it gets as much sunlight as the near side, just never faces us) was unseen until Soviet Luna 3 photographed it in 1959.
Tides — the Moon's gravitational pull
The Moon's gravity pulls Earth's oceans (and to a tiny extent, land) toward it. As Earth rotates, different points pass through this tidal bulge — creating high and low tides (~2 per day). The Sun also creates tides (~46% the strength). When Sun, Moon, and Earth align (new/full moon): spring tides (highest). When at right angles: neap tides (lowest range).
The 8 lunar phases — one complete cycle = 29.5 days
New Moon
Day 0 — invisible.
Waxing crescent
Days 1–6. Right side lit.
First quarter
Day 7. Right half lit.
Waxing gibbous
Days 8–13. Mostly lit.
Full Moon
Day 14–15. Fully lit.
Waning gibbous
Days 16–21. Left side lit.
Last quarter
Day 22. Left half lit.
Waning crescent
Days 23–29. Sliver left.
The Moon's movement through space is complex: It orbits Earth (27.3 days). Earth + Moon orbit the Sun together (365.25 days). The Moon's path around the Sun is always convex — it always curves toward the Sun, never loops backward, despite appearances. The Moon is also very slowly drifting away from Earth at ~3.8 cm/year (measured by laser retroreflectors left by Apollo missions).
Part E · Earth's atmosphere — the five layers
From the ground to space — altitude and what happens at each level
Exosphere700 km to ~10,000 km
The outermost layer — atmosphere so thin it merges imperceptibly into interplanetary space. Molecules can escape Earth's gravity. Satellites in very high orbits (GPS: ~20,200 km) orbit here. No weather phenomena. Temperature "definition" breaks down — molecules travel huge distances between collisions.
Thermosphere80–700 km
ISS orbits here (~400 km). Temperature rises dramatically with altitude (up to 2,500°C) — but air is so thin there are almost no molecules to transfer heat. Auroras (Northern/Southern Lights) occur here — solar wind particles excite atmospheric atoms. Re-entry vehicles experience intense heating here from air compression.
Mesosphere50–80 km
Temperature drops to −90°C at the mesopause (top) — coldest natural temperature on Earth. Most meteors burn up here (friction with air molecules). Noctilucent clouds form near the top at ~82 km — rare, beautiful electric-blue clouds of ice crystals, only visible at twilight. Too high for aircraft, too low for satellites — the "ignorosphere."
Stratosphere12–50 km
Contains the ozone layer (~15–35 km) which absorbs UV radiation — essential for life on Earth. Temperature increases with altitude here (the opposite of the troposphere) because ozone absorbs UV energy. Very dry, no weather. Commercial aircraft fly at ~10–12 km (just below/at the tropopause). Concorde flew at ~18 km in the lower stratosphere. Weather balloons reach ~30–40 km.
Troposphere0–12 km (8 km at poles, 16 km at equator)
All weather happens here. Contains ~75% of the atmosphere's mass and ~99% of its water vapour. Temperature decreases ~6.5°C per km. Mount Everest (8,849 m) is near the top. Beyond the tropopause (the boundary), temperature stops falling — this inversion "caps" convection and prevents weather systems from penetrating into the stratosphere.
The Kármán line (100 km) is the internationally recognised boundary of space — the point where the atmosphere is too thin for aircraft to generate lift and orbital mechanics take over.
Part F · key space distances — the numbers you need
Distances from Earth's surface — drag to explore
Part G · the universe's composition — what everything is made of
Composition of the observable universe
Dark energy — 68%
Dark matter — 27%
Ordinary matter — 5%
Of that 5% ordinary matter…
Intergalactic gas
~72%
Intragalactic gas & dust
~18%
Stars
~8%
Everything else (planets, you, etc.)
<1%
Everything you can see, touch, and measure — all galaxies, stars, planets, people, and light — accounts for just ~5% of the universe's total energy content. Dark matter doesn't interact with light (we detect it only via gravity). Dark energy is the name given to the force driving the universe's accelerating expansion. Both are among the deepest unsolved problems in physics.
The cosmic calendar — if the universe's age were one year
Carl Sagan's concept: compress 13.8 billion years into 1 calendar year. Each second = ~437 years.
Jan 1
Big Bang — the universe begins
13.8 billion years ago. Time, space, and all matter created. Temperature: 10³² K. Cosmic inflation expands the universe by a factor of 10²⁶ in a fraction of a second.
Jan 22
First stars ignite
~13.5 billion years ago. Population III stars — enormous, metal-free, extremely short-lived. Their explosions seeded the universe with heavier elements forged in stellar cores.
Mar 16
Milky Way begins to form
~12 billion years ago. The Milky Way is relatively old as galaxies go — it has been assembling via mergers for most of cosmic history. Its central supermassive black hole (Sagittarius A*, 4 million solar masses) likely formed early.
Sep 2
Sun and solar system form
4.6 billion years ago. A giant molecular cloud collapses. The Sun ignites. Planets form from the protoplanetary disc within ~100 million years. Earth's Moon forms from a giant impact with a Mars-sized body (the "Theia" hypothesis) within the first ~50 million years.
Sep 14
First life on Earth appears
~4 billion years ago. Microbial life (likely chemosynthetic bacteria) in ancient oceans. Life appeared remarkably quickly after Earth cooled enough to have liquid water — within the first ~500 million years.
Dec 17
Cambrian explosion
540 million years ago. Multicellular animal life diversifies explosively. Most major animal body plans appear in the fossil record within ~20 million years.
Dec 25
Dinosaurs go extinct (K-Pg event)
66 million years ago. Chicxulub impactor (~10 km diameter) hits the Yucatán Peninsula. 75% of all species extinct. Mammals diversify into the vacated ecological niches.
Dec 31 11:59:59
All of recorded human history
The last ~10,000 years (agriculture, writing, all civilisations, the entire scientific enterprise) occupies the final 23 seconds of the cosmic year. Modern Homo sapiens (~300,000 years old) appears at 11:58:43 pm. You exist in the final fraction of a second.
Part H · landmark space missions — 10 missions that changed everything
Part I · SpaceX — the private space revolution
Founded 2002 by Elon Musk with $100 million of his own capital
SpaceX entered an industry dominated by governments and legacy contractors, with the explicit goal of making humanity multiplanetary. Its core innovation was vertical integration (building almost everything in-house) and reusability — the two factors that have driven launch costs from ~$50,000/kg (Space Shuttle) toward a target of <$100/kg with Starship. As of 2025, SpaceX conducts more orbital launches per year than all other countries combined.
Launch vehicles
Key missions & programmes
Part J · interactive calculators — from other modules
1. Venus is closer to Earth than Mars, yet Mars has been visited by more spacecraft. Why is Mars a better destination for exploration?
Venus is hellish in ways Mars is not. Venus's surface temperature is ~465°C — hot enough to melt lead — due to an extreme greenhouse effect. The atmosphere is 90× denser than Earth's (equivalent to 900m underwater) and composed of carbon dioxide with sulphuric acid clouds. Every spacecraft that has landed on Venus has been destroyed within 2 hours by the pressure and heat. Mars, while cold (avg −63°C) and with a very thin atmosphere (1% of Earth's), is survivable for robotic missions. Rovers (Curiosity, Perseverance) have operated for years. Mars also has evidence of ancient water (dried riverbeds, polar ice caps) making it interesting for life-detection. There are also serious proposals for human missions to Mars. A human mission to Venus would require floating in the upper atmosphere — the surface is inaccessible with current technology.
2. Why does the Moon always show the same face to Earth, and what would you see if you stood on the far side?
Tidal locking: Earth's gravity created tidal bulges in the Moon's solid crust. The gravitational pull on these bulges created a torque that gradually slowed the Moon's rotation over billions of years until its rotation period exactly equalled its orbital period (27.3 days). At that point, the same face perpetually points toward Earth — the system is in its lowest energy state, and there's no longer a torque to change it. From the far side of the Moon, you would never see Earth — it would always be below your horizon. You would see the stars and Sun in a completely black sky (no atmosphere means no scattered light). The far side has a much more heavily cratered surface — the near side's mare (dark volcanic plains) are largely absent. China's Chang'e 4 became the first mission to land on the far side in January 2019. Communication requires a relay satellite positioned at the Earth-Moon L2 Lagrange point because the Moon itself blocks direct radio contact.
3. If you were on the ISS (400 km altitude), would you experience weightlessness because you're "far from Earth's gravity"?
No — this is one of the most common misconceptions in space science. At 400 km altitude, Earth's gravity is still about 88–89% of its surface value. You feel weightless on the ISS not because gravity is weak, but because you are in continuous freefall. The ISS (and everything in it) is falling toward Earth at the same rate — but moving horizontally fast enough (~27,600 km/h) that Earth's curved surface keeps falling away beneath it at the same rate it falls toward Earth. This is what an orbit is: perpetual freefall in which you miss the ground because you're moving sideways so fast. Astronauts experience microgravity because every object in the ISS — the station itself, their bodies, their breakfast — is all falling at exactly the same rate. There is no relative gravitational force between them. True weightlessness (absence of gravity) doesn't exist in the solar system — gravity has infinite range, just diminishing with distance.
4. Saturn has rings — why doesn't every gas giant? And what are the rings actually made of?
All four gas giants actually have rings — but Saturn's are uniquely prominent and bright. Jupiter, Uranus, and Neptune all have ring systems, but they are faint and dark (composed of dust and dark rocky material). Saturn's rings are made primarily of water ice particles ranging from tiny grains to chunks several metres across — and ice is highly reflective (albedo ~0.9 vs dark rocky material's ~0.05). This is why Saturn's rings are brilliant white and visible from Earth with a basic telescope while the other planets' rings are nearly invisible. The rings likely formed from the tidal disruption of an icy moon (or captured comet) that came within Saturn's Roche limit — the distance at which tidal forces overcome an object's self-gravity. Saturn's rings are only ~10–100 metres thick despite being 282,000 km wide — if scaled to the thickness of a sheet of paper, the rings would be several kilometres across. Saturn's rings are also surprisingly young — estimated at only 10–100 million years old, meaning dinosaurs existed before Saturn had its current ring system.
5. How far away is the nearest star, and how does that distance put the solar system's scale in perspective?
Proxima Centauri is 4.24 light-years away — ~40 trillion km, or ~268,000 AU. To put this in perspective: if you shrank the Sun to the size of a grapefruit (13 cm), Earth would be a grain of sand 14 metres away, Jupiter a marble 73 metres away, and Neptune a pea 440 metres away. The Oort Cloud's outer edge would be about 50 km away. Proxima Centauri, at the same scale, would be 3,800 km away — roughly the distance from London to New York. The nearest star is as far from our solar system as a different continent is from another, while our entire solar system (planets) fits on a football pitch. The Voyager 1 probe, launched in 1977 and travelling at ~17 km/s (the fastest human-made object at the time), is currently ~24 billion km from Earth — which sounds enormous but is only ~0.000025% of the way to Proxima Centauri. At its current speed, it would take ~73,000 years to reach there (it's not heading that direction).
6. What is the "Fermi Paradox" and why does the scale of the universe make it so puzzling?
The Fermi Paradox is physicist Enrico Fermi's famous question: "Where is everybody?" Given the universe contains ~2 trillion galaxies, each with ~100–400 billion stars, many with planetary systems, and the universe is 13.8 billion years old — there has been ample time and space for intelligent civilisations to arise, spread, and make themselves detectable. Even at 1% of the speed of light (far slower than theoretical limits), a civilisation could colonise the entire Milky Way in ~10 million years — a blink of cosmic time. Yet we detect nothing: no signals, no megastructures, no visitors. Proposed resolutions include: (1) The Great Filter — some evolutionary step is extraordinarily rare, and either we have already passed it (life, eukaryotes, multicellular life, intelligence — take your pick), or it lies ahead of us; (2) The civilisation lifetimes hypothesis — intelligent civilisations self-destruct before going interstellar; (3) The "Dark Forest" hypothesis — civilisations deliberately hide; (4) We simply haven't looked hard or long enough. The silence of the cosmos is one of the deepest unsolved questions in all of science.