The Sun leads
The Sun contains about 99.8% of the solar system's mass, so its gravity dominates the orbits of the planets.
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Our solar neighbourhood
Eight planets orbit the Sun. Their true sizes and distances are far too different to fit accurately in this small diagram.
The planets in order
Remember: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
If the Sun were a grapefruit, Earth would be roughly the size of a tiny sprinkle more than 15 metres away. Space is mostly space!
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What holds it together?
Gravity shapes planets, moons, solar systems, stars, and galaxies. It is an attraction between objects with mass and energy.
Orbit means falling
Earth is continually falling toward the Sun while moving sideways fast enough to keep missing it.
One Moon, many phases
The Moon's phases happen because we see different portions of its sunlit half as it orbits Earth—not because Earth's shadow covers it.
A distance made from speed and time
What is a light-year?
A light-year is a unit of distance, not time. It is how far light travels through space in one year—about 9.46 trillion kilometres.
The nearest star system beyond our Sun, Alpha Centauri, is about 4.37 light-years away.
distance = speed × time
Light in vacuum travels about 299,792 kilometres every second. Nothing carrying information is known to travel faster.
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A star's long life
Stars shine because nuclear fusion in their cores releases energy. A star's mass strongly affects how it changes over time.
Stellar nursery
Gravity pulls gas and dust together inside a cold cloud. The growing centre becomes hotter and denser.
Main sequence
For most of its life, a star fuses hydrogen into helium in its core. Our Sun is in this stage.
Giant changes
After core hydrogen becomes scarce, stars expand. Sun-like stars later leave behind dense white dwarfs.
Massive endings
Some much more massive stars explode as supernovae and may leave neutron stars or black holes.
We are star material
Many atoms in your body, including carbon and oxygen, were produced in earlier generations of stars.
Galaxies
Our Sun is one of hundreds of billions of stars in the Milky Way, which is one galaxy among vast numbers.
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How do we know?
Almost everything we know about distant space comes from carefully collecting and decoding light and other signals.
Telescopes
Different telescopes detect radio waves, infrared, visible light, ultraviolet, X-rays, or gamma rays.
Spectra
Splitting light into wavelengths reveals chemical elements, temperature, motion, and other properties.
Spacecraft
Robotic probes photograph, measure, sample, and explore places that are difficult or dangerous for humans to reach.
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Earth, Moon, and Sun cycles
Many sky patterns come from rotation, revolution, and the changing geometry of sunlight.
Day and night
Earth rotates once in about 24 hours. The side facing the Sun experiences daytime while the other side experiences night.
Seasons
Earth's axis is tilted. As Earth orbits the Sun, each hemisphere receives changing sunlight angles and day lengths—not because Earth moves dramatically nearer or farther.
Eclipses
A solar eclipse occurs when the Moon blocks the Sun from some locations. A lunar eclipse occurs when the Moon passes through Earth's shadow.
Tides
The Moon's gravity—and the Sun's—creates patterns of ocean tides as Earth rotates through gravitational bulges.
Months and years
Calendars are built around astronomical cycles, though months no longer match one lunar orbit exactly in many calendars.
The changing sky
Different constellations are visible in different seasons because Earth's nighttime side points in changing directions during its orbit.
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Worlds beyond our solar system
Planets orbiting other stars are called exoplanets. Astronomers usually detect them indirectly.
Transit method
If a planet crosses in front of its star, the star's measured light dips slightly. Repeating dips can reveal orbital timing and planet size.
Wobbling stars
A planet's gravity tugs its star. Tiny back-and-forth changes in the star's light can reveal this motion.
Habitable zone
This is the range where surface liquid water might be possible under suitable conditions. Being in the zone does not prove a world has life.
Atmospheres
When starlight passes through an atmosphere, its spectrum may reveal gases. Several explanations must be tested before claiming evidence of life.
Robotic exploration
Orbiters, landers, rovers, and sample-return missions extend human senses and test instruments in distant environments.
Human spaceflight
Living in space requires air, water, food, temperature control, radiation protection, exercise, and careful teamwork.
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Scale your imagination
Space uses measurements far beyond everyday experience.
1 AU ≈ 150 million kmAstronomical unit
One AU is approximately the average Earth–Sun distance, useful inside solar systems.
1 light-year ≈ 9.46 trillion kmLight-year
A distance useful between stars—not a measurement of time.
look-back time = travel time of lightSeeing the past
We see distant objects as they were when their light began travelling toward us.
universe age ≈ 13.8 billion yearsCosmic history
Measurements of expansion, ancient light, and stellar ages support this approximate age.
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Quick knowledge check
Make your best prediction before revealing the explanation.
Do Moon phases come from Earth's shadow?
Usually no. Phases are the changing portions of the Moon's sunlit half visible from Earth. Earth's shadow causes lunar eclipses.
Why do astronauts appear weightless in orbit?
They and their spacecraft are continuously falling together around Earth, so no floor supports them in the usual way.
Is the Sun burning like a wood fire?
No. The Sun releases energy mainly through nuclear fusion in its core, not ordinary chemical burning.
Does finding a planet in a habitable zone prove life exists there?
No. It identifies one potentially useful condition. Atmosphere, chemistry, geology, stability, and much more also matter.