Explore the Universe: Galaxies, Solar Systems, and Cosmic Structure

A galaxy is a massive collection of billions of stars, gas, dust, and dark matter all held together by gravity. Our own galaxy is called the Milky Way, a barred spiral galaxy that contains between 100 billion and 400 billion stars. Our solar system sits about two-thirds of the way from the center, in one of the Milky Way's spiral arms.

Galaxies come in three main shapes. Spiral galaxies have a pinwheel shape with curved arms extending from a bright central bulge the Milky Way and Andromeda are both spiral galaxies. Elliptical galaxies are smooth, oval-shaped systems with older stars and very little gas or dust for new star formation. Irregular galaxies have no defined shape and often result from collisions between galaxies.

Galaxies are not scattered randomly they group together into galaxy clusters. The Milky Way belongs to a cluster called the Local Group, which contains about 50 galaxies including the Andromeda and Triangulum galaxies.

A solar system consists of a star and all the objects that orbit it, including planets, moons, asteroids, and comets. Our solar system is centered on the Sun, a medium-sized star that provides heat and light through a process called nuclear fusion where hydrogen atoms combine to form helium, releasing enormous energy.

The eight planets orbit the Sun in this order from closest to farthest: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. A helpful mnemonic is "My Very Educated Mother Just Served Us Nachos." The inner four planets (Mercury, Venus, Earth, Mars) are small and rocky. The outer four (Jupiter, Saturn, Uranus, Neptune) are enormous gas or ice giants with no solid surface.

The force that keeps every planet in its orbit an elliptical (slightly oval) path is gravity, the attractive force between objects with mass. Without gravity, planets would fly off into space in straight lines.

Beyond the planets, you will find other important regions. The asteroid belt is a zone of rocky debris between Mars and Jupiter. The Kuiper Belt extends beyond Neptune and contains icy bodies including the dwarf planet Pluto. The Oort Cloud is a distant, spherical shell of icy bodies at the outermost edge of our solar system and is the source of many long-period comets.

A moon (natural satellite) is any natural object that orbits a planet due to the planet's gravity. Earth has one moon, but Jupiter and Saturn each have dozens. A comet is an icy body that develops a glowing tail when it travels close to the Sun and its ice vaporizes. A meteoroid is a small rocky body in space; when it enters Earth's atmosphere and burns up, it becomes a meteor ("shooting star"); if it lands on Earth's surface, it is called a meteorite.

Earth's rotation spinning on its own axis once every 24 hours creates day and night. Earth's revolution orbiting the Sun once every 365.25 days defines one year. Earth's seasons are caused by its tilted axis (about 23.5°), not by its distance from the Sun.

Universe: All of space, time, matter, and energy that exists including every galaxy, star, and planet. It is the largest structure of all.

Galaxy: A huge collection of billions of stars, gas, dust, and dark matter held together by gravity. The Milky Way is your home galaxy.

Solar System: A star and all the objects that orbit it, including planets, moons, asteroids, and comets. Our solar system orbits the Sun.

Light-year: A unit of distance (not time) equal to the distance light travels in one year approximately 9.46 trillion kilometres. Astronomers use it because space is so vast that kilometres alone would produce unmanageably large numbers.

Orbit: The elliptical (slightly oval) path that a planet or other object follows around another object due to gravity.

Spiral Galaxy: A galaxy with a pinwheel shape featuring curved arms extending from a bright central bulge. The Milky Way and Andromeda are spiral galaxies.

Elliptical Galaxy: A smooth, oval-shaped galaxy containing mostly older stars and very little gas or dust, meaning very little new star formation occurs.

Irregular Galaxy: A galaxy with no defined shape, often caused by gravitational collisions between galaxies.

Nebula: A large cloud of gas and dust in space where new stars and planets can form. Gravity causes the material to collapse and eventually ignite into a star.

Star Cluster: A group of stars held together by gravity and moving through space as a unit.

Gravity: The attractive force between objects with mass. Gravity keeps planets in orbit around the Sun and holds galaxies together.

Nuclear Fusion: The process that powers the Sun hydrogen nuclei combine under extreme heat and pressure to form helium, releasing light and heat energy.

Asteroid Belt: A region of rocky debris orbiting the Sun between Mars and Jupiter, separating the inner rocky planets from the outer gas giants.

Kuiper Belt: A disk-shaped region beyond Neptune containing icy bodies, comets, and dwarf planets such as Pluto, Eris, and Makemake.

Oort Cloud: A vast, spherical shell of icy bodies surrounding the outermost edges of our solar system, thought to be the source of long-period comets.

Comet: An icy body from the outer solar system that develops a glowing coma and tail when solar heat vaporizes its ice as it approaches the Sun.

Meteor / Meteoroid / Meteorite: A meteoroid is a rocky body in space; a meteor is what you see when it burns up in Earth's atmosphere (a "shooting star"); a meteorite is any piece that survives and lands on Earth's surface.

Moon (Natural Satellite): A natural object that orbits a planet due to the planet's gravity. Moons do not produce their own light.

Rotation: A planet spinning on its own axis. Earth's rotation takes about 24 hours and creates the cycle of day and night.

Revolution: A planet's orbital journey around the Sun. Earth's revolution takes about 365.25 days and defines one year.

Elliptical Orbit: A slightly oval-shaped path that planets follow around the Sun, as discovered by Johannes Kepler. Earth's orbit is nearly circular but is technically an ellipse.

Local Group: The galaxy cluster to which the Milky Way belongs, containing roughly 50 galaxies including the Andromeda and Triangulum galaxies.

Dwarf Planet: An object that orbits the Sun and is roughly spherical but has NOT cleared its orbital neighborhood of other debris. Pluto is the most well-known dwarf planet.

Proxima Centauri: The closest known star to Earth, located approximately 4.2 light-years away and part of the Alpha Centauri star system.

You can strengthen your understanding by practicing these key skills. Try ranking structures from smallest to largest: planet solar system galaxy galaxy cluster supercluster. You can also practice naming the eight planets in order using the mnemonic "My Very Educated Mother Just Served Us Nachos."

Challenge yourself to identify galaxy types from descriptions if it has curved arms, it is spiral; if it is smooth and oval, it is elliptical; if it has no shape, it is irregular. You can also practice explaining why Pluto is a dwarf planet: it orbits the Sun and is spherical, but it has not cleared its orbital neighborhood in the Kuiper Belt.

As you continue your science journey, you will explore how Earth's surface changes over time in Plate Tectonics, Global Patterns, Introduction, building on your understanding of Earth as a planet in our solar system.

This topic does not require specific prerequisite topics you are beginning your exploration of space science here. The concepts you learn about the universe, galaxies, and solar systems will form the foundation for understanding Earth's place in space and how planetary processes connect to what happens on Earth's surface.

Your next step after mastering space structure is to explore Plate Tectonics, Global Patterns, Introduction, where you will see how the same forces that shape planets also reshape Earth's crust over millions of years.

Understanding the universe's structure connects directly to your next topic: Plate Tectonics, Global Patterns, Introduction. In that topic, you will zoom in from the cosmic scale to Earth itself and discover how the planet's surface is constantly moving and changing. Knowing that Earth is a rocky inner planet in our solar system gives you important context for understanding why Earth behaves the way it does geologically.

The progression from space science to Earth science shows you how scientists study systems at every scale from the entire universe down to the ground beneath your feet. The same force of gravity that holds galaxies together also keeps Earth's layers in place and drives the movement of tectonic plates.