The Solar System

• The solar system consists of our sun, eight planets with their moons, dwarfs planets, asteroids, comets, and other celestial bodies.
• The planets are orbiting around the sun in paths known as elliptical orbits.
• The four inner planets (Mercury, Venus, Earth, Mars) also referred to as terrestrial planets, are made primarily of rock and metal.
• The four outer planets (Jupiter, Saturn, Uranus, Neptune), called gas giants and ice giants, are composed largely of gases and hold numerous moons.

Stars and Galaxies

• A star, like the Sun, is a hot sphere of gas that emits light and energy generated through nuclear reactions.
• Stars of different sizes, ages and temperatures have different colours. The hottest stars are white or blue, while the cooler stars are red.
• All the stars we see at night are located in our own galaxy, the Milky Way.
• There are billions of galaxies in the Universe, some of which are spiral-shaped like the Milky Way, while others are elliptical or irregular in shape.

The Universe

• The universe is the entirety of space and time, and all matter and energy in it.
• The universe is believed to have originated around 13.8 billion years ago in the Big Bang.
• The universe has been expanding since its formation, a concept known as the expanding universe theory.
• Dark matter is hypothesised to make up about 85% of the matter in the universe.
• Dark energy, a mysterious form of energy, is thought to be responsible for the acceleration in the expansion of the universe.

Observational Astronomy

• Telescopes are used to observe distant objects in space, with some collecting visible light and others collecting different types of electromagnetic radiation.
• Radio astronomy uses large antennas to detect radio waves from space.
• Space probes are robotic spacecraft sent away from Earth to explore other planets and celestial bodies up close.

Key Astronomers

• Nicolaus Copernicus first proposed that the Earth revolves around the sun, contradicting the long-held belief that Earth was the centre of the Universe.
• Galileo Galilei is known for improvements to the telescope that enabled detailed observations of the planets, leading him to support Copernicus's model.
• Edwin Hubble discovered that there are many galaxies outside the Milky Way and provided substantial evidence of the expanding universe.