The Universe (SQA National 5 Physics): Revision Note

The universe

• The universe is everything that exists - all matter, energy, space, and time

• It contains billions of galaxies, each with billions of stars, many of which have their own planets and moons

Astronomical terms

• Planet

  • An object that orbits a star, but does not emit its own light

• Dwarf planet

  • An object that orbits a star and is similar to a planet, but is not large enough to clear its orbital path of debris

• Moon

  • A natural satellite that orbits a planet or a dwarf planet

• Sun

  • The star at the centre of our solar system

• Asteroid

  • A small, rocky, irregularly shaped object that orbits the Sun

• Solar system

  • A star and the objects that orbit it

• Star

  • A large ball of hot gas that is undergoing nuclear fusion and emits heat and light as a result

• Exoplanet

  • A planet outside the solar system

• Galaxy

  • A large collection of stars

• Universe

  • A large collection of galaxies separated by empty space

Our place in the universe

• The Milky Way is one of many billions of galaxies making up the universe

• The Sun is one of many billions of stars making up the Milky Way

• Other stars in the Milky Way galaxy are much further away from Earth than the Sun is

• Some of these stars also have planets which orbit them, known as exoplanets

Hierarchy of the universe

The Solar System

• The Solar System consists of:

  • the Sun

    • A star which lies at the centre of the solar system and contains 99% of the total mass

  • eight planets

    • Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

  • natural and artificial satellites

    • Natural satellites are objects that orbit planets, e.g. the Moon

    • Artificial satellites are man-made and can orbit any object in space, e.g. the International Space Station (ISS)

  • dwarf planets

    • The gravitational field around a dwarf planet is not strong enough to pull in nearby objects, unlike a planet

  • asteroids and comets

    • Both are small objects which orbit the Sun

    • Asteroids are made of rock and metal and are typically found in the asteroid belt between Mars and Jupiter

    • Comets are made of dust and ice and typically have highly elliptical orbits