Definitions of Astronomical Objects (OCR A Level Physics)

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Katie M


Katie M



Definitions of Astronomical Objects

  • Everything that exists is contained within the universe, including:
    • Galaxies
    • Stars
    • Solar systems
    • Planets
    • Planetary satellites
    • Comets
    • Plus many more structures
  • The universe is a large collection of billions of galaxies
    • It is the largest known structure
  • The observable universe is the portion of the universe from which electromagnetic radiation has had time to reach Earth since the formation of the universe
    • The universe is expanding


Hierarchy of the Universe

  • A galaxy is a cluster of billions of stars held together by gravity
    • Earth and the Solar System are located in a spiral galaxy called the Milky Way
    • The Sun takes ~230 million years to complete one revolution of the galaxy
    • Galaxies are moving away from one another; the further apart they are, the faster apart they move
  • Stars fuse hydrogen into helium by nuclear fusion, releasing vast amounts of energy as electromagnetic radiation
    • Stars are formed from gas and dust pulled together under the force of gravity
    • The Sun is the star closest to Earth
    • The Sun is an average-sized small-mass star with a total mass of 2 × 1030 kg
  • Solar systems consist of a star and the gravitationally bound objects that orbit it
    • Earth is the third of eight planets orbiting the Sun
    • The Earth takes 365.25 days to orbit the Sun
    • Planetary satellites, comets and asteroids, as well as planets, orbit the Sun
  • Planetary satellites are bodies that orbit a planet
    • The Moon is a planetary satellite of Earth
    • The Moon takes 27.3 days to orbit the Earth which is the exact time it takes to revolve once on its own axis
    • Artificial satellites, such as GPS and communication satellites, orbiting Earth are planetary satellites


  • Comets are made from ice and rock, and travel in elliptical orbits around the Sun
    • Comets originate from the Oort Cloud
    • Halley’s Comet completes one revolution every 76 years

Scale of the Universe

  • Evidence suggests that the universe was created 13.8 billion years ago and that the observable universe spans a diameter of 93 billion light years
    • One light-year is the distance light travels in one Earth year
    • 1 light year = 9.5 × 1015 m
    • Speed of light in a vacuum, c = 3.00 × 108 m s-1


Exam Tip

You will be given the values of a light year, and the speed of light in your data booklet, so you do not have to learn them. Although it is handy if you do!

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.