Definitions of Astronomical Objects (OCR A Level Physics): Revision Note

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