5.10.1 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