5.5.1 Stars

• Explain the term ‘Planets‘?

  An object in orbit around a star with three important characteristic:

  • has a mass large enough for its own gravity to give it a round shape (unlike irregular shape asteroids.)

  • has no fusion reactions (unlike a star).

  • has cleared orbits of most other objects (asteroids, etc).
• Explain the term ‘Planetary satellites’?

  A body in orbit around a planet. Moons and mad-made satellites.
• Explain the term ‘Comets’?

  Small irregular bodies made up of ice, dust, and small pieces of rock In an (often highly eccentric elliptical) orbit around the Sun - as they approach Sun, develop tails.
• Explain the term ‘Solar Systems’?

  Consisting of a star and at least one planet in orbit around it (e.g. our solar system contains Sun and all the objects/planets that orbit it.)
• Explain the term ‘Galaxies’?

  A collection of stars and interstellar dust and gas bound together by their mutual gravitational force.
• Explain the term ‘Universe‘?

  Everything that exists within space and time.
• Formation of star

  Nebulae -> Protostar -> Main sequence star ->

  • (mass greater) -> Red supergiant -> black hole or Neutron star

  • (low mass) -> Red giant -> White dwarf -> Black dwarf
• Describe the formation of a star from interstellar dust and gas in terms of gravitational collapse?

  Dust and gas that is present will slowly come together, through gravitational attraction between individual atoms, form denser clumps of matter. Gradually become more dense as more matter is attracted due to inward movement of material - gravitational collapse.
• Gravitational collapse

  The inward movement of material in a star due to gravitational force caused by its own mass.

  Occurs in a mature star when the internal gas and radiation pressure can no longer support the star’s own mass.
• Radiation pressure

  The momentum of photons released in fusion reactions (core of star), and acts outwards (in the direction of the energy flow) to counteract the pressure from g force pulling the matter in the star inwards.
• Gas pressure

  In stars, the pressure of the nuclei in the core pushing outwards and counteracting the gravitational force pulling the matter in the star inwards.
• Describe the formation of a star from interstellar dust and gas in terms of fusion of H into He? 


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• Planetary nebula

  The outer layers of a red giant that have drifted off into space, leaving the hot core behind at the centre as a white dwarf.