Stars And The UniverseCopy

O Level / IGCSE Physics Cheat Sheet – 6.2 The Sun, Stars, and the Universe

6.2.1 The Sun as a Star

• Sun Characteristics:
  • Medium-sized star (main sequence).
  • Composed mainly of hydrogen and helium.
  • Emits energy mostly in infrared (IR), visible, and ultraviolet (UV) parts of the electromagnetic spectrum.
• Source of Energy:
  • Powered by nuclear fusion.
  • In stable stars: Hydrogen nuclei fuse into helium, releasing massive energy.
  • Fusion occurs at extremely high temperatures and pressures in the star’s core.

6.2.2 Stars

• Basic Star & Galaxy Facts:
  • (a) A galaxy contains billions of stars.
  • (b) The Sun is a star in the Milky Way galaxy.
  • (c) Other stars in the Milky Way are much farther from Earth than the Sun.
  • (d) Light-year = Distance light travels in 1 year in a vacuum (~9.46 × 10¹² km)
• Life Cycle of a Star:
  1. Formation:
     • Stars form from interstellar clouds of gas and dust, mainly hydrogen.
  2. Protostar:
     • Collapsing cloud → temperature increases due to gravitational attraction.
  3. Stable Star:
     • Fusion begins when core temperature is high enough.
     • Star becomes stable when gravity in = fusion pressure out.
  4. Fuel Exhaustion:
     • Star eventually runs out of hydrogen.
  5. Red Giant / Red Supergiant:
     • Low-mass stars → expand into red giants.
     • Massive stars → become red supergiants.
  6. End Stages:
     • Red giant → forms planetary nebula with a white dwarf at the center.
     • Red supergiant → explodes in supernova:
       • Leaves behind neutron star or black hole.
       • Produces a nebula with heavier elements.
  7. Rebirth:
     • The supernova nebula may lead to new star formation with orbiting planets.

6.2.3 The Universe

• Milky Way Galaxy:
  • One of billions of galaxies in the Universe.
  • Approximate diameter: 100,000 light-years.
• Redshift:
  • Redshift = Increase in observed wavelength of EM radiation from an object moving away.
  • Indicates that stars and galaxies are receding from us.
  • The greater the redshift, the faster the object is moving away, and the farther it is.
• Evidence for Big Bang Theory:
  • Redshift shows galaxies are moving away in all directions.
  • Suggests the Universe is expanding.
  • Supports the idea that everything started from a single point in a Big Bang.