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unit 6
6.1 earth, the moon, and the solar system 6.2 stars and the universe
6.2

stars and the universe

6.2.1 the sun

  • the sun is a medium-mass star
  • it is made mainly of hydrogen and helium
  • energy is released by nuclear fusion of hydrogen into helium
  • energy is emitted as visible light, infrared, and ultraviolet
  • the sun contains most of the mass of the solar system
  • distances in space are measured in light-years
  • a light-year is the distance travelled by light in one year
  • 1 light year ≈ 9.5 × 1015 m

6.2.2 stars

  • galaxies are made of billions of stars
  • the sun is a star in the milky way galaxy
  • the milky way has a diameter of about 100,000 light years
  • the sun is the closest star to earth

life cycle of stars

  • stars form from clouds of gas and dust containing hydrogen
  • gravity causes the cloud to collapse forming a protostar
  • temperature increases until nuclear fusion begins
  • a stable star forms when outward pressure balances gravity
  • all stars eventually run out of hydrogen fuel
  • average stars expand into red giants
  • massive stars expand into red supergiants
  • red giants form planetary nebulae leaving white dwarfs
  • red supergiants explode as supernovae
  • supernovae leave neutron stars or black holes
  • nebulae can form new stars

6.2.3 the universe

  • the universe is expanding
  • expansion is observed using redshift
  • redshift is an increase in wavelength of light from receding galaxies
  • speed of galaxies can be found from change in wavelength
  • brightness of supernovae can be used to estimate distances
  • the hubble constant describes the rate of expansion
  • h0 = v / d
  • h0 ≈ 2.2 × 10-18 s-1
  • 1 / h0 estimates the age of the universe
  • cosmic microwave background radiation exists throughout space
  • cmbr formed shortly after the universe began