Topic 7 ~ Astronomy

• Weight


  W = mg, Weight (Newtons, N) = mass (kilograms, kg) x acceleration due to gravity (metres per second, m/s)
• The greater the value of g


  The greater the weight of the object
• The greater the mass of the planet


  The greater value of g on that planet
• More mass = stronger gravitational pull = higher value of gravitational acceleration = higher value of g
• Our Solar System consists of

  • The Sun
  • Eight planets that orbit the sun
  • Natural satellites orbit the planets (e.g. the Moon for Earth)
  • Dwarf planets orbit the sun (including Pluto, Ceres)
  • Asteroids and comets
• Planets in our Solar System

  • Mercury
  • Venus
  • Earth
  • Mars
  • Jupiter
  • Saturn
  • Uranus
  • Neptune
• Smaller planets are made of primarily rock, then the larger planets are primarily gas
• All planets orbit the Sun on the same plane
• All planets rotate, just at different speeds
• Some planets rotate in the opposite direction or on a skewed axis to the other planets, and this may be due to past collisions throwing its axis off balance
• Larger planets have rings, as their gravitational field is so strong it attracts debris
• Geocentric model


  Initial model suggested Earth was at the centre, the planets, our moon, and the sun, orbited the Earth
• Heliocentric model


  Model with the sun at the centre
• Evidence for heliocentric model

  • Mars' "retrograde motion"
  • Galileo observing moons orbiting Jupiter showed not everything orbited the Earth
  • Kepler showed that the planets orbited in ellipses, and not circles
• As the planet orbits the sun

  • The gravitational force causes the planet to change direction constantly (it moves in a circle around the sun)
  • The speed of the planet is constant
  • The velocity is always changing
• For a stable orbit:


  • If the planet moves closer to the sun, (i.e. its orbital radius decreases)
  • The gravitational attraction to the sun increases
  • The orbital speed of the planet increases
• Red shift


  Light appears red shifted from galaxies which are moving away from Earth
• The emission spectrum shows the different wavelengths emitted from a star, with black absorption lines showing absorbed wavelengths. The spectra from distant galaxies show the black lines in the emission spectra shifted towards the red end of the emission spectrum, showing a red shift
• The red shift increases as the distance away from the earth increases which is evidence of an expanding universe
• Big Bang


  The start of the universe, where galaxies on the surface of an un-stretched balloon expand as the universe expands
• The distance between galaxies expands, so light from a galaxy has its wavelength "red-shifted" as it appears to move away from us
• As wavelength appears to get larger (more in the red-end of the visible spectrum), frequency appears to decrease, as each time a wavelength is emitted, the source (galaxy) is further away
• Evidence for the Big Bang

  • Red Shift
  • CMB (Cosmic Microwave Background radiation)
• The CMB proves that the hot young universe has cooled and expanded since
• The Big Bang accounts for all the experimental evidence, so it is the most accepted model currently
• Steady State Theory


  As universe expands, matter is constantly being created so there is a constant density of matter in the universe
• The red shift supports the Steady State Theory
• The Steady State Theory suggests that the observable universe is the same at any time and any place
• Showing that different stars were present at different eras of the universe shows that the universe has evolved and not stayed the same, so the Steady State Theory cannot be correct
• The Steady State Theory does not account for CMB
• Life Cycle of Star

  1. Dust and gas cloud is present in a galaxy

  2. Interstellar matter clump together to form clouds called nebulae. It is in these clouds/nebulae that stars are formed

  3. The gravitational attraction between the gas/dust particles draws them together

  4. The cloud becomes more concentrated, as the particles get closer

  5. The temperature and pressure of the cloud increases as the particles get pushed so close together

  6. Eventually the pressure gets so great that the gas /dust particles are able to fuse together

  7. Fusion occurs as the light (mainly hydrogen gas) nuclei fuse together to form helium nuclei

  8. This creates a large amount of energy

  9. This release opposes the collapsing of the cloud due to gravity

  10. Eventually the star runs out of gas to fuse

  11. If the star is massive, it will collapse, increasing the pressure + temperature of the core, meaning heavier elements can fuse. Once all the fusion has happened, it is too massive to be stable, so the star collapses, rebounds on its centre and produces a supernova

  12. What remains is either a neutron star or black hole

  13. If the star is normal-sized, the same process happens, less fusion occurs however (less fuel to fuse), the star swells to produce a red giant star fusing heavier elements, and once all reactions are over the star contracts and cools into a white dwarf
• Observations can use any wavelength in the EM spectrum
• To measure X rays, Gamma, UV, these telescopes need to be outside of the atmosphere, as the atmosphere does not allow these to reach the ground
• Early refracting telescopes evolved to reflecting telescopes, which could be made larger and cheaper (as refracting lenses are expensive and heavy)
• Technological advances allowed other parts of the EM spectrum to also be measured and collected