Option D Astrophysics

• Stellar quantities


  One of the most difficult problems in astronomy is coming to terms with the vast distances between stars and galaxies and devising accurate methods for measuring them
• Objects in the universe

  • Stars
  • Nebulae
  • Galaxies
  • Planetary systems

  
• Stars


  • Have a 'birth', a 'lifetime' and a 'death'
  • For most of their lifetime, emit radiation from nuclear fusion of gas atoms at their core
  • The closest star to Earth is the Sun
  • Many stars exist in pairs (binary stars)
  • All stars are moving but their motion is not obvious to observers on Earth

  
• Constellations


  Named after a well-known group of visible stars within a region, allowing us to locate stars

  
• Stellar clusters


  • Collections of stars formed in the same nebula and moving as a group within their galaxy
  • Globular clusters have a spherical shape due to gravitational forces
  • Open clusters have fewer stars and a less well-defined shape

  
• Galaxies


  • Groups of billions of stars and other matter bound together by gravity
  • Rotate about their centre of mass
  • Classified by their shapes: elliptical, spiral, irregular

  
• Clusters of galaxies


  Approximately spherical groups of tens, hundreds or thousands of galaxies

  
• Superclusters of galaxies


  Among the largest known structures in the universe

  
• Planetary systems


  Planets, comets and other objects orbiting a star (the Sun)
• Planets


  Move in elliptical paths with periods depending on the mass of the star and distance from it
• Comets


  • Much smaller than planets with typically much longer periods and more elliptical paths
  • Composed of dust and ice
  • Develop a 'tail' of particles when close to the Sun
• Nuclear fusion in stars

  Dominant process is the fusion of hydrogen into helium, releasing energy
• Stellar equilibrium


  • Maintained by balance between outward thermal/radiation pressure and inward gravitational pressure
  • Lasts for a long time, the 'main sequence' of a star's life

  
• Binary stars


  Two stars orbiting their common centre of mass
• Astronomical distances


  Measured using trigonometry (stellar parallax), identifying stars of known power, or redshift measurements

  
• Units for astronomical distances

  • Astronomical unit (AU)
  • Light year (ly)
  • Parsec (pc)
• The scale of the universe is extremely vast, with distances ranging from the size of the solar system to the observable universe
• Stellar parallax


  Displacement in the apparent position of a star when viewed from different positions, used to determine its distance
• Stellar parallax has limitations in measuring distances to most stars due to their vast distances
• Units commonly used in astronomy


  • Metres/m
  • Astronomical units/AU
  • Light years/ly
• 1 AU


  1.50 × 1011 m
• 1 ly


  • 9.46 × 1015 m
  • 6.30 × 104 AU
• 1 pc


  • 3.09 × 1016 m
  • 2.06 × 105 AU
  • 3.26 ly
• The scale of the universe
• Stellar parallax and its limitations
• Parallax


  The displacement in the apparent position of an object (compared to its background) when it is viewed from different positions
• Determining distance to stars through stellar parallax

  1. Measure parallax angle, p

  2. Use equation p = 1/d (AU) to calculate distance, d

  3. Directly quote distance in parsecs (pc) as d (parsec) = 1/p (arcsecond)
• For stars further away than about 100 parsecs, the stellar parallax method cannot be applied because the parallax angle is too small (less than 0.01 arcseconds) to measure accurately
• Key concepts about stellar parallax

  • The distance to a 'nearby' star (within 100 pc) can be determined by using geometry and its stellar parallax angle
  • The distance to a star which has a stellar parallax angle of 1 arcsecond is called one parsec (pc)
• The luminosity, L, of a star is defined as the total power it radiates (in the form of electromagnetic waves)
• Luminosity


  Measured in watts, W
• Apparent brightness, b


  The intensity received from a star at the Earth, measured in W/m^2

  
• Apparent brightness, b


  Depends only on the luminosity of the star and its distance away

  
• For very distant stars, the assumption of no absorption in intervening space may lead to inaccuracies when using the apparent brightness equation

  
• Astronomers usually refer to the magnitudes of stars rather than their brightness
• The principles of physics developed on Earth have been applied with success to developing knowledge of the universe
• Advances in observational technologies have led to this increased understanding of the universe
• Stars can be considered to be black bodies and the continuous spectra emitted represented by intensity-wavelength graphs for different surface temperatures

  
• Wien's displacement law


  λmax T = 2.9 × 10–3 mK can be used to calculate the surface temperature, T, of a star if the wavelength at which the maximum intensity is received can be measured
• Absorption spectrum


  The absorption of particular wavelengths of the continuous spectrum emitted by a star's core as the radiation passes through the cooler outer layers