astrophysics

• discuss one problem associated with the determination of distance from the Earth to a quasar

  • distant quasars are very faint due to inverse square law
  • so Hubble's law isn't applicable over such large distances
• give two reasons why the secondary mirror in the Cassegrain telescope affects the clarity of the image

  • secondary mirror blocks light so less light hits objective mirror
  • light diffracted passing secondary mirror affects image
• define absolute magnitude, M


  the apparent magnitude an object would have at a distance of 10 parsecs away from Earth
• What is the initial stage of a star similar to our Sun?

  Protostar
• How does a protostar evolve on the HR diagram?

  It gradually heats up, moving left
• What occurs when a protostar reaches a sufficient temperature?

  It becomes a main sequence star
• What happens to a main sequence star after it exhausts hydrogen in its core?

  It moves up and right on the HR diagram
• What is the next stage after a main sequence star becomes a red giant?

  Red giant
• How does a red giant compare to a main sequence star?

  Brighter and cooler than a main sequence star
• What happens when a red giant uses up all the helium in its core?

  It becomes a white dwarf
• How does a white dwarf compare to a main sequence star?

  Hotter and dimmer than a main sequence star
• What is the overall path of a star similar to our Sun on the HR diagram?

  Protostar to main sequence to red giant to white dwarf
• What is the purpose of the HR diagram in stellar evolution?

  To track the evolution of stars
• Why is nuclear fusion important in a star's life cycle?

  It allows the star to produce energy
• How does the temperature affect a star's stage in evolution?

  Higher temperatures enable nuclear fusion
• X-ray telescopes compared to optical telescopes in terms of structure, positioning and use, as well as comparisons of resolving and collecting powers

  • structure - parabolic mirrors used to reflect X-ray waves
  • positioning - space-based in a high Earth-orbit as X-rays absorbed by atmosphere 
  • use - discovering supermassive black holes
  • higher resolving power than optical telescopes due to having shorter λs
  • lower collecting power than optical telescopes as they have smaller objective diameters
• U-V telescopes compared to optical telescopes in terms of structure, positioning and use, as well as comparisons of resolving and collecting powers

  • structure - constructed using a primary concave mirror and a secondary convex mirror
  • positioning - space-based in a low Earth-orbit as U-V rays absorbed by ozone
  • use - analysing quasars, observing star formation rate in distant galaxies
  • higher resolving power than optical telescopes due to having shorter λs
  • similar collecting power to optical telescopes as they have similar diameters
• I-R telescopes compared to optical telescopes in terms of structure, positioning and use, as well as comparisons of resolving and collecting powers

  • structure - large concave mirrors which focus radiation to a detector, require coolants to prevent interference from heat
  • positioning - mostly space-based but on Earth, high elevations in dry places are required (to avoid water vapour)
  • use - finding exoplanets, observing after effects of gamma ray bursts
  • lower resolving power as they have longer λs
  • similar collecting power to optical telescopes as they have similar diameters
• similarities between radio telescopes and optical telescopes

  
  

  
  

  similarities:

  • similar to design of reflecting telescopes (parabolic concave surface reflects radio waves to an aerial)
  • both are ground-based
  • both can be manoeuvred to focus on different sources of radiation
• difficulty in the direct detection of exoplanets

  • difficult as light from exoplanets tends to be obscured by the much brighter star they orbit and the planet and star are too close to be able to resolve them
• sketch a light curve for an exoplanet transiting in front of its star


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• sketch a typical light curve for eclipsing binary star systems

  explain, in terms of the movement of the two stars, how this light curve is produced

  • lowest value of apparent magnitude occurs when both stars can be seen
  • first (smaller) dip occurs when the hotter star is in front of the cooler star
  • second (larger) dip occurs when the cooler star is in front of the hotter star

  
• detection techniques for exoplanets: radial velocity (variation of doppler shift):

  • when a planet orbits a star, the gravitational pull causes the star to “wobble”
  • this causes a periodic variation in Doppler shift in the star’s spectral lines 
  • this Doppler shift is used to calculate the radial velocity of the star as it moves about the centre of mass
• detection techniques for exoplanets: transit method

  • works by detecting a dimming in the star's apparent magnitude as an exoplanet passes in front of the star
  • repeated transits are used to determine the existence of an exoplanet
• why is red shift or z quoted as negative?

  • wavelength decreases if the source is approaching
  • so red shift is treated as negative if source is approaching and positive if it is receding
• what is my name

  hamdi