P7

P7

1. Coordinates in astronomy

   • The positions of stars are measured by angles seen from earth
   • The two fixed positions to measure from:
     • The Pole Star
     • The celestial equator
   • The two angles used to measure positions in the sky are:
     • Declination
       • From the celestial equator up to the star
         • In degrees
         • 'y'
     • Right ascension
       • The 'how much across' angle
         • In degrees or time
         • 'x'

2. Retrograde motion

   • When an outer planet appears to change direction (therefore 'going east to west'
     • This makes a loop/ squiggle in its track
   • Happens when the Earth 'overtakes' the outer planet

3. Lenses

   • Focal length
     • The distance between the middle of the lens and its focal point
     • The more powerful the lens, the more strongly it converges the parallel rays of light, so the shorter the focal length

4. Telescopes

   • Simple refracting telescopes
     • Made up of two convex lenses
       • Objective lens
         • Less powerful
       • Eyepiece
         • More powerful

5. Diffraction

   • All waves spread out at the edges when they pass through and gap or past an object
   • The narrower the gap, or the longer the wavelength, the more the wave spreads out

6. Aperture

   • The diameter of the objective lens
   • To detect faint sources you need a wide aperture
     • This is because, the bigger the aperture, the more radiation that can get into the telescope and the better the image formed
   • The aperture must be much wider than the wavelength of radiation you want to look at because this way the radiation passes through the aperture with very little diffraction

7. A diffraction grating

   • Has very narrow slits
   • When white light passes through the gaps the different wavelengths of coloured light are all diffracted by different amounts
     • This creates a spectrum of different coloured light
       • Astronomers can use these spectra to analyse the light coming from stars

8. Parallex

   • Measured in arcseconds
     • 1 arcsecond= (1/3600) degrees

9. Distances in space

   • Parsecs
     • 1 parsec= about 3 light years
     • Used to measure distances between stars
   • Megaparsecs
     • Used to measure intergalactic distances

10. Brightness of stars

    • The luminosity/ intrinsic brightness
      • How bright a star would seem if you were right next to it
    • The observed intensity/ brightness
      • How bright a star seems at a distance (from Earth)

11. Cepheid variables

    • Get brighter and dimmer over a period of several days
    • The greater the luminosity, the longer the time between pulses
    • Astronomers can work out the distance to a Cepheid variable by comparing the luminosity and the observed brightness of the star

12. The Curtis Shapley Debate

    • In the 1920s there was a debate about the size and structure of the Universe, alongside what nebulae were and where they were
    • Shapley's Agument:
      • The Universe is just one galaxy
      • Our Sun and our Solar Sytem are far from the centre of the galaxy
      • Nebulae are relatively nearby and part of the Milky Way
    • Curtis' Argument:
      • The Universe is made up of many galaxies
      • The Sun is at or very near the centre
      • The spiral nebulae are other very distant galaxies, separate from the Milky Way

13. The peak frequency

    • The largest type of frequency emitted by a star
    • The higher the temperature, the higher the peak frequency

14. Stars

    • Hot
    • Cold

15. Main sequence star

    • Red giant
      • White dwarf
    • Red supergiant
      • Supernova
        • Neutron star
        • Black Hole

16. The Hertzsprung- Russell Diagram

    • Temperature on the x axis
      • In Kelvin
      • Goes from hot to cold
    • Luminosity on the y axis
      • Measured by a scale, with '1' being the brightness of the Sun
    • You don't see unstable phases, such as supernovas, on the diagram because they happen too quickly

17. Hubble

    • Helped solve the Curtis Shapley debate with his observations of the Andromeda nebula
      • He concluded that the nebula contained many stars and that it was much further away than any other stars in our galaxy
        • Therefore the nebula must be spiral galaxies