P7- Observing the Universe

A brief summary of the physics module P7- Observing the Universe, from the OCR 21st Century Science textbook

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  • Created by: R_Hall
  • Created on: 27-05-11 10:32

Observing the sky with a naked eye

  • The sun appears to travel east-west across the sky once in every 24 hrs. The stars appear to travel east-west across the sky once in every 23hrs and 56 mins. The moon appears to travel east-west across the sky once in every 25 hrs. The naked eye planets (Mercury, Venus, Mars, Jupiter and Saturn) appear to move with the stars but change their positions in complicated patterns
  • Positions of astronomical objects are measured in terms of angles as seen from the Earth
  • A Sidereal Day (rotation of 360) of the Earth is different from a Solar Day due to the orbital movement of the Earth. A sidereal day in 4 minutes less than a solar day
  • Different stars are seen in the night sky at different times of the year, due to the movement of the Earth
  • Planets move in complicated patterns relative to the fixed stars
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  • Solar Eclipse- The Moon blocks the Sun's light
  • Lunar Eclipse- The Moon moves into the Earth's shadow
  • Both the Earth and the Moon have shadows. Because the sun is an extended source of light, these shadows do not have hard edges
  • The region of total darkness= Umbra. Region of partial darkness= Penumbra
  • The Moon orbits the Earth once a month, but you do not see a lunar eclipse every month.
  • The moon's orbit is tilted relative to the plane of the Earth's orbit by about 5 degrees. Usually, the Earth, Sun and Moon are not in line= no eclipse
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How does a telescope work?

  • Convex/ Converging lenses bring parallel light to a focus
  • More powerful lenses of the same material have more curved surfaces
  • Power of a lens  (dioptres) = 1/ focal length (metres)
  • Astronomical objects are so distant that light from them is effectively parallel
  • A simple telescope has 2 converging lenses of different powers, more powerful lens as the eyepiece
  • Magnification of a telescope= Focal length of objective lens/ focal length of eyepiece lens
  • Most astronomical telescopes have concave mirrors , not convex lenses, as their objectives
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Parallax and Parsecs

  • Parallax makes some stars seem to move relative to others in the course of a year
  • The parallax angle of a star is half the angle moved against a background of distant stars in 6 months
  • A smaller parallax angle means that the star is further away
  • A Parsec (pc) is the distance to a star with a parallax angle of one second of arc
  • A parsec is similar in magnitude to a light year
  • Typical interstellar distances are a few parsecs
  • Intergalactical distances are measured in megaparsecs (Mpc)
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Luminosity and Cepheid variable stars

  • The luminosity (intrinsic brightness) of a star depends on its temperature and size
  • The observed intensity of light (observed brightness) of a star (as seen on Earth) depends on its luminosity and its distance from Earth
  • Cepheid variable stars pulse in brightness, with a period related to their brightness
  • This relationship allows astronomers to estimate the distance to Cepheid variable stars
  • Cepheid variable stars have helped to establish the scale of the universe and the nature of most nebula's as distant galaxies
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What are the objects we see in the night sky and h

  • Telescopes revealed that the Milky Way consists of many stars and lead to the realisation that the Sun was a star in the Milky Way galaxy
  • Telescopes also revealed the existence of many fuzzy objects in the night sky, and they were originally called nebulae
  • The main issue in the Curtis- Shapely debate- whether nebulae were within the Milky Way or separate galaxies
  • Hubble's observations of Cepheid variables in one nebula indicated that it was much further away than any star in the Milky Way- it was in a separate galaxy
  • Cepheid variable data in distant galaxies has given accurate values of the Hubble constant
  • Speed of recession (km/s)= Hubble constant (km/s per Mpc) x Distance (Mpc)
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The composition of stars

  • All hot objects (stars) emit a continuous range of electromagnetic radiation, whose luminosity and peak frequency increases with temperature
  • The spectrum of a star also contains some specific spectral lines, and these provide evidence of the chemical elements present in it
  • Emission spectrum- (of an element) The electromagnetic frequencies emitted by an excited atom as electron energy levels fall
  • Absorption spectrum- (of a star) Consists of dark lines superimposed on a continuous spectrum. It is created when light from the star passes through a cooler gas that absorbs photons of particular energies
  • The removal of electrons from atoms is called ionisation and that electron movement within atoms produces line spectra
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How gases behave

  • When the volume of gas is reduced, its pressure increases
  • The pressure and volume of a gas varies with temperature- pressure and volume of gas both decrease with the temperature
  • Absolute zero of temperature- -273c or 0K gases condense to form a liquid before they reach this point
  • Temperature in K= temperature in c + 273
  • When a gas is cooled down, the particles of the gas lose energy, so they move more slowly
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Atoms, Nuclei and Nuclear Fusion

  • Nuclear processes discovered in the early 20th century provided a possible answer to the mystery of the Sun's energy source
  • A nucleus contains positive protons, neutral neutron and negative electrons
  • Protons are held together in the nucleus by a strong force much greater that the repulsive electrical force between them
  • Geiger and Marsden conducted an alpha particle scattering experiment, they fired alpha particles at a gold foil in vacuum chamber (particles not absorbed by the air)
  • Most of the particles passed through the gold foil, but a few were reflected back. The positive charge within the gold atom repelled the positive alpha particle
  • Hydrogen nuclei can fuse into helium nuclei, releasing energy, if brought closer together
  • Strong nuclear force- A fundamental force of nature that acts inside atomic nuclei
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Changing stars

  • Protostars from when a cloud of gas and dust in space starts to contract, pulled together by gravity. The particles attracts every other particle, so the cloud collapses towards its centre- forming a rotating disc
  • Compressing the gas (in a protostar) will raise its temperature
  • A star contains a hotter core (fusion takes place), a radiative zone (energy  transported to surface by photons of radiation), a convective zone (energy transported to surface by convection) and the photosphere (energy is radiated into space)
  • All stars change where there is insufficient hydrogen in the core for fusion to continue
  • Small stars (Sun) become red giants when the core hydrogen is depleted- larger stars become red supergiants
  • Red giants and supergiants release energy by fusing helium into larger nuclei (carbon, nitrogen and oxygen)
  • Red giants lack the mass to compress the core further at the end of the helium fusion, and they shrink into hot white dwarfs, which gradually cool
  • Fusion in red supergiants continues to larger nuclei due tot higher pressures in the core
  • Fusion in large stars ceases when the core has been largely converted into iron, and the star then explodes in a supernova, leaving a dense neutron star or black hole 
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How do astronomers work together?

  • Major optical and infrared observatories on Earth are mostly situated in Chile, Hawaii, Australia and the Canary Islands
  • Large telescopes are needed to collect the weak radiation from faint or distant sources
  • Radiation is diffracted by the aperture of the a telescope and that the aperture must be larger than the wavelength of radiation detected by the telescope to produce sharp images
  • International collaboration is needed in terms of economy and pooling of expertise
  • Non-astronomical factors such as cost, environmental and social impact near the observatory and working conditions for employees are taken into consideration
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Very helpful, thanks :-)


emily wrote:

You're very welcome :D

alex colton

Ro i Couldn't find a mistake did you spellcheck spoilsport. HeHe



Well-summarised Genius


Haha, the voice says h r s as seperate letters


Thanks :D helped a lot.


thank you :)

Gracefield Jones

Thanks it helped loads!! :-)

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