astrophysics

• • a universe is a large collection of billions of galaxies
• • a galaxy is a large collection of billions of stars
• • a solar system is a collection of plants orbiting a common star
• • our solar system is in the milky way galaxy
• gravitational field strength

  • force per unit mass on a body in a gravitational field
  • measured in N/kg - newtons per kilograms
  • varies with mass and size of the body; therefore different on other planets and the moon compared to earth
• • gravitational force allows various bodies to orbit around others
• moons orbit planets

  • the orbits are slightly elliptical with near constant orbital speed
• planets and comets orbit the sun

  • the orbits of planets are slightly elliptical with near constant orbital speed
  • orbits of comets are highly elliptical
• • artificial satellites orbit the earth
• orbital speed = (2 x pi x orbital radius)/time period
• comets

  • comets have a greater speed nearer to the star because the ice inside them melts as they get closer & warmer
  • this causes their mass to decrease so they travel faster
• stellar evolution

  1. star begins as cloud of dust and gas - nebula
  2. the particles experience a weak attraction towards each other due to gravity & clump together
  3. they continuing clumping together until the pressure and temperature is large enough that nuclear fusion can occur
• stellar evolution (fusion)

  4. hydrogen nuclei fuse together to form helium nuclei which releases large amount of energy and causes high outward pressure

  5. the outwards pressure balances with the inwards pressure due to gravity

  6. the star is now stable and called a main sequence star
• stellar evolution (stage 3)

  7. eventually, all the hydrogen in the star is used up so there's no longer enough outward pressure from fusion process

  8. the star collapse under its own gravitational attraction - becomes unstable
• stellar evolution - star of similar mass to sun

  9. expands massively and becomes a red giant

  10. then becomes a white dwarf and finally cools into black dwarf
• stellar evolution - stars of larger mass than sun

  9. expands and becomes red super giant, before exploding in a supernova

  10. either a neutron star or black hole is produced - depends on size of explosion so black hole is only formed if explosion was exceptionally massive
• classification:

  • stars are classified according to their colour
  • hotter stars are bluer
  • cooler stars are redder
• absolute magnitude (TRIPLE):

  • brightness of star depends on where it's measured from
  • a star's absolute magnitude is equal to apparent magnitude that the star would have from 10 parsecs (32.6 light-years) away
  • apparent magnitude is how bright a star appears from a certain point in space
• hertzsprung-russell diagrams (TRIPLE):

  • shows the relationship between star's surface temperature and brightness
  • temperature in kelvins shown on x-axis; absolute magnitude shown on y-axis
  • supergiants shown in top right
  • red giants shown below supergiants
  • main sequence stars shown in a diagonal strip from top left to bottom right
  • white dwarfs shown in bottom left
• cosmology (TRIPLE):

  • big bang theory states that the universe expanded outwards from single point
  • currently the only theory that accounts for all experimental evidence therefore the only accepted model as of present
• red-shift (TRIPLE):

  • red shift of light from galaxies shows they are all moving away from us
  • those that are furthest away are moving fastest - suggests universe was formed from an explosion at a single point
  • principle of doppler effect
• cosmic microwave background radiation (CMBR):

  • after big bang, lots of short wavelength radiation should have been released
  • this radiation over time would have been stretched to become microwaves
  • there is CMBR present wherever a telescope is pointed in the sky - evidence for big bang
• doppler effect (TRIPLE):

  • if a wave source is moving relative to an observer, a change in frequency and wavelength will be observed due to doppler effect
  • e.g when siren of ambulance is high-pitched as it approaches you and low-pitched as it moves away
  • responsible for the red-shift of light from galaxies which are moving away from earth, which is when the wavelength of the light coming from them increases
  • the faster a galaxy is moving, the more its light is red-shifted