Astrophysics

• The 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 planets orbiting a common star

  • our solar system is in the Milky Way galaxy
• the gravitational field strength is the force per unit mass on a body in a gravitational field

  • its measured in newtons per kilogram
• gravitational field strength varies with mass and size of the body & is therefore different on other planets & the moon compared to the earth
• weight = mass x gravitational field strength

  • w = m x g
• gravitational force enables the various bodies to orbit around others
• moons orbit planets

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

  • the orbits of planets are slightly elliptical with near constant orbital speed
  • the orbits of comets are highly elliptical
• Artificial satellites orbit the earth
• the greater the orbital radius or smaller the time period, the greater the orbital speed:

  • orbital speed = 2 x pi x orbital radius / time
• orbital speed = 2 x pi x orbital radius / time
• 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
• Nuclear fusion is when small nuclei fuse together to make heavier nuclei

  • lots of energy is released
  • fusion can only happen at very high temperatures & pressures
  • if the temperatures or pressures aren’t high enough, positive nuclei repel each other
  • these conditions are found in stars
• A star begins as a cloud of dust & gas called a nebula = the particles experience a weak attraction due to gravity & begin to clump together

  • they continue to clump together until pressure & temperature is enough for nuclear fusion to occur
  • after this, the hydrogen in the star is eventually used up = there is no longer enough outward pressure from nuclear fusion & it collapses under its own gravitational attraction, becoming unstable
• nuclear fusion?

  hydrogen nuclei fuse together to form a helium nuclei which releases a large amount of energy & causes a great outwards pressure

  • this outwards pressure balances with the inward pressure due to gravity & the star is now stable = called a main sequence star
• if a star has a similar mass to the sun = expands massively and becomes a red giant

  • it then becomes a white dwarf
  • and finally cools into a black dwarf
• if the star has a mass larger than the sun, it expands and becomes a red super giant before exploding in a supernova

  • what remains is either a neutron star
  • or if it was exceptionally massive, a black hole
• stars can be classified according to their colour

  • the colour is related to its surface temperature
  • hotter stars = bluer
  • cooler stars = redder
• stellar evolution?

  star begins as nebula

  • particles begin to clump together due to weak attraction
  • continue to clump until pressure & temperature is enough for nuclear fusion
  • main sequence star formed
  • eventually hydrogen is used up & there is not enough outward pressure
  • star collapses & becomes unstable
• hotter stars give out a higher frequency & are white or blue in colour

  • they emit visible light of a higher frequency
  • eg white dwarfs are very hot & very small stars
• cooler stars give out lower frequencies of light & emit reddish light

  • eg red giants or supergiants are cool stars
• As a plant orbiting the sun gets closer to the sun, its speed increases

  • this is as it is being pulled by the gravitational force of the sun