Stars and planets

• Mercury

  Venus

  Earth

  Mars

  Jupiter

  Saturn

  Uranus

  Neptune
• The asteroid belt lies between the orbits of Mars and Jupiter
• The closest four planets to the sun are rocky planets
• the four outer plants are gaseous
• A moon is a body that orbits a planet
• an asteroid is a small rocky objects that orbits the sun
• a comet is a lump of frozen gas, rock and dust that orbits the sun
• A comet has a tail that points away from the sun
• a comet has a circular orbit
• A planetary system comprises a star and all the objects which orbit it
• A galaxy is a huge collection of billions of stars and their solar systems
• For distance in space we use more suitable units: astronomical units and light years
• One astronomical unit is the mean distance from the sun to the earth
• a light year is the distance that light will travel in a year
• Stars are formed from massive clouds of dust and gas in space
• Gravity pulls the dust and gas together to form a protostar
• when the star is hot enough, nuclear fusion begins and the star becomes a main sequence star
• Low-mass main sequence stars such as out sun follow the sequence - red giant, white dwarf
• High-mass sequence stars follow the sequence - supergiant, supernova and neutron star or black hole
• Nuclear fusion is where smaller nuclei fuse to form heavier nuclei
• Apart from hydrogen, all elements have been produced through fusion in stars
• In main sequence, hydrogen nuclei fuse to form helium nuclei
• Elements heavier than iron are produced in supernova explosions
• The solar system was formed around 4.6 billion years ago from the collapse of a cloud of gas and dust, including elements ejected in supernovae
• Gravitational forces caused the matter to get closer together creating the sun and the planets
• A hertzsprung-Russell (H-R) diagram is a way of displaying the properties of stars and representing their evolutionary paths
• As a star evolves, its temperature and luminosity change
• The evolution of a star can be shown on a Hertzsprung-Russell
• A radioactive tracer must have a short half-life so that once inside the body, it decays quickly.
• The radiation emitted must penetrate the body easily and do as little damage as possible.
• Radioactive isotopes can also be used to treat cancer
• A radioactive material can be placed inside the tumour to kill the cancer cells. The radiation emitted needs to be:

  • strongly ionising so that it kills the cells
  • moderately penetrating so it can pass through the tumour to kill the cancer cells without damaging healthy ones.
• A relatively short half-life is required but long enough, so the isotope remains active enough to treat the tumour.