astrophysics <3

• The solar system is not to scale in diagrams because the sizes of the sun and planets are very different, and the distances involved are extremely large
• Main parts of the solar system

  • Sun
  • 8 main planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)
  • Dwarf planets (including Pluto)
  • Moons
  • Asteroids
  • Comets
• Moons


  Natural satellites which orbit planets
• Most planets in the solar system have at least one moon, and some have a large number of moons
• Galaxy


  Massive group of stars, e.g. the Milky Way contains hundreds of billions of stars
• The solar system is just a tiny part of the Milky Way galaxy, and there are hundreds of billions of galaxies in the universe
• How stars like the Sun form

  1. Cloud of dust and gas (nebula) collapses due to gravity

  2. Collapsing cloud becomes a protostar

  3. Protostar undergoes nuclear fusion, releasing energy

  4. Gravity acting inwards is balanced by outward force from fusion energy, putting the star in equilibrium
• Nuclear fusion is the process where hydrogen nuclei join together to form helium
• Life cycle of stars - Stage 1

  Stars initially form from a cloud of dust and gas called a nebula.
• Stage 2


  The force of gravity pulls the dust and gas together to form a protostar. The temperature rises as the star gets denser and more particles collide with each other. When the temperature gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei. This gives out huge amounts of energy which keeps the core of the star hot.
• Stage 3


  The star enters a long stable period where the outward pressure caused by nuclear fusion that tries to expand the star balances the force of gravity pulling everything inwards. In this stable period, it is called a main sequence star and it typically lasts several billion years. The Sun is in the middle of this period.
• Stage 4


  Eventually, the hydrogen begins to run out. The star then swells into a red giant if it is a small star or a red super giant if it is a larger star. It becomes red because the surface cools. Fusion of helium (and other elements) occurs. Heavier elements (up to iron) are created in the core of the star.
• Stage 5


  A small-to-medium-sized star like the Sun then becomes unstable and ejects its outer layer of dust and gas. This leaves behind a hot, dense solid core - a white dwarf.
• Stage 6


  As a white dwarf cools down, it emits less and less energy. When it no longer emits a significant amount, it is called a black dwarf.
• Stage 7


  Big stars, however, start to glow brightly again as they undergo more fusion and expand and contract several times, forming elements as heavy as iron in various nuclear reactions. Eventually they'll explode in a supernova, forming elements heavier than iron and ejecting them into the universe to form new planets and stars. Stars and their life cycles produce and distribute all naturally occurring elements.
• Stage 8


  The exploding supernova throws the outer layers of dust and gas into space, leaving a very dense core called a neutron star. If the star is massive enough, it will become a black hole - a very dense point in space that not even light can escape from.
• Planets


  Large objects that orbit a star. There are 8 in our solar system. Their gravity has to be strong enough to have pulled in any nearby objects apart from their natural satellites.
• Dwarf planets


  Planet-like objects that orbit stars but don't meet all of the rules for being a planet.
• Moons


  Type of natural satellite that orbit planets.
• Artificial satellites


  Satellites placed into orbit by humans.
• Milky Way galaxy


  Collection of billions of stars all held together by gravity.
• Orbital movement


  If an object is travelling in a circle then it is constantly changing direction, meaning that it is constantly accelerating. This also means that it is constantly changing velocity. For an object to accelerate, there must be a force acting on it. This force is directed towards the centre of the circle. This force would cause the object to fall towards whatever it was orbiting, but as the object is already moving, it just changes direction. The object keeps accelerating towards what it's orbiting but the instantaneous velocity keeps it travelling in a circle. The force that makes this happen is provided by the gravitational force between the planet and the Sun/its satellites.
• Size of orbit and speed

  The stronger the force, the faster the orbiting object needs to travel to remain in orbit. For an object to be in a stable orbit, if the speed of the object changes, the size (radius) of the orbit must do so too. Faster moving objects will move in a stable orbit with a smaller radius than slower moving ones.
• Red-shift


  When we look at light from most distant galaxies, we find that the wavelength has increased. The wavelengths are all longer than they should be - they are shifted towards the red end of the spectrum. This suggests the source of light is moving away from us. Measurements of the red-shift indicate that these distant galaxies are moving away from us very quickly. More distant galaxies have greater red-shifts than nearer ones, meaning that more distant galaxies are moving faster away than nearer ones.
• Big Bang


  All the matter in the universe occupied a very small space which was very dense and very hot. Then it "exploded" - space started expanding, and the expansion is still going on.
• Observations of supernovae from 1998 to present day

  Distant galaxies are moving away from us faster and faster (the speed at which they're receding is increasing).
• Dark matter


  Unknown substance which holds galaxies together but does not emit any electromagnetic radiation. It is thought to be responsible for the accelerated expansion of the universe.