The Solar System

Within our solar system there is one star, the Sun, plus the eight planets and the dwarf planets that orbit around the Sun. Natural satellites, the moons that orbit planets, are also part of the solar system.

Order of the Planets on the Solar System

Our solar system is a small part of the Milky Way galaxy.

The Sun was formed from a cloud of dust and gas (nebula) pulled together by gravitational attraction.

At the start of a star’s life cycle, the dust and gas drawn together by gravity causes fusion reactions

In a main sequence star, fusion reactions lead to an equilibrium between the gravitational collapse of a star and the expansion of a star due to fusion energy.

  • a nebula of dust and gas
  • is pulled together by gravity
  • expansion due to fusion energy
  • in equilibrium with gravitational collapse

A star goes through a life cycle. The life cycle is determined by the size of the star.

The life cycle of a star dependent on its size:

  • the size of the Sun: nebula – protostar – main sequence star – red giant – white dwarf – black dwarf
  • much more massive than the Sun: nebula – protostar – main sequence star – red supergiant – supernova – neutron star – (possibly) black hole
Lifecycle of A Star

Fusion processes in stars produce all of the naturally occurring elements. Elements heavier than iron are produced in a supernova.

The explosion of a massive star (supernova) distributes the elements throughout the universe.

Fusion processes lead to the formation of new elements. When the star becomes a red super/giant there is no hydrogen left for fusion so it fuses larger elements to create elements up to the size of iron.

elements heavier than iron are formed only when a massive star explodes

  •  fusion (processes in stars) produce new elements
  • cloud of gas / hydrogen and dust OR nebula
  • pulled together by gravity
  • causing increasing temperature (to start the fusion process)
  • (to become a) protostar
  • hydrogen nuclei fuse to form helium nuclei
  • and the star becomes main sequence
  • hydrogen begins to run out
  • helium nuclei fuse to make heavier elements
  • up to iron
  • the star expands (to become a)
  • red super giant
  • (the star collapses rapidly) and explodes
  • called a supernova
  • creating elements heavier than iron
  •  and distributing them throughout the universe leaving behind a neutron star
  • or a black hole.

Orbital Motion

Gravity provides the force that allows planets and satellites (both natural and artificial) to maintain their circular orbits.

Planets orbit stars. Moons orbit Planets.

The artificial satellites that orbit earth usually occupy one of two types of orbit:

  • Geostationary Orbit: The satellite has an orbital period the same as the Earth so it is always in the same position above the Earth. GPS and Communication satellites are in geostationary orbit.
  • Low Polar Orbit: The satellite will orbit the planet as the planet rotates so it can see the entire planet over time. Spy satellites and mapping satellites use this type of orbit.

For circular orbits, the force of gravity can lead to changing velocity but unchanged speed; this is because velocity is a vector and has direction.

For a stable orbit, the radius must change if the speed changes. If a satellite slows down it will move to an orbit further from Earth to remain stable.

  • gravity causes the satellite to accelerate (towards the Earth)
  • the acceleration causes a change in direction
  • velocity changes because direction changes

Origins of The Universe

There is an observed increase in the wavelength of light from most distant galaxies. The further away the galaxies, the faster they are moving and the bigger the observed increase in wavelength. This effect is called red-shift.

The observed red-shift provides evidence that space itself (the universe) is expanding and supports the Big Bang theory.

The Big Bang theory suggests that the universe began from a very small region that was extremely hot and dense.

Since 1998 onwards, observations of supernovae suggest that distant galaxies are receding (moving away) ever faster.

There is still much about the universe that is not understood, for example dark mass and dark energy.

hot and dense

The further away a galaxy is from Earth, the fastest the galaxy is moving away from Earth.

Red Shift

The distance for earth

The speed the galaxy is moving away from earth