Space physics

• Stars initially form from a cloud of dust and gas-a nebula.
• No one really knows what these things are, so there are lots of theories.
• 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 hot-a star is born.
• The star enters a long stable period where the outward pressure caused by the nuclear fusion that tried to expand the star balances the gravity pulling everything inwards.
• This is a main sequence star and typically lasts several billion years (the Sun is in this period).
• Eventually the hydrogen begins to run out.
• The star then swells into a red giant (small star) or a red supergiant (bigger star).
• It becomes red because the surface cools.
• Fusion of helium occurs.
• Heavier metals (up to iron) are created in the core of the star.
• A small star (like the Sun) then becomes unstable and ejects its outer layer of dust and gas.
• This leaves a hot, dense, solid core-a white dwarf.
• As a white dwarf cools down, it emits less and less energy.
• The force that makes an object accelerate is provided by the gravitational force between the planet and the Sun, or a planet and its satellites.
• For an object to accelerate, a force must act on it, the force is directed to the centre of the circle.
• The closer you get to a star or planet, the stronger the gravitational force is.
• Currently, scientists think the universe is mostly dark matter and dark energy.
• Faster moving objects will move in a stable orbit with a smaller radius than slower moving ones.
• Whenever scientists discover new evidence, they have to make a new theory or change an existing one.
• Dark matter is an unknown substance which holds galaxies together, but doesn’t emit any electromagnetic radiation.
• The size of the orbit depends on the object’s speed.
• There are still lots we don’t know about the universe.
• The red shift and the Big Bang theory suggest that the universe is expanding.
• Observations of supernovae from 1998 to the present appear to show that distant galaxies are moving away from us faster, and this speed they are receding at is increasing.
• Dark energy is thought to be the cause of expansion of the universe.
• The stronger the force, the faster the orbiting object needs to travel to remain in orbit.
• The Big Bang theory is the best guess so far.
• When it no longer emits a significant amount of energy, it is called a black dwarf.
• Big stars 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.
• Star and their life cycles produce and distribute all naturally occurring elements.
• The exploding supernova throws the outer layers of dust and gas into space, leaving a very dense core-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.
• The solar system is everything that orbits the sun: dwarf planets, moons, planets, artificial satellites, and our solar system is a tiny part of the Milky Way galaxy.
• The planets move around the sun in almost circular orbits (the same as the moon around the Earth).
• If an object is travelling in a circle it is constantly changing direction, which means it is constantly accelerating.
• This also means it is constantly changing velocity (but not speed).