Space (physics paper 2

What is within our solar system

• One star- the sun

• 8 planets that orbit the sun

• dwarf planets that orbit around the sun

• natural satellites- moons- the orbit planets

• asteroids

• comets

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

What is a galaxy?

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

• A galaxy is a massive group of stars (hundreds of billions).

• In the Universe there are hundreds of billions of galaxies.

How do stars such as the sun form?

1. Stars form from clouds of dust and gas (a nebula)

2. Main gas in a nebula is hydrogen.

3. Gravity causes the cloud of dust and gas to collapse.

4. As the dust particles move faster, the temperature rises to millions of degrees Celsius.

5. This collapsing cloud of hot gas is called a protostar.

6. If the temperature of the protostar gets high enough then hydrogen nuclei join together to form helium (nuclear fusion).

7. At this point the protostar has turned into a star.

8. Nuclear fusion is now releasing a huge amount of energy.

9. Gravity tends to make star collapse and force from fusion tends to make star expand.

10. The energy released by nuclear fusion creates a force which acts outwards and balances the force of gravity acting inwards on the star.

11. Star is in equilibrium.

What is a nebula?

A cloud of dust and gas.

What happens to star after main sequence stage? depends on it’s size

• Stars same size as sun:

  • Hydrogen nuclei fuse together to make larger nuclei such as helium.

  • At some stage the hydrogen in the star begins to run out.

  • This means the outward force due to fusion energy is less than the inward force due to gravity so the star collapses inwards.

  • The collapse of the star causes it’s temperature to increase.

  • Now helium nuclei fuse together to form heavier elements,

  • The star expands to form a red giant.

  • At some point the red giant stops fusing helium and at this stage the star shrinks and forms a white dwarf.

  • because the white dwarf is no longer carrying out fusion, it gradually cools down.

  • Eventually it stops releasing any energy and it forms a black dwarf.

What happens to stars much bigger than the sun after main sequence phase?

• Once these run out of hydrogen then leave the main sequence stage and expand into red super giants.

• Once again helium nuclei fuse together to produce heavier elements but not heavier than iron.

• At some point the red super giant stops carrying out nuclear fusion and at this stage the star explodes- supernova.

• The temperature of a supernova is high enough to produce elements heavier than iron.

• When the supernova explodes, these elements are distributed throughout the universe.

• After the supernova, the remains of the star can form either a neutron star or a black hole.

• A neutron star consists of neutrons densely packed together.

• A black hole has such a large gravity that not even light can escape.