Stars (CIE IGCSE Physics)

• Galaxies are made up of billions of stars
• The Universe is made up of many different galaxies
• The Sun is one of billions of stars in a galaxy called the Milky Way
• Other stars in the Milky Way galaxy are much further away from Earth than the Sun is
• Some of these stars also have planets which orbit them

Our solar system is just one out of potentially billions in our galactic neighbourhood, the Milky Way. There are estimated to be more than 100 billion galaxies in the entire universe

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• Astronomical distances such as the distances between stars and galaxies, are so large that physicists use a special unit to measure them called the light-year

• One light-year is defined as:

The distance travelled by light through (the vacuum of) space in one year

• The speed of light is the universal speed limit, nothing can travel faster than the speed of light
• But over astronomical distances, light actually travels pretty slowly
• The diameter of the Milky Way is approximately 100 000 light-years
  • This means that light would take 100 000 years to travel across it

• One light year is equal to 9.5 × 10¹² km, or 9.5 × 10¹⁵ m

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1. Nebula

• All stars form from a giant interstellar cloud of hydrogen gas and dust called a nebula

2. Protostar

• The force of gravity within a nebula pulls the particles closer together until it forms a hot ball of gas, known as a protostar
• As the particles are pulled closer together the density of the protostar will increase
  • This will result in more frequent collisions between the particles which causes the temperature to increase

3. Main Sequence Star

• Once the protostar becomes hot enough, nuclear fusion  reactions occur within its core
  • The hydrogen nuclei will fuse to form helium nuclei
  • Every fusion reaction releases heat (and light) energy which keeps the core hot

• Once a star initiates fusion, it is known as a main-sequence star
• During the main sequence, the star is in equilibrium and said to be stable
  • The inward force due to gravity is equal to the outward pressure force from the fusion reactions

The outwards and inwards forces within a star are in equilibrium. The centre red circle represents the star's core and the orange circle represents the star's outer layers

• Once a protostar is formed, its life cycle will depend on its mass
• The different life cycles are shown below

Flow diagram showing the life cycle of a star which is the same size as the Sun (solar mass) and the lifecycle of a star which is much more massive than the Sun

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• A low-mass star will go through the following stages

The lifecycle of a low-mass star

4. Red Giant

• After several billion years the hydrogen causing the fusion reactions in the star will begin to run out
• Once this happens, the fusion reactions in the core will start to die down
• This causes the core to shrink and heat up
  • The core will shrink because the inward force due to gravity will become greater than the outward force due to the pressure of the expanding gases as the fusion dies down

• A new series of reactions will then occur around the core, for example, helium nuclei will undergo fusion to form beryllium
• These reactions will cause the outer part of the star to expand
• A low-mass star that is up to 8 times the mass of the Sun or smaller will become a red giant 
  • It is red because the outer surface starts to cool

5. Planetary Nebula

• Once this second stage of fusion reactions have finished, the star will become unstable and eject the outer layer of dust and gas
  • The layer of dust and gas which is ejected is called a planetary nebula

6. White Dwarf

• The core which is left behind will collapse completely, due to the pull of gravity, and the star will become a white dwarf
• The white dwarf will be cooling down and as a result, the amount of energy it emits will decrease

7. Black Dwarf

• Once the star has lost a significant amount of energy it becomes a black dwarf
• It will continue to cool until it eventually disappears from sight

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• A high-mass star will go through the following stages

Lifecycle of a high-mass star

4. Red Supergiant

• After several million years, the hydrogen causing the fusion reactions in the star will begin to run out
  • A high-mass star (one more than 8 times the mass of the Sun) will become a red supergiant
• Similar to a low-mass star, the fusion reactions in the core will start to die down
• The core will go through a series of periods of shrinking and heating up
  • As a result, the outer parts of the star will expand and contract
• This time, fusion reactions will form elements all the way up to iron
• Fusion reactions cannot continue once iron is formed

5. Supernova

• Once the fusion reactions inside the red supergiant cannot continue, the core of the star will collapse suddenly and cause a gigantic explosion
  • This is called a supernova

• At the centre of this explosion a dense body, called a neutron star will form
• The outer remnants of the star will be ejected into space during the supernova explosion, forming new clouds of dust and gas (nebula)
  • The nebula from a supernova may form new stars with orbiting planets

6. Neutron Star (or Black Hole)

• In the case of the biggest stars, the neutron star that forms at the centre will continue to collapse under the force of gravity until it forms a black hole
  • A black hole is an extremely dense point in space that not even light can escape from