The Milky Way
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
Astronomical Distances
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
o 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
, Star Formation
Life cycle of a star
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
o 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
o The hydrogen nuclei will fuse to form helium nuclei
o 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
o 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
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
Astronomical Distances
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
o 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
, Star Formation
Life cycle of a star
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
o 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
o The hydrogen nuclei will fuse to form helium nuclei
o 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
o 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
