STELLAR EVOLUTION NOTES MR.

HUSSIEN KHALED

Classification of starts
• Stars come in a wide range of sizes and colours, from yellow stars to red dwarfs,
from blue giants to red supergiants
o These can be classified according to their colour
• Warm objects emit infrared and extremely hot objects emit visible light as well
o Therefore, the colour they emit depends on how hot they are

• A star's colour is related to its surface temperature

o A red star is the coolest (at around 3000 K)
o A blue star is the hottest (at around 30 000 K)

• This is why white dwarfs, for example, are very hot and red giants are red because
they are cool
• Astronomical objects cool as they expand and heat up as they contract, such as
when a star reaches the end of its life and becomes a red giant
o This means that their colour will also change accordingly with their
temperature

1
STELLAR EVOLUTION NOTES MR. HUSSIEN KHALED

The life cycle of solar mass stars

1. Nebula

• All stars form from a giant 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 release heat (and light) energy which keeps the core
hot
• Thermal expansion from fusion reactions occur within its core due to fusion and the
force of gravity keeps the star in equilibrium
• At this point, the star is born, and it becomes 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 which
results from the expanding hot gases inside the 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
o The core will shrink because the inward force due to gravity will
become greater than the outward force due to the pressure dies down
• A new series of reactions will then occur around the core, for example, helium nuclei
will undergo fusion to form beryllium
• As the core shrinks, more reactions will cause the outer part of the star to expand
• It will become a red giant
o It is red because the outer surface starts to cool

2
STELLAR EVOLUTION NOTES MR. HUSSIEN KHALED

5. White Dwarf

• The star will eventually become unstable and eject the outer layer of dust and gas
• 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

• A large star is one that is much larger than the Sun

o Stars that are larger than the Sun have much shorter lifespans – in the region
of hundreds of millions of years (instead of billions)
• The life cycle of a star larger than the Sun starts in the same way as a solar mass
star

1. Nebula

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

3
STELLAR EVOLUTION NOTES MR. HUSSIEN KHALED

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
• Thermal expansion from fusion reactions occur within its core and the force of
gravity keeps the star in equilibrium
• At this point, the star is born, and it becomes 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 which
results from the expanding hot gases inside the star

4. Red Supergiant

• Eventually, the main sequence star will reach a stage when it starts to run out of
hydrogen gas in its core
• Once this happens, the fusion reactions in the core will start to die down
• This causes the core to shrink and heat up
o The core will shrink because the inward force due to gravity is greater than
the outward force due to the pressure of the expanding gases
• A new series of fusion reactions will then occur around the core, for
example, helium nuclei will undergo fusion to form beryllium
• These fusion reactions will cause the outer part of the star to expand and it will
become a super red giant
o A super red giant is much larger than a red giant

5. Supernova

• Once the fusion reactions inside the red supergiant finally finish, the core of the star
will collapse suddenly causing a gigantic explosion
o 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 a supernova

4
STELLAR EVOLUTION NOTES MR. HUSSIEN KHALED

6. Neutron Star (or Black Hole)

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

The brightness of the stars and absolute magnitude

• Astronomers measure the brightness of stars at a
standard distance using the absolute magnitude
scale
• This is because a really bright star far away will
look the same as a dim star nearby, so it is difficult
to measure the brightness directly
• This scale runs back to front:
o The brighter the star, the smaller the
magnitude
o The dimmer the star, the larger the
magnitude
• The brightness of a star depends on two main
factors:
o How much light the star emits
o How far away the star is (more distant stars
are usually fainter than nearby stars)
• The absolute magnitude of stars is a measure
of how bright they would be if they were all
the same distance away from Earth

5
STELLAR EVOLUTION NOTES MR. HUSSIEN KHALED

Hertzsprung-Russell Diagrams
• Stars are classified using the Hertzsprung-Russell (HR) diagram
• This is a plot of:
o The luminosity on the y-axis
▪ This is from dim (at the bottom) to bright (at the top)
o The temperature on the x-axis
▪ This is from hot (on the left) to cool (to the right)
▪ This is normally in Kelvin

• The main features of the Hertzsprung-Russell diagram are:

o Most stars lie on the Main Sequence. This is the band of stars going from top
left to bottom right
o Below the main sequence (and slightly to the left) are the White Dwarfs
o Above the main sequence on the right-hand side are the Red Giants
o Above those are the Red Supergiants

• This means the white dwarfs are dimmer and hotter than the red giants which
are brighter and cooler
• Sometimes, the absolute magnitude is on the y-axis instead
o Remember this will still go from dimmest (highest number) at the bottom to
brightest (lowest number) at the top!