The document outlines the characteristics and life cycles of stars, describing their formation, evolution, and eventual demise based on mass. It differentiates between small stars, which evolve into white dwarfs, and massive stars, which can result in supernovae, neutron stars, or black holes. Key stages include the formation of protostars, nuclear fusion, and various end states like planetary nebulae and black holes.

1. To my Friends and Family
Dr Shaheen Akhter-Hamid
London
UK

2. Stars
Stars are hot bodies of glowing gas, with diameters ranging
from 450x smaller to over 1000x larger than our Sun.
Masses range from a 20th to over 50x of the Sun
Surface temperatures range from 3,000oC to over 50,000oC.

3. The Hertzsprung Russell diagram
This plots luminosity (and magnitude) of stars against their
colour and effective temperature. The resulting plots show
that the relationship between temperature and luminosity
of a star was not random but fall into distinct groups.
It ranges from the hot
blue-white stars on the
left to the cooler red
stars on the right.

4. Life cycle of a star
The outcome of star evolution depends on the mass

5. Life cycle of small stars
Small stars have a mass up to one and a half times that of
the Sun.
Stage 1
Stars are born in nebula, which is a cloud of gas (hydrogen)
and dust in space. These condense into a huge globule of
gas and dust and contract under gravity.

6. Stage 2 – Protostar
A region of condensing matter will begin to heat up
Inside the protostar the core’s temperature increases as
more atoms try to share the same space.
While this is happening the gas pressure also increases

7. Stage 3 – Nuclear fusion starts
As the protostar reaches 10 million degrees it creates
energy. The energy comes from nuclear fusion processes.
Nuclear fusion is the process by which light nuclei combine
to form heavier nuclei, to produce energy from
combination of elements lighter than iron
Main sequence stars fuse hydrogen atoms to form helium
atoms in their cores.

8. Stage 4 - Main Sequence Star
It has a stable balance of outward pressure from core
nuclear fusion and gravitational forces pushing inward.
A star of one solar mass remains in main sequence for
about 10 billion years, until all of the hydrogen has fused
to form helium.

9. Stage 5 – Red Giant
This is a large star, 10 and 100 times the diameter of the Sun
The hydrogen runs out, so there is less fusion pressure and
the star becomes unstable.
Gravity causes the core to contract and heat up
Helium fusion starts to occur in the core to make carbon
Reactions begin to occur in a shell around the core.

10. Stage 6 – Planetary Nebula.
The helium core runs out, and the outer layers drift
of away from the core as a gaseous shell called a
planetary nebula

11. Stage 7 White Dwarf
These are the shrunken remains of stars, that have burned
up all of the hydrogen they once used as nuclear fuel.
A white dwarf is very dense: its mass is comparable to that
of the Sun, and its volume is comparable to that of the
Earth
The star eventually cools and dims and when it stops
shining, the now dead star is called a black dwarf.

12. The Life of a Massive Star
Massive stars have a mass 3x times that of the Sun. Some
are 50x that of the Sun
Stage 1 - Massive stars evolve in a similar way to a small
stars until it reaches its main sequence stage
The hydrogen rapidly fuses into helium (billions of years in a
small star, but only millions in a massive star).

13. Stage 2 – Red Supergiant
A red supergiant is an aging giant star which has used its
supply of hydrogen fuel in the core.
Helium accumulates in the core, and hydrogen undergoes
nuclear fusion in the outer shells.
These shells expand, and the cooler star becomes red.
They are the largest known stars.

14. Stage 3
Since no heat (by hydrogen burning) is generated in the core,
the core has no energy to resist gravity and contracts, and the
heavier it becomes the more it contracts, heating up as it
does so .
The core begins to burn carbon, converting it into the heavier
elements all the way up to iron, which is stable and cannot be
burnt by nuclear fusion

15. Stage 4 Supernova
The core collapses in less than a second, in which a shock
wave blows off the outer layers of the star.
Supernovas can briefly outshine entire galaxies and radiate
more energy than our sun will in its entire lifetime.
They are the primary source of heavy elements in the
universe.
On average, a supernova
will occur about once every
50 years in a galaxy the size
of the Milky Way.

16. Stage 5 –Neutron stars
A neutron star results from the gravitational collapse after a
supernova, where the core survives the explosion. Protons
and electrons to combine to produce a neutron star.
Neutron stars are the densest and smallest stars known to
exist, with a radius of only 12 km, they can have a mass of
about twice that of the Sun.
Pulsars are neutron stars that are spinning very rapidly.

17. Stage 5 – Black Hole
If the core is much greater than 3 solar masses, the core
contracts to become a black hole.
The gravitational pull in a black hole is so great that
nothing can escape from it, not even light.