1. Life Cycle of Stars
Objectives
Describe the complete life cycle of a star.
Explain how stars are formed.
Describe the processes involved in the eventual end of a star.
We’re going on a ‘star-fari’
Make sure you understand each step, take down key notes (purple boxes) and ask questions if you don’t get it.
You must be able to describe processes in your own words, and remember each stage in order!

2. NEBULA: giant cloud of gas – mostly hydrogen
Hydrogen is the most simplistic atom that exists – made of 1 proton and 1 electron
This is the 'Orion Nebula'. It would take light 1,344 years to get to it, 24 years to get across it and it contains 2000 times the mass of our sun.
Remember that light travels 300,000,000 metres in a single second. THAT’S BIG!

3. This one is called the 'Horse Head Nebula'.
In the nebula, gravity pulls hydrogen gas gradually closer together
As hydrogen gas moves closer together, particles rub together. Friction causes the nebula to get very hot

4. The spherical nebula eventually gets hot enough (through friction) to start glowing – it becomes a PROTOSTAR
This isn’t a ‘proper’ fully functioning star yet. Nuclear fusion hasn’t happened yet!

5. MASSIVE ENERGY IS RELEASED AS LIGHT AND HEAT!
The hydrogen atoms are being forced together under:
incredibly high pressure (gravity) and
incredibly high temperature (friction) – this causes NUCLEAR FUSION
MAIN SEQUENCE STAR: A fully formed star (like our Sun), where nuclear fusion is taking place, and hydrogen nuclei are forced together to form helium
Hydrogen nuclei (protons) = positive. They repel.
High temperature and pressure FORCE protons together
HYDROGEN  HELIUM – this is NUCLEAR FUSION
MASSIVE ENERGY IS RELEASED AS LIGHT AND HEAT!

6. There are two main forces acting in a main sequence star – you HAVE TO KNOW THESE
The force of gravity acts inwards
The pressure of nuclear fusion acts outwards
Both forces are equal and balanced – this is why the main sequence star is stable

7. Quick Snapshot – what do we know so far?
What’s it made of?
How does it form?
Huge cloud of hydrogen gas.
Pulls together due to gravity.
Nebula
What happens in this
stage?
What hasn’t happened
yet?
Friction makes gas hot,
protostar starts glowing
Nuclear fusion
Protostar
What is happening in
this stage?
Why is this a stable
sequence?
Nuclear fusion;
hydrogen  helium
Forces (gravity and fusion pressure) are balanced
Main Sequence

8. The life span of a star in its main sequence depends on its mass and therefore its size.
The larger the star, the:
HOTTER it is, as more gravity = more nuclear fusion
SHORTER its lifespan – they get through their fuel more quickly
Stars that are 2 times larger than our sun and above last only millions of years
- They are normally blue or white, as they’re so hot
Stars around the same mass as our sun last around 1-10 billion years
You might be given a data table of different sizes and lifespans of stars, and be asked to either DESCRIBE the pattern (really easy) or SUGGEST WHY there are differences (see purple box above)

9. Main Sequence
Small mass star (incl. our sun)
Large mass star
Red Giant
Super Red Giant

10. Red Giant (for stars the size of our Sun or smaller)
The star runs out of hydrogen fuel.
The outer layers collapse due to gravity, and these are used by fusion.
The star EXPANDS and COOLS
Star swells to 100s
of times its original size
Cools to become red

11. Super Red Giant (stars much larger than our Sun)
In very large stars, there’s enough
Gravity for fusion of hydrogen, through all elements up to iron!
The star swells to a MUCH LARGER size to form a red super giant
Exam tip: never confused red giants with red super giants
Red giants: happens in medium stars (like our Sun)
Red super giants: happens in very large stars

12. (more detail to follow)
Whistle-stop tour
(more detail to follow)
Red
Super giant
Red Giant
Star expands and
cools. Still fusing!
White Dwarf
SUPER
NOVA
Tiny, really hot,
glowing – but no
Fusion!
Fusion stops, star COLLAPSES then EXPLODES OUTWARDS
Black dwarf
Can’t see it???
Neutron star
Black hole OR

13. As MJ said, It Don’t Matter If You’re Black Or White (Dwarfs)
White, hot core.
Brown Dwarf
Cooling core.
Black Dwarf
Cold, solid core.
In all of these, NO NUCLEAR FUSION is happening.
You know how metal glows if you heat it? This is basically the same thing – it glows less as it cools

14. Quick Snapshot – what do we know so far?
Red Giant
Super Red Giant
Supernova
White Dwarf
Black Hole
Neutron Star
Black Dwarf
What happens to create a red giant? Nuclear fusion stops (temporarily) – star expands, cools and turns red
What is the difference between a white and black dwarf?
White dwarfs are hot, glow. Black dwarfs are cold, do not glow
What is the difference between a red giant and red super giant?
Stars the size of our sun form red giants, stars much bigger form red super giants
How is a supernova forms, and what happens inside it?
Fusion stops, star collapses, explodes outwards. Elements heavier than iron are formed by fusion
Compare black dwarves and neutron stars
Similarities:
Both are very dense (black holes more so)
Differences:
Light cannot escape a black hole
Neutron stars release X-rays

15. Supernova
Super Red Giant
Supernova: Fusion stops, star very quickly collapses. Huge amount of fusion starts again quickly, and elements heavier than iron are formed

16. Supernova continued:
A massive explosion ‘kicks out’ dust into space
Interestingly, this dust and gas that is created can be remade into ‘new generation’ stars and planets.
Our Sun is a third generation star – formed from the remains of 2 other stars before it!

17. The remains of the star after a supernova has a HUGE amount of mass.
Neutron Star
The remains of the star after a supernova has a HUGE amount of mass.
Gravity is very strong, and pulls atoms together. Electrons combine with protons to form neutrons
Neutron stars are only 10-20km across
They are very dense – a teaspoon of its material would weigh 20,000,000,000,000 kg (20 billion tonnes)

18. Radio waves and X-rays being emitted from the neutron star.
Neutron star at centre.
Exam note: A single mark question has been asked in the past about what it emitted from neutron stars and black holes.
The answer is: Radio waves and X-rays.

19. Here is a picture of a black hole.
Black Holes
Here is a picture of a black hole.
Only Joking!

20. Black holes are infinitely dense
Black hole: very large stars form black holes. They have such large gravitational fields that even light cannot escape (that’s why they are ‘black holes’)
Black holes are infinitely dense

21. Super Red Giant Supernova Black Hole Neutron Star
Fusion occurs in the core up to iron.
The fusion at the core slows down so shells fuse causing the star to expand.
Super Red Giant
The fuel for fusion runs out, the star collapses and explodes outward.
Elements heavier than iron are formed in the explosion and thrown into space.
Supernova
Black Hole
Neutron Star
Core of a large star that is now made from neutrons.
Protons and electrons come together to form neutrons.
Strong gravitational pull. Radio waves and x-rays emitted.
Collapsed core of a large star.
Very dense with a very strong gravitational pull.
Radio waves and x-rays are emitted from them.

22. Nebula
Protostar
Main Sequence
Small mass star (incl. our sun)
Large mass star
Red Giant
Super Red Giant
White Dwarf
Supernova
Black Dwarf
Black Hole
Neutron Star

23. Small mass star (incl. our sun) Large mass star
KEY TASK
Make a summary sheet by:
Labelling each stage
Writing summary bullet-point info for each stage around the sides N P
M S
Small mass star (incl. our sun)
Large mass star
R G
R S G
W D S
B D
B H
N S

24. Exam questions can ask you to describe and compare the processes of nuclear fusion, the process responsible for star formation and nuclear fission, the process of large unstable nuclei/atoms breaking down into smaller atoms.