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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!
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!
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
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!
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!
There are two main forces acting in a main sequence star – you HAVE TO
KNOW THESE
1. The force of gravity acts inwards
2. The pressure of nuclear fusion acts outwards
Both forces are equal and balanced – this is why the main sequence star is
stable
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.
What happens in this
Friction makes gas hot,
stage?
protostar starts glowing
What hasn’t happened Nuclear fusion
yet?
What is happening in
this stage?
Why is this a stable
sequence?
Nuclear fusion;
hydrogen  helium
Forces (gravity and fusion
pressure) are balanced
Nebula
Protostar
Main Sequence
The life span of a star in its main sequence depends
on its mass and therefore its size.
The larger the star, the:
a) HOTTER it is, as more gravity = more nuclear fusion
b) SHORTER its lifespan – they get through their fuel more quickly
Stars around the same
mass as our sun last
around 1-10 billion years
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
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)
Main Sequence
Small mass star (incl. our
sun)
Red Giant
Large mass star
Super Red Giant
Red Giant (for stars the
size of our Sun or smaller)
RED GIANT:
• 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
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
Whistle-stop tour
(more detail to follow)
Star expands and
cools. Still fusing!
Red
Giant
White
Dwarf
Tiny, really hot,
glowing – but no
Fusion!
Red
Super
giant
SUPER
NOVA
Fusion stops, star COLLAPSES
then EXPLODES OUTWARDS
Black
dwarf
Can’t see it???
Neutron
OR
star
Black
hole
As MJ said, It Don’t Matter If You’re Black Or White (Dwarfs)
White
Dwarf
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
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
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
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!
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)
Neutron
star at
centre.
Radio waves
and X-rays
being emitted
from the
neutron star.
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.
Black
Holes
Here is a picture of a black hole.
Only Joking!
Black
Holes
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
Fusion occurs
in the core up
to iron.
Super Red Giant
The fuel for
fusion runs out,
the star
collapses and
explodes
outward.
Black Hole
Collapsed core of a large star.
Very dense with a very strong
gravitational pull.
Radio waves and x-rays are
emitted from them.
Supernova
The fusion at the
core slows down so
shells fuse causing
the star to expand.
Elements heavier
than iron are
formed in the
explosion and
thrown into space.
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.
Nebula
Protostar
Main Sequence
Small mass star (incl. our sun)
Red Giant
White Dwarf
Black Dwarf
Large mass star
Super Red Giant
Supernova
Black Hole
Neutron Star
KEY TASK
Make a summary sheet by:
a) Labelling each stage
b) Writing summary bulletpoint info for each stage
around the sides
N
P
M
S
Large mass star
Small mass star (incl. our
sun)
R
G
W
B
R
D
D
S
G
S
B
H
N
S
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.