The Life Cycles of Stars
The Broncos Science Team
Twinkle, Twinkle, Little Star ...
How I Wonder What You Are ...
Stars have
• Different colors
 Which indicate different temperatures
The hotter a star is, the faster it burns
its life away.
Stellar Nursery
Space is filled
with the stuff to
make stars.
Stars start from clouds
Clouds of gas
and dust
called nebulas
provide the
material from
which stars
form.
But not this kind of dust
Rather: Irregular Grains
Of Carbon or Silicon
The Nebula Theory
All celestial objects
including stars and planets
are created from a cloud
of gas and dust called a
Nebula.
• Gravitational attraction
takes over causing the
gas/dust to stick together
to form stars and planets.
This is a picture of
a nebula
Nuclear Fusion !
At 15 million degrees Celsius in the center of
the star, fusion ignites !
Where does the energy come from ?
4 (1H) --> 4He + 2 e+ + 2 neutrinos + energy
Mass of four 1H > Mass of one 4He
E = mc2
A Balancing Act
The sun’s gravity pulls it together while
nuclear fusion blows it apart!
Energy released from nuclear fusion counteracts the inward force of gravity which
results in a “tug of war”.
Throughout its life, these
two forces determine the
stages of a star’s life.
New Stars are not quiet !
Remember Earth is
1A.U. from the Sun
Expulsion of gas from a young binary star system
All Types of Stars Shown Together
Stars have different properties including colour,
brightness and size
The Properties of Stars
1. Brightness (Luminosity): How bright does the star seem.
- Affected by distance from Earth (closer stars appear
brighter).
- Size of the star (larger stars produce more light).
2. Temperature and Composition: The colour of a star tells
us the star’s surface temperature and what elements the
star is made of (composition).
3. Size and Mass: The radius of a star is calculated from its
luminosity and temperature. Stars range in size from 1/10
the to over 1000X the size of our sun!
All Types of Stars  HR Diagrams
How bright
is the
star???
What colour
is the
star???
How hot is
the star???
These boxes on the BR-Diagram tell you what the
different axis mean.
All Types of Stars  HR Diagrams
Use the HR-Diagram to copy the characteristics of the following
groups of stars: Main Sequence, Red Giants, Low Mass Stars (i.e. Sun),
Supergiants, White Dwarfs and Red Dwarfs.
The Life Cycles of Stars
1. Sun-like Star Cycle (Small Mass)
2. Large Mass Star Cycle
A Red Giant  A Really “Superstar”
Betelgeuse
– A Red Giant Star
Betelgeuse is part of the Orion
Constellation
Which Brings us Back to ...
The end for Small type stars (Sun)
After Helium exhausted, outer layers of star expelled
Planetary Nebulae
White dwarfs
At center of Planetary Nebula lies a
White Dwarf.
• Size of the Earth with Mass of the Sun
“A ton per teaspoon”
• Inward force of gravity balanced by
repulsive force of electrons.
Fate of high mass stars
After Helium exhausted, core collapses again
until it becomes hot enough to fuse Carbon
into Magnesium or Oxygen.

12C
+ 12C --> 24Mg
OR 12C + 4H --> 16O
Through a combination of processes,
successively heavier elements are formed
and burned.
Periodic Table
Light Elements
Heavy Elements
28Si +4
12
416
1
12
16
12
16
12
4He
20
24
32
16
He
He
7(
3(
4
CO4(4+
He)
+
He)
+H)
C-N-O
C
O
C
O 56
Ni
C
Cycle
Ne
Mg
S
O
++ +
energy
energy
+++energy
energy
energy
energy 56Fe
Supernova !
Supernova Remnants: SN1987A
a b
c d
a) Optical - Feb 2000
• Illuminating material
ejected from the star
thousands of years
before the SN
b) Radio - Sep 1999
c) X-ray - Oct 1999
d) X-ray - Jan 2000
• The shock wave from
the SN heating the
gas
Supernova Remnants: Cas A
Optical
X-ray
Elements from Supernovae
All X-ray Energies
Calcium
Silicon
Iron
What’s Left After the Supernova
Neutron Star (If mass of core < 5 x Solar)
• Under collapse, protons and electrons
combine to form neutrons.
• 10 Km across
Black Hole (If mass of core > 5 x Solar)
• Not even compacted neutrons can
support weight of very massive stars.
Black Holes - Up Close and Personal
Accretion Disk
Singularity
(deep in center)
Event Horizon
Jet
(not always present)
SN interaction with ISM
Supernovae compress
gas and dust which lie
between the stars. This
gas is also enriched by
the expelled material.
This compression starts
the collapse of gas and
dust to form new stars.