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Life Cycle of a Star
Star Life Cycle: Stars are like humans. They are born, live and then die.
What exists BEFORE stars form?
 Interstellar Medium – the total mass of the gas and
dust between the stars.
What is a Nebula?
 Nebula – a dense cloud of gas and dust
What is a Protostar?
 The nebula begins to contract due to gravity
and the pressure and temperature increase
and becomes a protostar
What is a Main Sequence Star?
 When the temperature gets hot enough, nuclear fusion
begins and a main sequence star is born.
 Inward force of gravity = Outward force of nuclear
fusion
 90% of all stars
Quick Check
The fusion reaction that fuels a mainsequence star converts hydrogen into:
A. Carbon
B. Iron
C. Neon
D. Helium
What is a Red Giant?
 A star loses mass during fusion as energy is released. This
decreases the star’s gravity.
 A star will expand, becoming a Red Giant when the
outward force of fusion is greater than the inward force of
gravity
What is a White Dwarf?
 As the fuel runs out in a star, fusion slows down.
 When the outward force of fusion is less than the inward
force of gravity, the star will shrink in size, becoming a white
dwarf.
White Dwarf
 Pulsar – a rotating white dwarf
emitting radio waves.
Quick Check
There are many sizes and colors of stars.
Which of the following best describes the
category in which the Sun would be
placed?
 A. blue supergiant stars
 B. red giant stars
 C. yellow main sequence stars
 D. white dwarf stars

What is a Supernova?
 Supernova – an explosion that marks the end of a very massive
star’s life.
 When it occurs, the exploding star can outshine all of the other
stars in the galaxy in total for several days and may leave behind
only a crushed core.
How does a star’s life end?
 The life cycle of stars depends on their mass.
 Small and medium stars become black dwarves once they die.
 Larger stars become novae and die as: neutron stars
 A neutron star is an imploded
core of an exploded star made
up almost entirely of neutrons.
A teaspoon of their material
would weigh more than all of
automobiles in the U.S. together
http://chandra.harvard.edu/photo/2004/rxj1242/index.html
How does a star’s life end?
 The life cycle of stars depends on their mass.
 The most massive stars become
supernovae and die as a black hole
 A black hole is an extremely
massive remnant from which light
can not escape
http://chandra.harvard.edu/photo/2004/rxj1242/index.html
Quick Check
All stars end up as a supernova and black
hole
 A. True
 B. False

Why do larger/hotter stars burn their fuel
faster and live shorter lives than the Sun?
 Large, more massive stars have much more gravity than the
sun.
– This greater internal pressure causes fusion reactions
to occur quickly.
– This causes the largest stars to burn their fuel, and
eventually run out, much more quickly
 Larger stars live shorter lives.
 Bigger stars are brighter and hotter due to the rapid rate of
fusion.
Where do we get the elements??

All stars spend the majority of their lives fusing
hydrogen into helium: the main sequence.
 When all of the hydrogen in the central regions is gone
and converted to helium, the star will begin to “burn”
helium into carbon.
 Massive stars: Stars heavier than about 5 times the
mass of the sun can do this with no problem: they
burn hydrogen and then helium and then carbon,
oxygen, silicon, and so on…until Iron.
Where do we get the elements??
 Iron is the lightest element that doesn’t release energy
when you attempt to fuse it together. You actually end
up with less energy than you started with! So instead of
generating pressure to hold up the outer layers, the iron
fusion actually takes pressure out of the core. Thus,
there is nothing left to combat gravity from the outer
layers. The result: collapse!
 ALWAYS REMEMBER THE BALANCE BETWEEN FUSION
AND GRAVITY!!
 This collapse (supernova) happens very, very quickly:
about 15 seconds. During the collapse (supernova), the
nuclei in the outer parts of the star are pushed together,
so close that elements heavier than Iron are formed.
What is the Hertzsprung-Russell Diagram?
 The stars in the
universe are at
different stages in
their life cycles.
 Some stars are young
and hot; others are
older and colder.
 The HertzsprungRussell Diagram (or
H-R Diagram) gives
us a picture of a
star’s life.
What is the Hertzsprung-Russell Diagram?
 The H-R diagram plots the
luminosity (brightness) or
absolute magnitude of
stars against their surface
temperatures.
 Most stars fall into 4
distinct groups in the H-R
diagram, because the
groups represent stages in
the life cycles of the stars.
What is the Hertzsprung-Russell Diagram?
 X axis:
 Plots surface temperature
 Decreases as you move
from left to right
What is the Hertzsprung-Russell Diagram?
 Y axis:
 Plots luminosity
(brightness) or absolute
magnitude
 Increases as you move
from bottom to top
APPARENT MAGNITUDE
VS.
ABSOLUTE MAGNITUDE
Apparent magnitude
Measure of the amount of a
star’s light received on Earth
Absolute magnitude
Measure of the amount of light
a star actually gives off
What does COLOR indicate about a star?
Hot stars are……..Blue
Cool stars are……..Red
Medium temperature stars are……..Yellow
White dwarfs
small, hot stars
leftover centers of old
stars
Red-Giants
When star runs out of
energy, the center shrinks
and the outer part expands
outward. It will grow very
large and cool.
Supergiants
Very massive cool
giant star
Main Sequence
Stretches diagonal from the top
left to the right bottom
Our sun is an average main
sequence star
Blue Supergiants
Very massive blue stars
Quickly use up energy and turn
into Giants or Supergiants
Red Dwarfs
Low-mass stars
Oldest stars in
galaxy