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Life Cycle of a Star

The changes that a star goes through is
determined by how much mass the star has.
Two Types of Life Cycles:
Average Star- a star with relatively low mass
Massive Star- a star with relatively high mass
Stellar Nebula

All stars begin in
a cloud of gas
and dust called a
stellar NEBULA.
Eagle
Nebula
Lagoon
Nebula
Orion
Nebula
(Hubble Space Telescope)
It begins in a gas cloud called a nebula
Star formation is ongoing; star-forming regions
are seen in our galaxy as well as others:
•Gravity causes the nebula to
contract.
•The nebula separates into
smaller ‘chunks’. The gas and
dust in these ‘chunks’ condense
and becomes hotter.
•At 15 million ºC fusion begins
and the star emits light!!
•At this point, we call the new
star a “protostar”
The release of energy that causes the star to shine,
also stops the star from contracting. It counteracts
gravitational force.
We call these ‘Main Sequence’ stars.
Stars with one solar mass remain in the main
sequence for about 10 billion years, until all of the H
has fused to He.
FUSION
• http://youtu.be/oIe1EDExxyg
FUSION
Life Cycle of Stars – sketch this
http://www.seasky.org/cosmic/sky7a01.html
The Life of an Average Star
(one solar mass)


An Average Star (low mass
star) is condensed in a nebula
and begins a nuclear reaction
that causes hydrogen to form
helium, releasing energy in the
form of heat and light.
A low mass star will stay in this
MAIN SEQUENCE phase for a
about 10 billion years, until it
begins to use up all of its
hydrogen.
The Life of an Average Star
Towards the end of it’s MAIN
SEQUENCE phase, a star begins
to burn all of its hydrogen.
The He in the core is hot enough
to fuse into carbon.
Gasses start to expand.
The outer layers begin to cool,
causing a red color. We call this
a ‘red giant’.
The Life of an Average Star


The star begins to
quickly blow off its
layers of gas, forming a
cloud around the star
called a planetary
nebula.
The core of the star that
remains in the center of
the nebula is very hot
but not very bright.
Life of an Average Star

When a star has burned all
its fuel it will collapse
under the pressure of
gravity.

The remaining core is very
small and dense. It is
known as a ‘white dwarf’.
- When it stops
shining, it is
known as a black
dwarf.
Life of a Massive Star
http://www.seasky.org/cosmic/sky7a01.html
Stellar Nebula
All stars, regardless of size,
begin in a stellar nebula.
Life of a Massive Star

Stars with more
mass than the sun
(high mass stars)
burn their hydrogen
faster than low mass
stars, so their MAIN
SEQUENCE phase
is much shorter.

These stars burn
hotter and brighter
than low mass stars.
Life of a Massive Star

When the high mass star burns off it’s
hydrogen its outer layers begin to expand
and contract repeatedly and rapidly

Temperatures at the core are much higher
than a red giant. Nuclear fusion causes
elements to combine into an iron core at
amazing speeds.
Life of a Massive Star

There is no longer an
outward force to
counter gravity’s inward
force.
In less than a second,
the core collapses on
itself under the intense
gravity. A massive
explosion occurs called
a ‘supernova’.
A supernova is so energetic that it can
outshine its galaxy in terms of luminosity.
A supernova produces
more energy than the sun
will over the course of its
entire existence.
Life of a Massive Star

If the core survives the supernova explosion, and
the surviving core is less than 3 solar masses, it
becomes a neutron star.

Neutron stars are very tiny (10 km), very dense (1
trillion times more dense than a white dwarf) and
made up of neutrons.
Life of a Massive Star

If the surface of the neutron star is hotter than about a
million K, the surface would be liquid form, while if it's
cooler than that, it would be solid. Below that is a solid
crust, about a kilometer thick. This crust is very hard
and very smooth. Gravity would probably prevent any
irregularities larger than half a centimeter.
Life of a Massive Star
If the mass is too dense
(over 3 solar masses) it
will continue to collapse in
on itself, forming a black
hole. The gravitational pull
of a black hole is so great,
light can not escape…
more about black holes
later…let’s stick to stars
for now…
Some interstellar clouds are too small for fusion
ever to begin. They gradually cool off and
become dark.
Jupiter is a good example.
A protostar must have 0.08 the mass of the Sun
(which is 80 times the mass of Jupiter) in order
to become dense and hot enough that fusion
can begin.
Shock Waves and Star Formation
Shock waves from nearby star formation can be the trigger
needed to start the accretion process in an interstellar cloud:
Shock Waves:
Waves of matter driven
outward from high
temperatures and
pressures
Shell of gas rushing out
Shock Waves and Star Formation
Other triggers to start the collapse process in an interstellar
cloud:
• Death of a nearby average star
• Supernova
•Galaxy collisions
Shock Waves and Star Formation
This region may very well be several generations of star formation:
Characterizing
Stars
Temperature and Color (review)
Hottest = blue color
Medium = orange/yellow color
Coolest = red color
MAGNITUDE: Brightness Increases from bottom to top
The Hertzsprung-Russell (HR) Diagram
Are these stars
brighter or dimmer
than the sun
1 L is equal to the
brightness of the sun
REMEMBER: Temperature Increases from right to left
Sketch this HR
diagram (quickly).
Mark on the diagram
where you would find
the following:
Hot & Bright
Cool & Bright
Sun
Hot and Dim
Cool and Dim
HR Diagram Basics
Characteristics of Stars
The Hertzsprung-Russell (HR) Diagram

Temperature & Color
–
The color of a star indicates the temperature of
the star
–
Stars are classified by temperature
 Decreasing
 O,
termperature (bright to dim)
B, A, F, G, K, M [Oh Be A Fine Girl, Kiss Me ]
Spectral Type Classification
O
B
A
F
G
50,000 K
K
M
3,000 K
temperature
How bright a star appears to us on earth
What affects apparent magnitude?
The Inverse-Square Law
The farther a star is from Earth, the dimmer it looks to
us. Doubling the distance makes the star look onefourth as bright. Tripling the distance decreases the
star’s brightness by a factor of 9.
(Luminosity)
This is the real brightness of a star as
though all the stars were
At the same distance- Standard
comparison is 10 parsecs
Absolute Magnitude – the actual
brightness of a star
• Absolute magnitude
tells how bright a star
really is, no matter how
far from Earth it is.
• Are the car lights
actually dimmer as the
car moves away?