Download 1 - Stellar Life Cycle

Stellar Evolution:
The Life Cycle of a Star
Essential Question: How does the life cycle
of a star vary based upon its mass?
Star Sequence (Life Cycle)
 Stars develop in a stellar nebula - a giant cloud
of dust and gas.
 Sometimes part of the cloud compresses and
shrinks because of gravity.
 As it compresses it becomes hotter and when it
is hot enough, nuclear fusion can start in the
center, creating a protostar.
 The protostar then forms a star.
 Once nuclear fusion is producing heat in the
center of the new star, this heats stops the rest of
the star from collapsing.
 The balance between gravity trying to make the
star shrink and heat holding it up is called
Thermodynamic Equilibrium.
 The star then stays almost exactly the same for a
long time (about 10 billion years for a star like
the Sun).
 Depending on the star’s mass it becomes one of
three general types of stars:
 Brown Dwarf – never fully develops into a star –
not enough mass (not enough hydrogen for
fusion to continue)
 Average Star (Our Sun)
 MASSIVE Star
QUESTIONS
• What factor affects which type a star will
develop into?
• Describe the 3 different options that the stars
can develop into.
Average Stars
 Also known as Main Sequence stars – make up
90% of stars in space
 Become Red Giants as hydrogen resources are
depleted
 Then can break up into a ring
nebula (also called Planetary
Nebula, even though it has
nothing to do with planets)
 Remaining part of the star
becomes a White Dwarf as it cools
 Once all heat is gone it then
becomes a Black Dwarf and dies.
Massive Stars
 Become Red Supergiants as
hydrogen resources are depleted
 Then explodes into a Supernova
 Remaining part of the star
becomes either


a Neutron Star, or
a Black Hole and dies.
The life of a star
 During its “life” a star will not change very
much.
 However, different stars are different colors, size
and brightness.
 The bigger a star, the hotter and brighter it is.
Hot stars are Blue. Smaller stars are less bright,
cooler and Red.
 Because they are so hot, the bigger stars actually
have shorter lives than the small, cool ones.
Death of a star
 Eventually, the hydrogen (the “fuel” for the
nuclear fusion) in the center of the star will run
out
•No new heat is made and gravity will take over
and the center of the star will shrink.
•This makes the very outside of the star “float up”
and cool down, making the star look much bigger
and redder - a Red Giant star.
The second Red Giant stage
 As the center collapses, it becomes very hot
again, eventually getting hot enough to start a
new kind of nuclear fusion with Helium as the
fuel.
 Then the Red Giant shrinks and the star looks
“normal” again.
 This does not last very long, though, as the
Helium runs out very quickly and again the star
forms a Red Giant.
The end of a Sun-like star
 For a star like the Sun, no more nuclear fusion
can take place, so the center of the star will then
keep collapsing.
• Eventually it can become almost as small as the
Earth, but with the same mass as a whole star!
This very dense object is called a White Dwarf.
• A piece of White Dwarf the size of a cell phone
would weigh as much as an elephant on the
Earth!
Hertzsprung- Russell (H-R) Diagram
 Illustrates important things about stars:
 Brightness
 Absolute Magnitude
 usually shown on right-hand Y-axis
 Luminosity
 usually shown on left-hand Y-axis
 Temperature/Color
 Spectral Class
 shown on X-axis
Supernovae
Supernovae occur when a massive star
ends its life in an amazing blaze of glory.
For a few days a supernova emits as
much energy as a whole galaxy. When
it's all over, a large fraction of the star is
blown into space as a supernova
remnant. A typical supernova remnant is
at most few light-years across. (M 1
shown)
M 1 Supernova Remnant
M 1 Supernova was discovered in 1054 and
was visible for 23 days during the daytime,
and easily seen for 2 years at night. It
apparently was depicted in “cave paintings”