Star's Life Cycle

2. EVOLUTION AND LIFE CYCLE OF STARS

5. EXAMPLES
LOOK ON P. 764
in TEXT Book

6. STARS Life Cycles:
• A Star goes through a life cycle just like humans
would.
Birth Life Death

10. Birth of a star
A shockwave
causes
nebula gas
and dust to
collapse due
to gravity.
As the cloud
collapses and
shrinks due to
gravity, it
compresses the
gas and starts
to heat up
inside
(compression
creates heat).
It gets so hot
that nuclear
fusion starts
inside the
center of the
cloud (a
protostar
forms).

11. Birth of a Star
The high heat and
temperature from
the fusion reaction
increases the forces
of outward
pressure and the
protostar grows
and starts to
expand.
Eventually, a balance
between gravity
wanting to collapse
the protostar and the
heat and pressure
wanting to expand
the protostar is met.
A star is
born and
burns
happily.

12. Shockwave Gravity
compresses
star
Heats up
fusion
begins,
pressure
balances
with gravity.
Hydrogen is
burning and
balancing out
the
compression
from gravity.
REVIEW…

13. This balance between gravity and heat and
pressure from fusion must be kept in order for a
star to live peacefully.
If a star does not burn enough hydrogen, then
gravity will overcome it and it will start to collapse
in on itself again.
If a star makes too much energy, it will expand too
much, thus shutting down the fusion reactions,
ultimately causing the star to collapse again until
fusion starts back up
A star will continue to live happily as long as it balances internal pressure
and gravity.
Fusion for a star is like FOOD for us, With it we are sustained and can live.
Without it, we die.
HAPPY LIFE – Main Sequence

14. Implication
More Mass
• The more massive a star, the
harder it has to try to overcome
the force of gravity working to
collapse it.
More
Fusion
• The only way a star can overcome
the collapsing force of gravity, is
through the expanding heat and
pressure of fusion.
More
Hydrogen
• The more massive stars, need to
burn more hydrogen than less
massive stars.

15. Star live happy until it uses up its food: Hydrogen
Stars live happy
until they run out
of food. It can no
longer overcome
gravity and starts
to compress again.
Bigger stars need
more food to
overcome gravity
and thus use up
there food faster
(die earlier).
A dying star
changes from a
main sequence
start to another
type of star.
Depending on the
mass of the star.

16. What happens when a star eats all its food?
When it’s hydrogen inside its core runs out?
THE STAR EXPANDS
And is now
burning
Helium
BECOMES A
RED GIANT or
SUPER GIANT

17. DEATH
• At this time, gravity and heat and pressure are unbalanced.
• The star begins to collapse as it cannot overcome gravity
without fusion.
• What happens after this point depends on the MASS of
the star.

18. Medium – Low Mass Stars (like the sun).
The stars core
continues to
collapse
The star
collapses so
much that the
enormous
amount of heat
produced
causes leftover
hydrogen in a
cloud outside
the core and He
in the core to
burn.
This burning
causes the
outer
atmosphere
of the star to
be wisped off
into space
leaving just
the stars core
behind.
This core is
called a white
dwarf. White
dwarfs are
the same size
as earth but
way more
dense.

19. High Mass Stars
Hydrogen
runs out in
the core
much
more
quickly
because of
its high
mass,
collapsing
the core.
It
collapses
so much
that the
inside
heats up
much
more
than
inside a
medium
mass
star.
High mass
stars are
more
massive,
fusion of
elements
heavier than
H and He
take place,
causing the
star to
expand
rapidly into
a super giant
star.
Eventuall
y, the
star runs
out of
fuel in
the core
causing
fusion to
stop.
The star
collapses
in on itself
and then
explodes
outward
forming a
supernova
After the
explosion
however, the
core of the
star remain
and becomes
either a
black hole or
a neutron
star
depending
on just how
massive it is.

20. Supernova

21. From a Supernova… now what?
1.4 to 3 times the mass of
the sun
Core will collapse to
about the size of a small
city.
Pressure from neutrons
in the core stop the core
from collapsing further.
Neutron Star
Greater than 3.0 the mass
of the sun.
Core will collapse to about
the size of a small city.
Pressure from neutrons in
the core CANNOT stop the
core from collapsing.
Core shrinks down to a
point called a singularity.
Black Hole is
formed.

22. Neutron Stars
• Small
• Extremely Dense - teaspoon weighs a mountain
• Stupendous surface gravity
• Rotate 1000 times per second (pulsar)
neutron
star
marshmallow
3-megaton
explosion

23. Black Holes
• Massively dense core of a dead star compressed to a speck.
• There is an imaginary sphere around the black hole called an
event horizon.
• Anything that goes
into the event horizon
can not escape its
stupendous gravity.

24. Black Holes
If Sun became black hole (not possible)…orbits of
planets would be engulfed.
event horizon
BH