Transcript 201p Powerpoint Notes - Lives of Stars Page 1

1. Where Are We??
We live in the Milky Way
Galaxy, which is one of at
least 1 2 5 billion (maybe
up to 4 0 0 !) galaxies
in the universe.
The Milky Way is a barred
spiral galaxy, which is much
more complex than the earliest galaxies.
There are at least 4 0 0 billion stars and nebulae in
the Milky Way (maybe as many as 500 billion).
Our solar system is on the Orion Arm of the Milky
Way.
Nebulae
Nebulae
are large
clouds
of gas
and dust
Stellar nurseries
Stars are
born in
gaseous
bright
nebulae…
Stellar nurseries
and in
dark
nebulae.
The birth of a Protostar
Within a nebula,
a ball of
hydrogen gas
collapses under
the force of
gravity and
begins to heat
up – a protostar
is born!
Basic Star Formation….
A star is formed
when a contracting
cloud of gas and
dust becomes so
dense and hot that
nuclear fusion
begins.
•
Our Sun
Our Sun is a
small to
medium size
yellow star
Our Sun
1) A star is a huge nuclear
furnace, turning 400 million tons
of hydrogen
into helium
every
second!!
Hydrogen Fusion
Inside the sun,
4 Hydrogen nuclei
fuse to form 1
Helium nucleus,
releasing starlight,
heat and radiation.
Hydrogen Fusion
Note: Positrons
(anti-electrons) and
electrons
annihilate each
other to create
gamma rays &
photons of light
energy (starlight)
Proton-proton chain
reaction
Step 1:
2 1H1  1H2 + n (neutrino)
+ positron (e+)
Proton-proton chain
reaction
Step 2:
2 + H1 
H
1
1
3 + g (gamma ray)
He
2
Proton-proton chain
reaction
Step 3:
3 + He3 
He
2
2
4 + 2 H1
He
2
1
Proton-proton chain
reaction
Net Reaction:
4 1H1  2He4
Two Opposing Forces
1) The life of a star is marked by 2
opposing forces (see next page):
a) The pressure created by
thermonuclear
reactions in the
core of a star
pushing outwards
(
) and...
Two Opposing Forces
2) The crushing force of gravity
pulling the star’s
surface inwards.
(
)
Two Opposing Forces
Pressure from
Nuclear Reactions
pushing outward
Gravity pulling
inward
Two Opposing Forces
3) At first, gravity dominates
and causes the core of the
star to collapse inward.
Two Opposing Forces
4) As the core starts to
shrink, the core temperature
rises to 40 million C and
Hydrogen fuses to make
Helium.
Two Opposing Forces
5) The outward flow of
energy increases,
causing the star to
expand outward.
Two Opposing Forces
6) When a star runs out of
hydrogen, its nuclear “fuel”,
the force of gravity takes
over and the core of the star
collapses .
Two Opposing Forces
7) This time, the compression
caused by the gravitational
collapse heats the star’s core
even more & starts a whole
new chain of thermonuclear
reactions with Helium
replacing Hydrogen as the
nuclear fuel.
Two Opposing Forces
8) When stars finish “burning up”
both their hydrogen and helium
nuclear fuel, they begin to die out
again and, once again, they
collapse inward.
What happens next depends
on the size of the star…
(to be continued...)
The mass of a star determines its fate
Brown Dwarf
Tiny Stars
Sun-Sized Stars
Red Giant
White Dwarf
3 – 10 M Stars
Supergiant
20 – 30 M
Stars
Neutron Star
Supernova
Supergiant
Black Hole