Download The Life of a Star

The Life and
Death of a Star
A Newborn Star
A star is “born” when nuclear fusion begins.
Fusion creates a huge outward, expanding pressure as
the EM radiation journeys outwards away from the Sun’s
core.
This expanding pressure is countered
by the inward force of gravity,
determined by the density of the star.
Therefore, a star remains a constant
size while its supply of hydrogen
remains and nuclear fusion continues.
Fusion Video
Types of Stars
Red Dwarf: any star less than half a solar mass with minimal luminosity and
a long lifecycle
Red Giant: a star with a mass >/= 1 solar mass
•Becomes larger and redder as it runs out of fuel
Red Supergiant: a star with a mass >/= 10 solar masses
•Becomes larger and redder as it runs out of fuel
Blue Giant/Supergiant: a massive, hot and luminous star with a very short
lifespan
White Dwarf: a small, hot, dim star created by the remaining material that is
left behind when a red giant dies
Black Dwarf: the remnant of a white dwarf that has cooled down, is void of
any luminosity and as a result is invisible
Hertzsprung-Russell Diagram
A Star’s Lifespan
Depends on its initial mass
There are 4 different categories of stars:
1. Low mass stars
2. Medium mass stars
3. Large mass stars
4. Extremely Large mass stars
Low Mass Stars
Origin: Form in a very small
nebula
Lifespan: Fuses hydrogen into
helium for hundreds of
billions of years as a
red dwarf.
Old Age: After running out of fuel,
these stars contract due
to gravity and heat up
becoming white dwarfs.
Death: They will eventually cool to
black dwarfs.
Medium Mass Stars(Our Sun)
Origin: Formed in a small nebula
Lifespan: Mid-mass stars spend their lives
fusing hydrogen into helium in their
cores. (10-100 billion years)
Old Age: The core runs out of hydrogen
reducing the outward force
created by fusion enabling gravity
to overpower it
– Gravity compresses the core causing
secondary fusion
– As secondary fusion continues,
» the core and the shell contract
» the outer layers of the star
expand producing a red giant.
– As the star runs out of fuel, its
temperature decreases, leaving
behind its core (white dwarf)
Death: Gravity eventually wins causing the
white dwarf to become a black dwarf.
Medium Mass Stars:
Red Giant to White Dwarf
When the core of a medium mass star
reaches 1.0 x 106°C, helium fusion
begins in the core (secondary
fusion).
Once all fusion reactions stop, the star
throws its outer layers into space,
forming a planetary nebula
– This leaves behind the hot dense
core of the red giant.
– The remaining core is called a
white dwarf.
Over time, the white dwarf cools off and
becomes a black dwarf.
Planetary Nebula:
A collection of gas and dust that was
formed during the dying phases of a
star
Large & Extremely Large Stars
Origin: Formed in large or very large nebulae.
Lifespan: Large stars live relatively short lives (a few million years) because they
burn their hydrogen quickly
Old Age: After hydrogen fusion stops, the core compresses under gravity while
the outer layers expand and the star becomes a Red Supergiant
– In Red Supergiants, other fusion reactions occur from the products of the
previous fusion reactions (primary & secondary fusion reactions):
Hydrogen →Helium → Carbon → Neon → Silicon → Iron
– When ALL fusion reactions stop, gravity takes over and the star collapses
inwards.
Death: The outer layers falling
inward rebound off of
the dense iron core and
explode outward in an
explosion known as a
Supernova, leaving
either a Neutron Star or
a Black Hole behind.
Supernova
Only occurs in a large star when it uses up its fuel.
The star collapses on itself (due to gravity), bounces off the iron core and
then explodes outward with great force.
– During this time, it shines so brightly that it can be seen during the day.
– A star forming a supernova will either become a neutron star or black hole
depending on the initial mass of the star.
Supernova Clip
Neutron Star/Pulsar
Neutron Star:
The extremely dense core of a
star that has just undergone a
supernova, spinning very
quickly and emitting high
frequency EM radiation
The spinning generates a
magnetic field and the star
spews out radiation like a
lighthouse beacon.
Occurs when large and
extremely large stars die
– Solar mass >10
Neutron Star
Black Hole
Black Hole:
Created when an extremely
large star collapses due to
gravity into a single point
(singularity).
• A singularity’s pressure,
density and gravity are
infinite
• Nothing, including light, can
escape
A black hole consists of a huge
amount of matter packed into
a dense core
Occurs only in extremely large
stars
– Solar mass > 30
Lifespan Summary
Summary Video – begin at 5:50