1. The Life and Death of a Star

2. 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 2

3. 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
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

4. Hertzsprung-Russell Diagram

5. 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 5

6. 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. 6

7. 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. ( 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. 7

8. 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 8

9. 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. 9

10. 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 10

11. 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 11

12. 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 12

13. Lifespan Summary
Summary Video – begin at 5:50 13