1. Ch.2, Sec.2 – The Life Cycle of Stars
The Beginning and End of Stars
- stars begin their lives by coming together through the force of gravity inside of nebulas
- once there is enough gravity to pull hydrogen atoms together, they become extremely hot and fuse together through nuclear fusion
- a star is born and is often called a “protostar”!

2. Ch.2, Sec.2 – The Life Cycle of Stars
Different Types of Stars
- stars are classified by size, mass, brightness, color, age, spectrum, & temperature
- stars can be classified as main-sequence (like our Sun), giants, super giants, and white dwarfs
- after stars form, they enter the longest phase of their lives called the “main sequence”
- this is the stage where stars fuse hydrogen atoms into helium atoms, releasing massive amounts of energy!

3. Ch.2, Sec.2 – The Life Cycle of Stars
- after stars have fused all of their hydrogen atoms, they enter the 3rd stage of their lives called red giant
- red giants expand and cool, while their cores shrink and their atmospheres grow very large

4. Ch.2, Sec.2 – The Life Cycle of Stars
- in the final stages of a star’s life cycle, a star that has the same mass as the sun or smaller can be called a white dwarf
- white dwarfs have no hydrogen left for energy & can take millions of years to cool completely

5. Ch.2, Sec.2 – The Life Cycle of Stars
A Tool for Studying Stars
- two astronomers, Hertzsprung & Russell, developed a chart comparing stars’ surface temperatures & absolute magnitudes
- the H-R Diagram also shows how stars change over time
Ejnar Hertzsprung
Henry Norris Russell

6. Ch.2, Sec.2 – The Life Cycle of Stars
- most stars live in the main sequence stage of their lives!

7. Ch.2, Sec.2 – The Life Cycle of Stars
When Stars Get Old
- stars more massive than the sun can become stranger objects than red giants & white dwarfs!
- when massive blue stars use up all their energy, they explode into supernovas
- supernovas can end up being brighter than galaxies for several days

8. Ch.2, Sec.2 – The Life Cycle of Stars
- after supernovas occur, the materials eventually succumb to the force of gravity to become stars 2x the mass of our sun
- when these stars are created, all of their mass becomes neutrons and they’re called neutron stars

9. Ch.2, Sec.2 – The Life Cycle of Stars
- if a neutron star is spinning, it is called a pulsar
- pulsars send out pulses or beams of radiation as they spin

10. Ch.2, Sec.2 – The Life Cycle of Stars
- neutron stars can sometimes succumb to the overwhelming force of gravity to become black holes, which are usually the center of galaxies
- black holes have the greatest gravitational pull in the Universe
- no light can escape the pull of black holes, so they are very difficult to find!

11. Ch.2, Sec.2 – The Life Cycle of Stars