1. 28.3 LIFE CYCLE OF STARS
Stage 1
Stage 2
Stage 3

2. Stage 1 Stage 2 Stage 3 Main Sequence Star (stars like our Sun)
Main sequence stage, last
the longest
Star stays until all H2 fuel
is used up (converted to He)
He core shrinks and
Contracts to produce
additional heat.
H2 fusion starts in the outer
Layers causing the star to
swells– moves to next stage.
Main Sequence Star (stars like our Sun)
less than 8 solar masses
Nebula spins & flattens
To form a protostar
Protostar shrinks due to
gravitional attraction.
As temperature increases and pressure increases, light and heat are emitted
Energy is produced from
Nuclear fusion of H2 into
He begins – moves to next stage
Core temperature increases
to a point that He can fuse into
heavier elements (O2 and C)
H2 into He continues in the
outer layers.
When Hydrogen is exhausted
the outer layers blow away
leaving only the carbon-oxygen
core – White Dwarf.
Expelled layer absorbs the
white dwarfs ultraviolet emission
causing a halo affect called a
Planetary Nebula

3. Stage 1 Stage 2 Stage 3 Massive Star 8 solar masses or more
Same process however due
to the more massive size it
swells to an even larger size
than the main sequence star
does
Same process however
much more dust and gas
present
Fusion in core continues until iron
forms
Iron does not release energy so it
absorbs it, thus leading to a quick
collapse resulting in an explosion –
Supernova.
Massive star that is 8-15 solar
masses, creates a neutron star
following the supernova
Massive star that is more than
15 Solar masses, creates a
black hole following the supernova.

4. HR Diagram Gives you a picture of the star’s life
Luminosity versus temperature
Color indicates temperature

5. HR Diagram Groups Main Sequence Giant Stars 90% of all stars
Band across diagram with varying temps (colors)
Star is in middle of its life
Sun is here
Giant Stars
Next stage in Sun’s life
To the right above main sequence
More bright than main sequence because they’re bigger
Cooler stars

6. HR Diagram Groups Red Super Giants 4. Blue Super Giants White Dwarfs
Upper right hand corner (above Giant stars)
Massive stars at end of life
Very cool, but size gives them luminosity
4. Blue Super Giants
Very rare, large hot, bright stars at end of life
White Dwarfs
Lower left corner
Hot, dim main sequence stars at end of their life
small

7. Galaxies and the Universe
Look like hazy patches- need telescope to view
Classified based on shape
Spiral (ex. Milky Way—our galaxy)
Elliptical are near spherical to lens-shape
Irregular (ex. The two Magellanic Clouds

8. Origin of the Universe
Big Bang Theory– explains history/creation of the universe
Approximately billion years ago all matter in the universe existed in an incredibly hot and dense state, from which it expanded and cooled slowly condensing into stars and galaxies.
Evidence is Redshift of cosmic background discovered by Edwin Hubble