Stellar Evolution
‘The life-cycle of stars’
What is a star?
• LARGE GLOWING BALL OF GAS
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Composed of H and He
Fixed points of light in space
Undergo fusion in their cores
Generate heat and light
Give off tremendous radiation
Star Energy
• Nuclear Fusion – a nuclear reaction in which to
atoms are fused together…
• New elements are created and energy is
released.
• This process is responsible for creating ALL
elements found in the universe…
in other words, we are all made from star dust.
Star Energy (con’t)
• Hydrogen fusion
H + H  He +
Energy
• Helium fusion
He + He  Be +
More Energy
• As the mass of elements increases,
energy production increases
Properties of Stars
• Color & Temperature
• Brightness
– Apparent vs. Absolute
• Size & Mass
• Composition
Hertzsprung-Russel Diagram
• A graph showing the surface temperature
and absolute brightness of a group of stars
• Used to compare several properties of
stars and estimate their sizes and
distances
• Shows star color, size, temperature and
brightness
H-R Diagram
Protostar
Nebula
• A cloud of gas and
dust
• Gravity causes the
cloud to collapse and
condense
• Temperatures begin
to increase = Glows
• Fusion begins at
VERY high temps.
(Some of the extra gas and dust may form planets)
Protostar
• Gravity pulls a
nebula’s dust and
gas into a denser
cloud
• As a nebula heats
up, it contracts
• A contracting cloud
of dust with enough
mass to form a star
Main Sequence Stars
• Core reaches a temp
of ~15 million K.
• Hydrogen begins to
fuse into Helium in
the core.
• ~90% of stars lifetime
is spent in the main
sequence stage.
•Classified based on temperature and luminosity
Giant Stars
• Core decreases in size
as all (or most) H is
consumed
• He fusion is occurring –
producing more energy
• Diameter increases x10
• Surface temp decreases
as star expands
Super Giants
• Form from massive stars
• A chain of reactions take
place in the core producing
He, C, O, Ne, Mg, Si, S, Ar,
Ca, Ti, Cr, Fe
• Highest temperature = Blue
Super Giants
• Usually explode in a
tremendous event called a
supernova
Planetary Nebula
• Energy from star
becomes to
decrease; no more
elements to fuse
• Star begins to
collapse
• Dying star
surrounded by
gases
Nova/Super Nova
• The core of a giant star
produces too much
energy in it’s core and
causes the outside of
the star to expand.
• The outer layer of
gasses are blown out
to space (nova, or
supernova), leaving
behind a small, hot
core…
White Dwarf
• Stars decrease in size ~ same diameter as
earth, mass stays the same
• Mass of sun, size of Earth
DENSITY INCREASES TREMENDOUSLY
• Solid, but still hot, so it is glowing
• Luminosity decreases
Neutron stars
• Form from the remains of extremely
massive stars after a supernova.
• Very small ~ 30 km across
• Density of 2 x 1014
• Would be comparative to 1 sugar cube =
mass of humanity
Black hole
• If the core’s mass is even greater/denser
than a neutron star, it collapses.
• Surface gravity is so great that no matter
can escape it…not even electromagnetic
waves!
The fate of our Sun