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Unit 11: Stellar Evolution
Mr. Ross Brown
Brooklyn School for Law and
Technology
In this unit we will learn about:
• Principles of stellar evolution models
• How mass plays a role in the evolution and fate of
a star
• The lifetimes of low- & high-mass stars, & how to
calculate lifetime using luminosity & mass
• The changes that occur as stars consume hydrogen
• Why a star’s core shrinks as its atmosphere
expands as it leaves the main sequence
• The sequence of fusion reactions that take place in
a high-mass star as it nears the end of its life
What are the primary factors that
influence a star’s lifecycle?
• 23 March 2016
• Do now: What are the forces that will
influence a star’s lifecycle?
What are the primary factors that
influence a star’s lifecycle?
• Gravity
– From the moment of a star’s birth, gravity is the
force that drives a star towards its own collapse
– Resisted by generating energy and pressure
• Hydrostatic equilibrium
What are the primary factors that
influence a star’s lifecycle?
• Solar Nebula Theory
– How our Sun began
– Interstellar cloud
What are the primary factors that
influence a star’s lifecycle?
• When the Sun consumes about 90% of its
hydrogen……….. (after about 10 billion years)
– Core will shrink and grow hotter
– Remaining hydrogen burns faster
– That radiating energy make the Sun grow huge
– But its outer edges will cool, turning red
– After another billion years, Sun will shrink some
but grow hotter still, a yellow giant
What are the primary factors that
influence a star’s lifecycle?
• 24 March 2016
• Do now: How will a massive star’s lifecycle be
different from an average star such as our
Sun?
What are the primary factors that
influence a star’s lifecycle?
• Lifecycle of a low mass star
What are the primary factors that
influence a star’s lifecycle?
• Massive stars
– >8M☉
– May only live 100 million years
– Increased heat fuses elements into heavier
elements: HeC, CO, SiFe.
– Fe can’t release energy, star instantly collapses
– Boom!
What are the primary factors that
influence a star’s lifecycle?
• Lifecycle of a high mass star
What are the primary factors that
influence a star’s lifecycle?
• Stellar Recycling
– All the energy radiating out, and all the matter
after the Boom!
– Each time through the cycle, more heavy elements
are generated
– We wouldn’t have terrestrial planets if not for this
process
What are the primary factors that
influence a star’s lifecycle?
What are the stages of star formation?
• 29 March 2016
• Do now: Based on what we’ve discussed, what
are the early stages of star formation?
What are the stages of star formation?
• Interstellar gas clouds
– Gas and dust
– Hydrogen (71%) and Helium (27%)
– Dust is silicates, carbon, and iron compounds
– Cold; 10°K
– Gravity  collapse  disk
– Protostar
What are the stages of star formation?
What are the stages of star formation?
• Protostar
– Hotter; 1500°K
– As gravity increases, so too does temperature
– When temperature reaches ~1,000,000°K, nuclear
reactions begin
• H  He
What are the stages of star formation?
What are the stages in the life of a
star?
• 30 March 2016
• Do now: What are the limits on the sizes of
stars?
What are the stages in the life of a
star?
• Stellar mass limits
– 0.1 and 30M☉
Too little gravity
– Stars smaller than 0.1 are very rare. Why?
– These low mass stars are very dim and are called
“Brown Dwarf” stars due to their dim red light
What are the stages in the life of a
star?
• Very high mass stars
Too much gravity
– Very rare. Why?
– They emit so much radiation that their pressure
prevents additional material from falling into them
– As such, if there were a 200M☉ star it would
“shine” itself apart
What are the stages in the life of a
star?
• The lifetime of a main-sequence star
luminosity
mass
– Determined by _____________
& _____________
– Formula:
t=1010m/l years
What are the later stages in the life of
a star?
• 31 March 2016
• Do now: How do stars die?
What are the later stages in the life of
a star?
• The Death of a Star
– The core continues to shrink. H is converted to He,
He to C.
– Burns hotter, grows, becomes more luminous
– Due to size, outer layers cool considerably
– Carbon and silicon atoms condense into “flakes”
– “Mira” star streams gas through interstellar space
What are the later stages in the life of
a star?
• As the star dies, there’s a shell of glowing gas,
…but it’s not a planet!
called a planetary nebula
• Astronomers thought it was a bubble, ejecting
Nope, not a bubble
gas in all directions
• With better pictures (thanks Hubble) we now
know it ejects gas in line with its axis
What are the later stages in the life of
a star?
What are the later stages in the life of
a star?
What are the later stages in the life of
a star?
• 1 April 2016
• Do now: Where are the heaviest elements of
the periodic table produced?
What are the later stages in the life of
a star?
• The old age of massive stars
– As heavier and heavier elements are produced,
and as the core compresses down further and
further, ever heavier elements are created
What are the later stages in the life of
a star?
What are the later stages in the life of
a star?
• For example:
– 12C + 16O fuse into…..
–28Si
– As the necessary temperatures for these reactions
increase, the star develops a layered effect, like an
onion
What are the later stages in the life of
a star?
• Core collapse!
– Iron cannot burn, tightly bound nuclei
– Fusion stops
– Under this great gravity and pressure, protons and
electrons “merge” becoming neutrons, and the
core changes from a sphere of iron to a sphere of
neutrons
What are the later stages in the life of
a star?
• In less than a second, core shrinks from iron
ball size of the Earth to a ball of neutrons
about 6 miles across
• All this collapsing generates heat to billions of
degrees, and in a sudden rush it explodes in a
supernova
What are the later stages in the life of
a star?
What are the later stages in the life of
a star?
• Supernova explosion propels material at
10,000 km/ sec
• Supernova remnant continues to expand
What are the later stages in the life of
a star?
• How did astrophysical theories of stellar
evolution develop?
– 1850s, Robert Boyle, Jacques A. C. Charles, and
Joseph Gay‐Lussac et al develop ideal gas law
– 1924, Sir Eddington develops mass-luminosity
relationship