Lifecycle
• Lifecycle of a main sequence G star
• Most time is spent on the main-sequence (normal star)
Stellar Lifetimes• From the luminosity, we can
determine the rate of energy release, and thus rate of fuel consumption
• Given the mass (amount of fuel to burn) we can obtain the lifetime
• Large hot blue stars: ~ 20 million years
• The Sun: 10 billion years• Small cool red dwarfs: trillions
of years
The hotter, the shorter the life!
Mass Matters• Larger masses
– higher surface temperatures
– higher luminosities– take less time to form– have shorter main
sequence lifetimes
• Smaller masses– lower surface
temperatures– lower luminosities– take longer to form– have longer main
sequence lifetimes
Mass and the Main Sequence
• The position of a star in the main sequence is determined by its massAll we need to know
to predict luminosity and temperature!
• Both radius and luminosity increase with mass
Main Sequence Lifetimes
Mass (in solar masses) Luminosity Lifetime
10 Suns 10,000 Suns 10 Million yrs
4 Suns 100 Suns 2 Billion yrs
1 Sun 1 Sun 10 Billion yrs
½ Sun 0.01 Sun 500 Billion yrs
Is the theory correct? Two Clues from two Types of Star Clusters
Open Cluster
Globular Cluster
Star Clusters
• Group of stars formed from fragments of the same collapsing cloud
• Same age and composition; only mass distinguishes them
• Two Types: – Open clusters (young birth of stars)
– Globular clusters (old death of stars)
What do Open Clusters tell us?
•Hypothesis: Many stars are being born from a interstellar gas cloud at the same time
•Evidence: We see
“associations” of stars
of same age
Open Clusters
Why Do Stars Leave the Main Sequence?
• Running out of fuel
Stage 8: Hydrogen Shell Burning• Cooler core imbalance
between pressure and gravity core shrinks
• hydrogen shell generates energy too fast outer layers heat up star expands
• Luminosity increases• Duration ~ 100 million
years• Size ~ several Suns
Stage 9: The Red Giant Stage
• Luminosity huge (~ 100 Suns)
• Surface Temperature lower
• Core Temperature higher
• Size ~ 70 Suns (orbit of Mercury)
The Helium Flash and Stage 10
• The core becomes hot and dense enough to overcome the barrier to fusing helium into carbon
• Initial explosion followed by steady (but rapid) fusion of helium into carbon
• Lasts: 50 million years• Temperature: 200 million K
(core) to 5000 K (surface)• Size ~ 10 the Sun
Stage 11• Helium burning continues• Carbon “ash” at the core
forms, and the star becomes a Red Supergiant
•Duration: 10 thousand years
•Central Temperature: 250 million K
•Size > orbit of Mars
Deep Sky Objects: Globular Clusters
• Classic example: Great Hercules Cluster (M13)
• Spherical clusters
• may contain
millions of stars
• Old stars
• Great tool to study
stellar life cycle
Observing Stellar Evolution by studying Globular Cluster HR
diagrams• Plot stars in globular clusters in
Hertzsprung-Russell diagram
• Different clusters have different age
• Observe stellar evolution by looking at stars of same age but different mass
• Deduce age of cluster by noticing which stars have left main sequence already
Catching Stellar Evolution “red-handed”
Main-sequence turnoff
Type of Death depends on Mass
• Light stars like the Sun end up as White Dwarfs
• Massive stars (more than 8 solar masses) end up as Neutron Stars
• Very massive stars (more than 25 solar masses)
end up as Black HolesBlack Holes
Reason for Death depends on Mass
• Light stars blow out their outer layers to form a Planetary Nebula
• The core of a massive star (more than 8 solar masses) collapses, triggering the explosion of a Supernova
• Also the core of a very massive stars (more than 25 solar masses) collapses, triggering the explosion Supernova Supernova
Light Stars: Stage 12 - A Planetary Nebula forms
• Inner carbon core becomes “dead” – it is out of fuel
• Some helium and carbon burning continues in outer shells
• The outer envelope of the star becomes cool and opaque
• solar radiation pushes it outward from the star
• A planetary nebula is formedDuration: 100,000 yearsCentral Temperature: 300 106 KSurface Temperature: 100,000 KSize: 0.1 Sun
Deep Sky Objects: Planetary Nebulae
• Classic Example: Ring nebula in Lyra (M57)
• Remains of a dead, • exploded star• We see gas expanding in a sphere• In the middle is the dead star, a
“White Dwarf”
Stage 13: White Dwarf• Core radiates only by
stored heat, not by nuclear reactions
• core continues to cool and contract
• Size ~ Earth
• Density: a million times that of Earth – 1 cubic cm has 1000 kg of mass!
Stage 14: Black Dwarf
• Impossible to see in a telescope
• About the size of Earth
• Temperature very low
almost no radiation
black!
More Massive Stars (M > 8MSun)
• The core contracts until its temperature is high enough to fuse carbon into oxygen
• Elements consumed in core
• new elements form while previous elements continue to burn in outer layers
Evolution of More Massive Stars• At each stage the
temperature increases reaction gets faster
• Last stage: fusion of iron does not release energy, it absorbs energy cools the core “fire extinguisher” core collapses
Region of instability: Variable Stars
Supernovae – Death of massive Stars• As the core collapses, it
overshoots and “bounces”• A shock wave travels
through the star and blows off the outer layers, including the heavy elements – a supernova
• A million times brighter than a nova!!
• The actual explosion takes less than a second
Supernova Observation
Type I vs Type II Supernovae
Type I – “Carbon Detonation”• Implosion of a white dwarf after it accretes
a certain amount of matter, reaching about 1.4 solar masses
• Very predictable; used as a standard candle– Estimate distances to galaxies where they occur
Type II – “Core Collapse”
• Implosion of a massive star
• Expect one in our galaxy about every hundred years
• Six in the last thousand years; none since 1604
Supernova Remnants
Crab Nebula
From Vela Supernova
SN1987A
• Lightcurve
Formation of the Elements• Light elements (hydrogen, helium) formed in Big Bang
• Heavier elements formed by nuclear fusion in stars and thrown into space by supernovae– Condense into new stars and planets
– Elements heavier than iron form during supernovae explosions
• Evidence:– Theory predicts the observed elemental abundance in the
universe very well
– Spectra of supernovae show the presence of unstable isotopes like Nickel-56
– Older globular clusters are deficient in heavy elements