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