Death of Stars

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)

Lifecycle

• Lifecycle of a main sequence G star

• Most time is spent on the main-sequence (normal star)

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-Russel 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

SN1987A

• Lightcurve

What’s Left?

• Type I supernova– Nothing left behind

• Type II supernova– While the parent star is destroyed, a tiny ultra-

compressed remnant may remain – a neutron star

– This happens if the mass of the parent star was above the Chandrasekhar limit

More Massive Stars end up as Neutron Stars

• The core cools and shrinks• Nuclei and electrons are crushed

together• Protons combine with electrons to

form neutrons• Ultimately the collapse is halted by

neutron pressure, the core is composed of neutrons

• Size ~ few km• Density ~ 1018 kg/m3; 1 cubic cm

has a mass of 100 million kg!

Manhattan

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

Neutron Stars (“Pulsars”)

• First discovered by Jocelyn Bell (1967)– Little Green Men?!? Nope…

• Rapid pulses of radiation

• Periods: fraction of a second to several seconds

• Small, rapidly rotating objects

• Can’t be white dwarfs; must be neutron stars

The “Lighthouse Effect”

• Pulsars rotate very rapidly

• Extremely strong magnetic fields guide the radiation

• Results in “beams” of radiation, like in lighthouse

Super-Massive Stars end up as Black Holes

• If the mass of the star is sufficiently large (M > 25 MSun), even the neutron pressure cannot halt the collapse – in fact, no known force can stop it!

• The star collapses to a very small size, with ultra-high density

• Nearby gravity becomes so strong that nothing – not even light – can escape!

• The edge of the region from which nothing can escape is called the event horizon– Radius of the event horizon called the Schwarzschild

Radius

How might we “see” them?• Radiation of infalling matter

Evidence for the existence of Black Holes

• Fast Rotation of the Galactic Center only explainable by Black Hole

• Other possible Black Hole Candidates: – Cygnus X-1 (X-ray source), LMC X-3

• Observational evidence very strong

Black Holes – The Center of Galaxies?

• IR picture of the center of the Milky Way

Novae – “New Stars”

• Actually an old star – a white dwarf – that suddenly flares up– Accreted hydrogen

begins fusing

• Usually lasts for a few months

• May repeat (“recurrent novae”)

Review:The life of Stars