Download Integrative Studies 410 Our Place in the Universe

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 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
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
Duration: 100,000 years
Central Temperature: 300  106 K • A planetary nebula is formed
Surface Temperature: 100,000 K
Size: 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
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
From Vela Supernova
Crab Nebula
SN1987A
• Lightcurve
What’s Left?
• Type I supernova
– Nothing left behind
• Type II supernova
– While the parent star is destroyed, a tiny ultracompressed 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 ultrahigh 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