Title: Is the theory correct? Two Clues from two Types of Star Clusters
1Is the theory correct? Two Clues from two Types
of Star Clusters
? Open Cluster
Globular Cluster ?
2Star 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)
3What 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
4Why Do Stars Leave the Main Sequence?
5Stage 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
6Stage 9 The Red Giant Stage
- Luminosity huge ( 100 Suns)
- Surface Temperature lower
- Core Temperature higher
- Size 70 Suns (orbit of Mercury)
7The 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
8Stage 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 gt orbit of Mars
9Deep 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
10Observing 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
11Catching Stellar Evolution red-handed
Main-sequence turnoff
12Type 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
13Reason 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
14Light 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 formed
Duration 100,000 years Central Temperature 300
? 106 K Surface Temperature 100,000 K Size 0.1
? Sun
15Deep 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
16Stage 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!
17Stage 14 Black Dwarf
- Impossible to see in a telescope
- About the size of Earth
- Temperature very low
- ? almost no radiation
- ? black!
18More Massive Stars (M gt 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
19Evolution 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
20Supernovae 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
21Type I vs Type II Supernovae
22Type 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
23Type 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
24Supernova Remnants
From Vela Supernova
Crab Nebula
25SN1987A
26Whats 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
27More 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
28Formation 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
29Neutron 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
- Cant be white dwarfs must be neutron stars
30The Lighthouse Effect
- Pulsars rotate very rapidly
- Extremely strong magnetic fields guide the
radiation - Results in beams of radiation, like in
lighthouse
31Super-Massive Stars end up as Black Holes
- If the mass of the star is sufficiently large (M
gt 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
32How might we see them?
- Radiation of infalling matter
33Evidence 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
34Black Holes The Center of Galaxies?
- IR picture of the center of the Milky Way
35Novae 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)
36Review The life of Stars