Where do stars form?

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Where do stars form?
In H II regions along spiral arms HII regions
M51-HST
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NGC 3079
HST
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Stars form in nebulae
Orion A - NOAO
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Star Formation Main Steps

• 1. Gas cloud collapse
• 2. Main Sequence stage (H fusing or burning)
• 3. Red Giant or supergiant phase (He fusion)
• 4. Ejection Planetary nebula or Supernova
• 5. Core remnant stage
• white dwarf
• neutron star/pulsar or
• black hole

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Initial gas cloud collapse phase
Giant Molecular Cloud 105 106s M? cold gas dust
Protostar Conversion of GPE into KE
heat 20003000K IR microwaves emitted
T. Tauri stage star-like. Strong jets along
rotational axes. Star starts fusion
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Protostar T Tauri track
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Starbirth in nearby galaxies
Large Magellanic Cloud 6 degrees, 160,000 ly
Small Magellanic Cloud 5.6 degrees, 240,000 ly
30 Doradus (Tarantula nebula)
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Tarantula nebula in the LMC (HST)
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Barnard 86, a Bok globule
NGC 6520, an open cluster
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ORION
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The Orion nebula - our nearest stellar nursery
Visible light is absorbed by dust gas
IR light travels through the dust gas, allowing
us to view star birth
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Proplyds (protoplanetary disks) these are
possible precursors to solar systems
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M16 - a stellar nursery
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T Tauri type stars
T Tauri (CFHT)
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T Tauri type stars
Material ejected along rotational axes
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AFGL 2591 A Massive Star Acts Up
Young star AFGL 2591 is putting on a show. The
massive star is expelling outer layers of
dust-laced gas as gravity pulls inner material
toward the surface. AFGL 2591 is estimated to be
about one million years old -- much younger than
our own Sun's 5 billion-year age -- and has
created a nebula over 500 times the diameter of
our Solar System in just the past 10,000 years.
The above image in infrared light is one of the
first from the new NIRI instrument mounted on one
of the largest ground-based optical telescopes in
the world Gemini North. Sharp details are
discernable that are blocked by opaque dust
in visible-light images. Close inspection of the
image reveals at least four expanding rings,
indicating an episodic origin to the mysterious
activity. AFGL 2591 lies about 3000 light years
away toward the constellation of Cygnus.
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Stars form in clusters
The Pleiades, an open cluster
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The Jewel box open cluster
M. Bessell (MSSSO)
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Mass determines a stars Main Sequence
luminosity!!!
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Lifetimes of main-sequence stars
Heavier MS stars have shorter lives
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Formation of stars planets
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Stellar Evolution and the HR Diagram

• Our Sun as a star
• Nuclear fusion and energy transport in the sun
• Stages of stellar evolution for low and
  intermediate mass stars
• The Hertzsprung Russell diagram

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Light Travel from the Sun The speed of light is
c 3x108 ms-1. A photon leaving the surface of
the sun reaches the earth after a time T
distance/c 8 minutes.
How does the Sun burn? The Sun must be at least
as old as the earth (4.6 billion years). It has a
luminosity of L 3.9 x 1026 Joules s-1. Its
mass composition is H 74
He 24
rest 2 What produces
the Suns energy?
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SOHO image of the solar chromosphere in
ultraviolet light.
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Some Solar Values
1/2o
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A star is a balancing act between
P Pressure T Temperature acting outwards
P,T
Gravity acting inwards
Hydrostatic equilibrium
The internal pressure gradients must counteract
the gravitational force G. (What happens
otherwise?) This is a fundamental requirement
for all stars.
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Solar Energy Source Some early ideas
Normal chemical reactions - such as the
combustion of coal Large numbers of meteorite
impacts (10,000 years) Slow gravitational
collapse (20 million years)
In the 1930s a major breakthrough in astronomy
was the understanding that the energy source in
stars is from Nuclear fusion reactions at high
temperatures and pressures.
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Nuclear Fusion in the Sun
Core temperature 1.5 x 107 K Core radius 0.25
Rsurface Core mass 10 total stellar mass
The suns energy is generated in the core by
nuclear fusion reactions which convert Hydrogen
to Helium 4 1H 1 4He energy
(photons and
neutrinos) Energy released ?mc2
3.85 x 1026 J/s
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Fusion Hydrogen ? Helium
This Proton-Proton chain is the energy source of
stars like our Sun
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CNO cycle
Uses C-12 as nuclear catalyst to convert 4
protons into He-4 Dominates in more massive
(hence hotter) MS stars
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Comparison of PP CNO
CNO contributes only 1 Suns energy
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What mass of hydrogen is converted to helium?
Mass s-1 luminosity / c2 4 x 109 kg s-1
How long can the sun survive by burning hydrogen?
Hydrogen burning lifetime H mass available in
core
Rate of conversion This gives a timescale of
approximately 1010 years, ie 10 billion
years. Our Sun is roughly half-way through its
hydrogen burning phase.
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Energy transport from the core to the visible
surface of the Sun
1. Core region R lt 0.25 Rsun Nuclear fusion zone
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2. Radiative region 0.25 lt R lt 0.75Rsun photons
diffuse through hot gas.
3. Convective Region 0.75 lt R lt Rsun Energy
transported by bulk gas motions.
4. Photosphere - the visible surface of the sun.
Thickness 500 km. T 6000K
Energy from the suns interior is released as
photons (particle of light) and as neutrinos
(zero or very low mass particles).
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Stellar Evolution
Stars form with masses between 0.1 and 100 times
the mass of the sun. For most of their
lifetimes they burn by the nuclear fusion of
hydrogen to helium. These are the Main Sequence
stars. Low mass stars convert hydrogen more
slowly and spend longer in this phase. They are
also cooler and smaller in size. Main sequence
lifetime 1010 years for Mstar Msun Main
sequence lifetime 106 yrs for Mstar 30
Msun What happens when the core hydrogen runs
out??
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Later Stages
As the core is used up the stellar core contracts
under gravity. This raises the central gas
density, pressure and temperature. At a
temperature of 2 x 108 K the stellar core
ignites Helium in the triple-alpha reaction 3
4He 12C ? (gamma ray). To balance
the pressure gradients across the star the outer
layers expand greatly and cool down. The star
is now a luminous Red Giant.
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Red Giant Stars
Red Giant stars have dense compact cores and much
lower density expanded atmospheres.
Core helium burning
Outer hydrogen atmosphere
R
By the time a star has become a Red Giant, its
radius has become about 150 times larger than in
the core-hydrogen burning stage. The Red Giant
stars are very luminous L 4?R2 ?Teff 4 The
surface temperatures are typically 3000 K
(reddish)
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Triple alpha (helium) flash
Fuses He into C, releasing energy. Red Giant phase
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Asymptotic Giant Branch Stars
For stars of one solar mass, the Red Giant phase
lasts for approximately 107 years. After the
helium core burning phase ends, the stellar
energy is supplied by nuclear fusion in two
layers around the core. In this double-shell
burning stage the star is known as an Asymptotic
Giant Branch (AGB) star. The AGB stars have
extremely strong STELLAR WINDS. The stellar
winds remove most of the stellar atmospheres
which are blown outwards into the interstellar
medium. The mass-loss rates of AGB stars are
typically 1018 kg s-1. This is a billion times
higher than for the sun.
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Nucleosynthesis in stars

• Mass is the key factor!
• Low mass stars convert hydrogen into helium H?He
• Stars like our Sun hydrogen into helium, then
  helium to carbon and oxygen
• High mass stars (gt5xSun) H?He, He ?C,O, ?Ca, Fe,
  Ni, Cr, Cu others! Then SUPERNOVA ? heavier
  elements

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Very high core temperature 4 x 109 K
Can fuse up to iron
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Fusion products in MS Red Giants
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The death of stars

• Once all fusion has occurred and outer layers
  expelled the final remnant of the star depends on
  the mass of the remaining core
• 1. If mass lt 1.4 M? ? white dwarf
• 2. For mass 1.4 M? lt M? lt 3.0 M? ? neutron star
  (pulsar)
• 3. If M? gt 3.0 M? ? black hole!

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Planetary Nebulae
At the end of the AGB phase the stars have lost
so much matter that their dense central cores
become visible. Nuclear burning now occurs in an
outer hydrogen layer. Ultraviolet photons from
the core sweeps up and ionises some of the
stellar wind into a shell around the core. The
swept up shell is seen as a PLANETARY
NEBULA. Planetary nebulae can have very exotic
and beautiful shapes.
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Two examples of circular planetary nebulae -
HST images
IC 3568
NGC 6369
For many examples of P. Nebulae - see the HST web
pages
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The bipolar planetary nebula M2-9
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The Making of the Rotten Egg Nebula
1.4 light yrs
Fast expanding gas clouds mark the end for a
central star in the Rotten Egg Nebula. The
once-normal star has run out of nuclear fuel,
causing the central regions to contract into a
white dwarf. Some of the liberated energy causes
the outer envelope of the star to expand. The
result is a proto- planetary nebula.
As the million-kilometer per hour gas rams into
the surrounding interstellar gas, a supersonic
shock front forms where ionized hydrogen and
nitrogen glow blue.
Credit V. Bujarrabal (OAN, Spain), WFPC2, HST,
ESA, NASA
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The Cats Eye Nebula
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The death of a star - formation of a planetary
nebula
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White Dwarfs
At the end of the planetary nebula stage, the
outer hydrogen is largely depleted. The star is
left with an extremely hot (30,000K), compact
(Rstar Rearth), and dense core (?star
106?earth). The star is now a WHITE DWARF. Very
little nuclear fusion occurs in White Dwarf
stars. The stars support themselves against
gravitational collapse by electron degeneracy
pressure. This is where electrons are forced
into high energy states. White Dwarfs cool very
slowly and gradually fade into darkness.
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The Evolution of Solar Mass Stars
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What about more massive stars?
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Eta carinae
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Their fate?
They go out in a bang as a supernova these
exploding stars outshine an entire galaxy for a
few weeks Massive stars live fast die
young!!! (and leave spectacular remnants)
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Crab nebula - a supernova remnant
VLT
The precursor star went supernova in AD 1054,
recorded by Chinese astronomers
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Core of Crab nebula
HST
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The centre of the Crab nebula contains a rapidly
rotating neutron star - a pulsar. It contains
about twice the mass of the Sun but is only10 km
across, spinning dozens of times a second!
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The Cygnus Loop supernova remnant This expanding
cloud carries heavy elements out into space. You
and I are made up of elements formed and
transported in this way! The accompanying
shockwave can trigger new bouts of star formation
as it passes through other nebulae.
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Even more massive stars end up as black holes!
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The Hertzsprung Russell Diagram
The HR diagram was first plotted by Hertzsprung
(1911) and Russell (1913). It is used to study
the evolution and properties of stars. The HR
diagram is a plot of Stellar Luminosity or
Absolute Magnitude (y-axis) against Stellar
(surface) Temperature or colour (x-axis).
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Supergiants
Main Sequence
Giants
White dwarfs
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Hertzsprung Russell Diagram for Nearby Stars
To plot the HR diagram we need to know the
individual stellar distances - or use a group of
stars in a star cluster which are known to be at
the same DISTANCE.
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HR Diagram for close well known stars
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Schematic view of the evolutionary path of a one
solar mass star.
Asymptotic Giant Branch
Planetary nebulae
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Red Giant
Luminosity (solar units)
1
Sun-like star
Main Sequence
White Dwarf track
Red
Blue
10-3
20000
3000
6000
Effective Temperature (K)
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HR sequence for cluster of stars I
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II
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III
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IV
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HR diagram for the globular cluster M5 - plotted
as V magnitude against B-V colour.
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B - V
B-V
The globular clusters contain old (population
II), highly evolved stars. This cluster shows
well-defined giant and horizontal branches.
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As a cluster ages the turn-off point moves
further down the Main Sequence. This can be used
to determine the age of a stellar cluster.
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Our Sun
Final mass after mass-loss
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