Forging the elements

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Forging the elements
Tales of the Sun and stars

• A new view of the Universe IV
• Fred Watson, AAO, with thanks to Jessica Chapman,
  ATNF
• April 2005

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Main Sequence Stars and Beyond
Our sun as a star Nuclear fusion and the main
sequence The Hertzsprung Russell Diagram
Evolution beyond the main sequence More
examples of the H-R diagram What happens to
massive stars?
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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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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 (energy per second) of
L 3.9 x 1026 Joules s-1. Its mass
composition is H 74
He 24
rest 2 (0.2 by number)
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Hydrostatic Equilibrium
P Pressure T Temperature
P,T
Gravity
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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Nuclear Fusion in stars like the Sun
Core temperature 1.5 x 107 K Core radius 0.25
Rsurface
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
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Some simple calulations
What mass of hydrogen is converted to helium in
one second?
Mass s-1 luminosity / c2 4 x 109 kg s-1
How long can the sun survive by burning hydrogen?
Hydrogen burning lifetime Total mass available
for conversion
Rate of conversion Lifetime mass
available x c2 / L 1010 years. Our sun is
roughly half-way through its hydrogen burning
phase.
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Hydrogen burning
Stars form with masses between about 1/10 and 100
times the mass of the sun. For most of their
lifetimes they burn by the nuclear fusion of
hydrogen to helium. Stars with higher masses
are more luminous L Mn where n 3.5 for
sun-like stars So - more massive stars have
shorter hydrogen burning lifetimes.
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Hydrogen fusionI. Masses lt 1.5 solar massesThe
proton-proton chain
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The PP-I chain
The net effect of the PP-I chain is 4 1H
1 4He 2 positrons 2 neutrinos 2 gamma rays

• The by-products provide the source of luminosity
• Positrons anti-electrons (e) collide with
  electrons (e-)
• Neutrinos rapidly escape from the star
• Gamma rays (photons) travel outwards through
  starinteracting many times with atomic gas.

Energy is also provided by the PP-II and PP-III
chains
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Energy transport from the core to the visible
surface of low-intermediate mass stars
1. Core region R lt 0.25 Rstar Nuclear fusion zone
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2. Radiative region 0.25 lt R lt
0.75Rstar photons diffuse through hot gas.
3. Convective Region 0.75 lt R lt Rstar Energy
transported by bulk gas motions.
4. Photosphere - the visible surface of the
star. Thickness 500 km. T 6000K
Energy from a stars interior is released as
photons (particle of light) and as neutrinos
(zero or very low mass particles).
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Hydrogen fusionII. Masses gt 1.5 solar massesThe
C-N-O cycle
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The C-N-O cycle
4 1H 1 4He 2 positrons 2 neutrinos 3
gamma rays The C-N-O cycle becomes dominant at
temperatures above 18 million K.
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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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Hertzsprung Russell Diagram for Nearby Stars
The hydrogen burning stars lie on the main
sequence. The sun has a surface temperature of
6,000 K.
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Main Sequence stellar classification

• Stars are often classified from their surface
  properties using a temperature sequence
• O B A F G K M
• Hot Cool
• Blue Red
• 30,000K 3,000K
• The sun is a G-type star.

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Evolved stars
What happens when the core hydrogen runs out? As
the hydrogen is used up the central core of the
star becomes smaller, denser and hotter. The
outer layers of the star expand hugely. Hydrogen
ignites in a shell around the core. Helium then
ignites in the core and burns to carbon
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Becoming a giant
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
Hydrogen shell burning (initially)
Core helium burning
Outer hydrogen atmosphere
The radius of a red giant star is 0.5 AU (half
the sun-earth distance!) The surface temperature
is 3000 K The core temperature is 108 K
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Explosive consequences

• As the star evolves, heavier elements are created
  through nuclear fusion processes in the core and
  in shells around the core
• (H, He, C, N, O, Mg..Fe).
• The mass in the core of the star continually
  increases.
• If the core mass reaches 1.4 solar masses the
  star will explode and/or collapse.
• For stars with initial mass below about 8 solar
  masses this does not happen.

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STELLAR MASS LOSS
Evolved stars LOSE about HALF of their MASS
through their stellar winds. The winds are
mostly made up of hydrogen. Molecules such as
H2O (water) and OH (hydroxyl) form in the
stellar winds at large distances from the star.
Stellar wind
OH Molecules
star
SiO molecules
H2O molecules
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Mass loss from an evolved star
Silicon monoxide maser emission showing mass-loss
near the surface of the variable star TX
Cam. This movie is made from 44 images over a
period of several years.
Phil Diamond et al.
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OH30.1-0.7
110
88
77
81
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Planetary Nebulae
Giant stars lose so much hydrogen that eventually
their small central cores become visible. The
stellar winds then stop. Ultraviolet photons
from the core sweep up the stellar wind into a
shell around the core. The swept up shell is
seen as a PLANETARY NEBULA. Planetary nebulae
can have very 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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Planetary Nebulae Morphologies
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White Dwarfs
At the end of the planetary nebula stage the star
is left with an extremely hot, dense core (a
million times denser than the earth). The star
is now a WHITE DWARF. White Dwarfs cool very
slowly and gradually fade into darkness. White
dwarfs are supported by electron degeneracy
pressure.
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A white dwarf

• Typical mass of the central core is somewhere
  between 0.5 to 1.0 solar masses, with a size
  close to that of the Earth.
• All nuclear burning ceases have a white dwarf
• They cool and dim and after billions of years
  become undetectable (become a black dwarf).
• Over 95 of the stars in our Galaxy will become
  white dwarfs

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By-product a huge diamond
BBC A diamond that is almost forever (Feb 2004)
Crystalised carbon IS diamond. Recently
discovered one 50 light-years away in Centaurus.
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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 diagrams for nearby stars show that there are
a greater number of lower mass stars than high
mass stars in the solar neighbourhood.
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Globular cluster M80
To plot an 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 the globular cluster M5 - plotted
as V magnitude against B-V colour.
V
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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The Jewel Box Cluster A cluster of young stars
at the same distance
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The HR diagram for the young open cluster h and
chi Persei
Most of the stars in the cluster are still on the
Main Sequence
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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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Massive stars

• Massive stars (gt 8 solar masses) will also
  develop very strong stellar winds after the
  hydrogen-burning stage.
• However the winds are not sufficient to stop the
  stars finally exploding in supernovae explosions.
  In most cases supernovae occur when stars try to
  ignite iron.

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Eta Carina
This shows a huge nebula around the very massive
star Eta Carina. Eta Carina may be a binary
system with two massive stars at the centre of
the nebula.
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Eta Carina a radio movie
This shows radio emission from a region around
the star near the centre of the nebula.
S. White, B. Duncan
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The Toby Jug Nebula (IC 2220)
This shows mass loss around a bright and massive
supergiant star.
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SN 1987A
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The Crab Nebula
The crab nebula was formed in a supernovae
explosion in 1054. There is a strong pulsar at
the centre of the nebula.
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Massive stars - overview

• Hydrogen burning
• Supergiant star Helium core burning
• Further fusion processescreate heavier elements
• Supernova
• Neutron star - pulsar

(in some cases)
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Conservation of angular momentum
http//cassfos02.ucsd.edu/public/tutorial/SN.html
Sun has r 7x108m and rotational period P 1
month If the Sun becomes a white dwarf, r
6400km, P 3 min (typical white dwarf rotation
from 33 sec upwards) If the Sun became a neutron
star, r10km, P 0.5 ms (typical neutron star
rotation from 1ms upwards)
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Neutron stars and pulsars
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Black holes

• If core mass is greater than 3 M0 then neutron
  degeneracy pressure cannot apply core collapses
  to black hole.
• General relativity required to describe the space
  around a black hole

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Observing black holes

• Cannot observe black holes directly using current
  astronomical techniques
• Cygnus X-1 is believed to be a black hole binary
  with a 20-35 solar mass black hole and a stellar
  companion orbital period of 6 days.