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A new view of the Universe IV
Fred Watson, AAO, with thanks to
Jessica Chapman, ATNF
April 2005
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?
SOHO image of the solar chromosphere in ultraviolet light.
Some Solar Values
1/2o
Distance to the
sun
Radius
Value
Notes
150 million km 1 astronomical unit
700,000 km
109 x Rearth
2 x1030 kg
300,000 Mearth
Mean density
1.4 x 103 kg m-3
(1.4 g cm-3)
0.25 <ρearth>
Surface
Temperature
6000 K
strongest emission at
yellow wavelengths
Mass
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)
Hydrostatic Equilibrium
P: Pressure
T: Temperature
Gravity
P,T
The internal pressure gradients must counteract the
gravitational force G. (What happens otherwise?)
This is a fundamental requirement for all stars.
Nuclear Fusion in stars like the
Sun
Core temperature = 1.5 x 107 K
Core radius = 0.25 Rsurface
The sun’s 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
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.
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.
Hydrogen fusion
I. Masses < 1.5 solar masses
The proton-proton chain
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 star
interacting many times with atomic gas.
Energy is also provided by the PP-II and PP-III chains
Energy transport from the core to
the visible surface of low-intermediate mass stars
1. Core region: R < 0.25 Rstar
Nuclear fusion zone
2. Radiative region:
0.25 < R < 0.75Rstar
photons diffuse through hot gas.
2
3. Convective Region: 0.75 < R < Rstar
Energy transported by
bulk gas motions.
4. Photosphere - the visible surface of
the star. Thickness ~ 500 km. T = 6000K
Energy from a star’s interior is released as photons (‘particle of light’) and as
neutrinos (zero or very low mass particles).
Hydrogen fusion
II. Masses > 1.5 solar masses
The C-N-O cycle
The C-N-O cycle
4 1H
rays
1 4He + 2 positrons + 2 neutrinos + 3 gamma
The C-N-O cycle becomes dominant at temperatures
above 18 million K.
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).
Hertzsprung Russell Diagram for Nearby Stars
Main sequence
Sun
The hydrogen burning stars lie on the ‘main
sequence’. The sun has a surface temperature of
6,000 K.
Main Sequence stellar classification
• Stars are often classified from their surface
properties using a temperature sequence:
O B A
Hot
Blue
30,000K
F
G
The sun is a G-type star.
K M
Cool
Red
3,000K
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
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.
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
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.
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
Mass loss from an evolved star
Silicon monoxide maser
emission showing massloss 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.
121
110
88
81
77
OH30.1-0.7
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.
Two examples of ‘circular’ planetary nebulae - HST images
NGC 6369
IC 3568
For many examples of P. Nebulae - see the HST web pages
Planetary Nebulae Morphologies
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’.
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
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.
Schematic view of the evolutionary path of a one
solar mass star.
Luminosity (solar units)
Asymptotic Giant
Branch
103
Planetary nebulae
Red Giant
Sun-like star
1
Main Sequence
White Dwarf track
10-3
Red
Blue
20000
6000
Effective Temperature (K)
3000
HR diagrams for nearby stars show that there are a greater
number of lower mass stars than high mass stars in the solar
neighbourhood.
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.
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.
The Jewel Box
Cluster
A cluster of young
stars at the same
distance
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
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.
Massive stars
• Massive stars (> 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.
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.
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
The Toby Jug Nebula (IC 2220)
This shows mass
loss around a
bright and massive
supergiant star.
SN 1987A
SN 1998aq
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.
Massive stars - overview
Hydrogen burning
Supergiant star – Helium core burning
Further fusion processes…create heavier elements
Supernova
(in some cases)
Neutron star - pulsar
Conservation of angular momentum
2
1
r
L  I 
P1
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, r~10km, P = 0.5 ms (typical
neutron star rotation from 1ms upwards)
Neutron stars and pulsars
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
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.