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A new view of the Universe IV
Fred Watson, AAO, with thanks to Jessica Chapman, ATNF
April 2005
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
Value Notes Distance to the sun
150 million km 1 astronomical unit
Radius 700,000 km 109 x Rearth
Mass 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
1/2o
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,T
Gravity
The internal pressure gradients must counteract thegravitational force G. (What happens otherwise?)
This is a fundamental requirement for all stars.
P: PressureT: Temperature
Nuclear Fusion in stars like the Sun
Core temperature = 1.5 x 107 KCore 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
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.
Some simple calulations
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 fusionI. Masses < 1.5 solar masses
The proton-proton chain
The PP-I chainThe 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
Energy transport from the core to the visible surface of low-intermediate mass stars
2
1. Core region: R < 0.25 Rstar
Nuclear fusion zone
2. Radiative region: 0.25 < R < 0.75Rstar
photons diffuse through hot gas.
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 fusionII. Masses > 1.5 solar masses
The C-N-O cycle
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.
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
The hydrogen burning stars lie on the ‘main sequence’. The sun has a surface temperature of 6,000 K.
Sun
Main sequence
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.
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
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
Hydrogen shell burning (initially)
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.
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.
star
Stellar wind
STELLAR MASS LOSS
H2O molecules
OH Molecules
SiO molecules
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.
121
110
7781
88
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.
NGC 6369 IC 3568
Two examples of ‘circular’ planetary nebulae - HST images
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.
Lum
inos
ity
(sol
ar u
nits
)
Effective Temperature (K)
RedBlue
1
103
10-3
6000 300020000
Sun-like star
Planetary nebulae
Red Giant
White Dwarf track
Asymptotic Giant Branch
Main Sequence
HR diagrams for nearby stars show that there are a greater number of lower mass stars than high mass stars in the solarneighbourhood.
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 clusterwhich are known to be at the same DISTANCE.
HR diagram for the globular cluster M5 - plotted as V magnitude against B-V colour.
The globular clusters contain old (population II), highly evolved stars.
This cluster shows well-defined giant and horizontal branches.
B - V
B-V
V
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
S. White, B. Duncan
This shows radio emission from a region around the star near the centre of the nebula.
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 - overviewHydrogen burning
Supergiant star – Helium core burning
Further fusion processes…create heavier elements
Supernova
Neutron star - pulsar(in some cases)
Conservation of angular momentum
1
21
P
rIL
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)
http://cassfos02.ucsd.edu/public/tutorial/SN.html
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.