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.
Light Travel from the Sun The speed of light is c = 3x10 8 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 10 26 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 the gravitational force G. (What happens otherwise?) This is a fundamental requirement for all stars. P: Pressure T: Temperature
Nuclear Fusion in stars like the Sun Core temperature = 1.5 x 10 7 K Core radius = 0.25 R surface The sun’s energy is generated in the core by nuclear fusion reactions which convert Hydrogen to Helium: 4 1 H 1 4 He + energy (photons and neutrinos) Energy released = mc 2
What mass of hydrogen is converted to helium in one second? Mass s -1 = luminosity / c 2 : 4 x 10 9 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 c 2 / L ~ 10 10 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 ~ M n 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 1 H 1 4 He + 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 2 1. Core region: R < 0.25 R star Nuclear fusion zone 2. Radiative region: 0.25 < R < 0.75R star photons diffuse through hot gas. 3. Convective Region: 0.75 < R < R star 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 1 H 1 4 He + 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 10 8 K the stellar core ignites helium in the ‘triple-alpha’ reaction: 3 4 He 12 C + (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 ~ 10 8 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 H 2 O (water) and OH (hydroxyl) form in the stellar winds at large distances from the star. star Stellar wind STELLAR MASS LOSS H 2 O 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.
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
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) Effective Temperature (K) Red Blue 1 10 3 10 -3 6000 3000 20000 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 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. The globular clusters contain old (population II), highly evolved stars. This cluster shows well-defined giant and horizontal branches. 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.
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 Neutron star - pulsar ( in some cases )
Conservation of angular momentum Sun has r = 7x10 8 m 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
Black holes If core mass is greater than 3 M 0 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.