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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.