Download The Life and Death of Stars

The Life and Death of Stars
John Irwin
Low Gillerthwaite Field Centre
Photo credits with thanks to NASA and Hubble telescope
The Life and Death of Stars
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Basic concepts
How are stars formed?
What provides a star’s fuel?
What happens when the fuel runs out?
Different types of star
Basic Concepts
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What is light?
Energy – Mass equivalence
Atomic structure
Fission and fusion
Fundamental forces of nature
Timeline of the Universe
What is light
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Ancient Greeks – a beam emitted from the eye
1000 Al-Haytham – streams of particles from the sun
1672 Isaac Newton – minute particles
1799 Thomas Young – wave
1846 Michael Faraday, 1873 Robert Maxwell – wave
1905 Albert Einstein – photon - particle and a wave
Light is electromagnetic radiation:
Radio – TV – Infra-red – Light – Ultra Violet – X-ray – Gamma ray
Electromagnetic radiation
gine.gsfc.nasa.gov/science/toolbox/emspectrum1.html
Basic concepts - Speed of light
• Light has a fixed speed in a vacuum
• C = 300,000 km/sec or 186,000 miles/sec
or 3 x 1010 cm/sec
• Sun is 150 million km from earth
• Sunlight takes 500 secs to reach earth
An experiment to measure the speed of light
• 3 everyday items to measure the speed of light
– Measuring device
– Source of electromagnetic radiation
– Medium to see the effects of the radiation
– Ruler
– Microwave oven
– Bar of chocolate
An experiment to measure the speed of light
λ = 6.5 x 2 cm =13cm
ν = 2450 mhz = 2.45 x 10 9 cycles/sec
c = λν
= 13 x 2.45 x 10 9 = 31.85 x 10 9 cm/sec = 318,500 km/sec
Basic concepts - What is a light year?
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The distance light travels in a year
It is a measure of distance – not a measure of time
Speed of light is 300,000 km/sec
Light takes 500 secs to reach us from the sun
A light year is 9.5 trillion km (9.5 x 1012 km)
Alpha Centauri is 4.4 light years away
Basic concepts - Energy – Mass equivalence
• The most famous equation in Physics
• E = mc2
E - energy (Joules)
m - mass (kgm)
c - speed of light (m/sec)
Albert Einstein 1905
• Burn 1 kg coal -> 32 million Joules (3.2 x 107 Joules)
• Convert 1kg coal to energy -> 9 x 1017 Joules
Energy – Mass equivalence
• 1000 MW Power Plant
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Coal
Oil
Gas
U235
– 9000 tons/day
– 40,000 barrels/day
– 240 million cu.ft/day
– 3 kg/day
(2 train loads per day)
(1 oil tanker per week)
(30,000 barns/day)
Basic concepts - Atomic structure
• Atomic Nucleus contains most of the mass
– Protons – positive charge
– Neutrons – no charge
• Orbiting the nucleus
– Electrons – negative charge
http://www.bbc.co.uk/schools/gcsebitesize/science/
add_aqa_pre_2011/atomic/atomstrucrev1.shtml
Attribution: Yzmo
Atom statistics
• Atom
• Nucleus
10-8 cm
10-13 cm
(i.e. atom is about 100,000 times larger than the nucleus)
• Proton and Neutron have similar mass (1.7 x 10-24 gm)
• Electron is 1/2000 the mass of a proton
• For an atom the size of this room
the nucleus would be 1/100 cm
• A Hydrogen atom is 99.9999999999 % empty space
Basic concepts – Nuclear Fission and Fusion
• Fission
– An atom is split into two or more smaller atoms
eg U235 + n -> Ba + Kr + 3n + 202 MeV
• Fusion
– Two or more atoms fuse to create a larger atom
e.g. H2 + H3 -> He + n + 17.6MeV
• Mass is lost and is released as energy : E = Mc2
Basic concepts – Fundamental Forces of Nature
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Strong nuclear force
Electromagnetism
Weak Nuclear force
Gravity
Basic concepts – Fundamental Forces of Nature
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Strong nuclear force
Electromagnetism
Weak Nuclear force
Gravity
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10-6
10-16
10-41
Timeline since Big Bang
Big Bang
NASA/WMAP Science Team
Sun and Earth
formed
4.6 billion years
13.8 billion years
Basic concepts – Timeline of the Universe
• If the Universe were a year old, then it would have begun on
January 1st at 00:00:00 with the Big Bang
• Sept 1st – Solar System formed
• Oct 1st – bacteria begin photosynthesis
• Dec 18th – first animals moved from the sea to land
• Dec 26th – first dinosaurs
• Dec 30th – dinosaurs became extinct
• Dec 31st 23:40 Neanderthals
• Dec 31st 23:52 Homo sapiens
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http://www.thestargarden.co.uk/Timeline.html
Basic concepts – Timeline of the Universe
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Dec 31st
23:59:54
23:59:58
23:59:59.25
23:59:59.52
23:59:59.75
23:59:59.85
24:00:00.00
Pythagoras (537 BC)
Battle of Hastings
Newton’s theory of Gravity (1687)
John Dalton – atomic structure (1805)
Einstein – Special Relativity (1905)
Most of us in this room were born
Now!
Stars
• ~ 200 billion stars in our galaxy (The Milky Way)
• ~ 200 billion galaxies
• ~ 4 x 1022 stars
• Grains of sand on all the World’s beaches ~ 2 x 1022
Formation of stars
• Stars are formed in Nebulae
• A nebula is a cloud of dust and gas
• For example the Nebula in Orion
– 1350 light years away
• Dust and gas coalesce and becomes denser and denser
• What force drives this collapse?
GRAVITY
Nasa Hubble Telesco
Nasa Hubble Telescope
Nasa Hubble Telesco
Nasa Hubble Teles
Nasa Hubble Telescope
A star is born
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Gas cloud temperature – a few degrees Kelvin
Gravity causes the gas and dust to coalesce
This compression causes the gas and dust to heat up
A protostar is formed
Eventually it reaches 12 million degrees Kelvin
As this point nuclear fusion starts
Protons are forced together and Helium nucleii are formed
Nuclear fusion starts to fuel the star
Starts main sequence phase
Nuclear fusion in a star like our sun
Attribution: Borb
Energy released
• Atomic bomb (fission) – Hiroshima
14 kilotons TNT = 1gm mass converted
• Hydrogen bomb (fusion)
50 megatons TNT = 2.3kg mass converted
• Sun burns 564 million tons Hydrogen/sec (564 billion kg)
to create 560 million tons Helium/sec
(560 billion kg)
• So the sun loses 4.3 million tons/sec
(4.3 billion kg)
• Equivalent to 2 billion Hydrogen bombs every second
Life sequence of a star
All stars follow a similar life sequence:
Gas cloud -> Protostar -> Main Sequence ->
Giant / Supergiant -> Planetary Nebula or Supernova ->
Remnant
90-95% of stars are Main Sequence stars
Star size varies from 1/10 to 150 solar masses
Brown dwarf - less than 1/10 solar mass
Star sizes
• Star size varies from 1/10 to 150 solar masses (SM)
• The heavier the star:
– the hotter it is – the bluer it is – the faster it burns – the shorter it exists
• Examples
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Proxima Centauri
Sun
Sirius
Regulus
Zeta Puppis
1/
7 SM
1 SM
2 SM
3.5SM
22 SM
3000K red
6000K yellow
10,000K white
12,000K blue
42,000K blue
trillion years
9 billion years
1 billion years
100m years
2-5m years
• Star size determines what happens at the end of its life
Our Sun
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The sun converts 4.3 million tons/sec into energy
It has been doing this for 4.5 billion years
Has enough Hydrogen in the core for 5 billion years
What happens when all the core hydrogen is burnt?
– the core will collapse, become denser and hotter
– Helium starts to fuse into Carbon and Oxygen
• It will expand enormously into a Red Giant
• It will lose its outer layers as a Planetary Nebula
• Ultimately it will cool and become a white dwarf which
could last for trillions of years
A Planetary Nebula – NGC6543
Nasa Hubble Telescope
White dwarf
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White dwarf created by stars after Red Giant phase
Stars which have 1/10 to 8 solar masses
Small – about the size of the earth
Similar mass to our Sun
Therefore very dense
– A teaspoon of white dwarf matter weighs 5 tons (5000 kg)
• Gravity is 200,000 x earth gravity
• Lasts for billions/trillions of years
• Will eventually lose all heat and become a black dwarf
http://www.slideshare.net/Teach5ch/majorfeatures-of-the-universe-andie
Massive stars
• More than 8x mass of our sun
• Examples
– Antares – 700 times the diameter of our sun
– Betelgeuse –10m years old
– Rigel – 10m years old
• Massive stars have short life span
– 30 million down to a few hundred thousand years
• Supergiant stars
Relative sizes of stars
Red Giants
Pollux
Arcturus
Supergiants
Rigel
Betelgeuse
Antares
http://sci.gallaudet.edu/Science/
relativesizes.html
What happens to Supergiants
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Like our sun they fuse Hydrogen to Helium for fuel
When core Hydrogen runs out the star collapses (cf our Sun)
It will start fusing Helium to Carbon
Eventually core Helium runs out and star collapses again
Gets even hotter than before
Carbon fuses into heavier elements
Process of: star collapse – fusion keeps repeating
This creates heavier and heavier elements in the core
Stellar nucleosynthesis
https://en.wikipedia.org/wiki/Stellar_nucleosynthesis
The end of a Supergiant
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Fusion of elements up to Iron releases energy
This holds the star up against the pull of gravity
Fusion of iron requires energy
But there is nothing to burn
The outer edge of the core collapses catastophically
(.25 sec)
• Bounces off the core creating an enormous shockwave
The end of a Supergiant - Supernova
• The shockwave travels to the surface of the star (2 hrs)
• On its way it fuses elements heavier than iron
– Gold, silver, platinum, uranium
• The outer surface and the new elements are blasted into
space
• The supernova will be incredibly bright for a few months
• After a few years it will fade away
• It will leave a shockwave of gas and dust hurtling outwards
• It will also leave a remnant of the star behind
Supernovae remnants
• A Supernova leaves behind a remnant of the star
• Depending on the size of the remnant it will form:
• Remnant size:
– <1.4 SM
– 1.4 – 3 SM
– >3 SM
White dwarf
Neutron star
Black hole
Neutron star
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Created as remnant of a Supernova explosion
Very small – about 10 miles across
1.4 to 3 x the mass of our Sun
Therefore incredibly dense
– A teaspoon of neutron star matter weighs 10 million tons
• Gravity is 200 billion x earth gravity
• Neutron stars also spin very rapidly
– Up to 700 times per second
What is a neutron star made of
• Mainly neutrons !!
• The protons and electrons are fused to create neutrons
• It is held up against the force of Gravity by neutron
pressure
https://s-media-cacheak0.pinimg.com/originals/3d/24/b0/3d24b060731583cfc28e5ba6c1fe1223.jpg
Black holes
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Created as remnant of a Supernova explosion
At least 3 x the mass of our Sun
Gravity wins – it collapses to a point
Singularity
This is known as a Black Hole
The escape velocity is greater than the speed of light
Nothing can escape the Event Horizon, including light
Black Holes
• We can’t see Black Holes
• We can detect their presence
• Event Horizon size
– From 6 miles up to the size of our Solar System
• Black holes are very compact
• At the centre of every galaxy is a Supermassive Black
Hole
Stardust
The universe started with a Big Bang 13.8 bn years ago
Shortly after the Big Bang by mass the Universe consisted
of (by atoms):
– 75% Hydrogen (92%)
– 25% Helium
(8%)
– Traces of Lithium
Today the mass of the Universe consists of (by atoms):
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74% Hydrogen (92%)
24% Helium
(7%)
1% Oxygen
(0.1%)
0.5% Carbon (.05%)
0.5% other elements (0.6%)
Stellar Nucleosynthesis – Periodic Table
Attribution: Cmglee
Chemical elements in a human body
https://askabiologist.asu.edu/content/atoms-life
A final thought
• We don’t know what most of the stuff in the Universe is !
• Stars, planets, gas clouds, dust etc make up 5% of the
Universe
• Dark Matter makes up 25% of the Universe
• Dark Energy makes up 70% of the Universe
• What is Dark Matter and Dark Energy?
• Nobody knows!!!
ANY QUESTIONS