February 24, 2015, jeb

Outline

• Life After Death for White Dwarfs
• Supernovae
• Death of a High Mass Star
• Physics Lesson:
  
  Iron is the end of the line for Fusion
• Formation of the Elements
• Types of Supernovae
• Neutrinos from Supernovae
• Supernova 1987A (type II)
• Neutrinos from SN1987A
• Type I Supernova - Binary Star System
• Supernova Remnant
• Crab Nebula
• Neutron Star
• Pulsars

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Life After Death for White Dwarfs

White dwarfs can spring back to life in a couple of ways:
• Novae
• Supernovae (Type I)

Novae: Mild Stellar Explosions

"New Star"
1572 Tycho Brahe
1604 Kepler
......Increased Brightness by 50,000 in few days

Nova :
• Star with sudden outburst of energy
• Brightens hundreds to many thousand-fold
• Binary star system:
  • degenerate partner collects hydrogen from red giant
  • eventually ignites
  • Nova will eject material
    
    
    NASA animation of Nova
    
    
• Same star may flare up into Nova many times during its lifetime

Supernovae

The explosions of stars with the resulting release of tremendous amounts of radiation.




Recorded explosions visible to naked eye:
Year (A.D.) Where observed Brightness 185 Chinese Brighter than Venus 369 Chinese Brighter than Mars or Jupiter 1006 China, Japan, Korea, Europe, Arabia Brighter than Venus 1054 China, SW India, Arabia ->Crab Nebula Brighter than Venus 1572 Tycho Nearly as bright as Venus 1604 Kepler Brighter than Jupiter 1987 Ian Shelton (Chile) .



Many more are observed by telescope in distant galaxies 
International Supernovae Network

Death of a High Mass Star (Greater than 10 M_SUN)

Electron degeneracy will not halt collapse (Chandrasekhar limit) 

Gravitational compression heats core following each burning phase 



Temperature range for fusion processes
600 x 106K Carbon "burning" ->Neon, Magnesium 109 K Neon "burning" -> Oxygen, Magnesium 1.5 x 109K Oxygen "burning" -> Sulfur, Silicon 2.7 x 109K Silicon "burning" -> Iron



At this point the star has an iron core, with shells of different elements burning at different temperatures

Iron cannot provide energy through thermonuclear fusion
see below

Density of core builds and catastrophic collapse sets in 
. . . => Supernova Explosion 

Physics Lesson:


Iron is the end of the line for Fusion

(Very Important) Principle Behind Supernovae Explosion:

Iron (26 protons) cannot fuse and release energy


PHYSICS LESSON : 
Light nuclei (hydrogen, helium, carbon, oxygen, etc.) can fuse and release energy. 

Heavy nuclei (uranium, plutonium, etc) can split (fission) and release energy. 

Iron is midway between and has nowhere to go.

ergo IT'S STABLE.

A supernova is about 10¹⁰ times as bright as the Sun.
It may even outshine its galaxy
Fades away in months or a few years
(leaving behind supernova remnant



Evolutionary stages of a 25 MSUN star

Stage	Temperature (K)	Density (g/cm3)	Duration of stage
Hydrogen burning	4x 107	5	7 x 106 years
Helium burning	2 x 108	700	5 x 105years
Carbon burning	6 x 108	2 x 105	600 years
Neon burning	1.2 x 109	4 x 106	1 year
Oxygen burning	1.5 x 109	107	6 months
Silicon burning	2.7 x 109	3 x 107	1 day
Core collapse	5.4 x 109	3 x 109	1/4 second
Core bounce	2.3 x 1010	4 x 1014	milliseconds
Explosive	about 109	varies	10 seconds



Note: star core gets hotter to start each burning stage. 
Each successive stage lasts for a shorter period 
When core collapses, density reaches astonishing 400,000,000,000,000 g/cm³  

Therefore, in a heavy star, we eventually build an IRON CORE and energy production of core ceases. 

When mass of Iron core reaches 1.4 M_SUN (the Chandrasekhar Limit) it collapses into a 
Supernova Explosion!
 

During the violent explosion the enormous expanding shock wave causes fusion to heavier nuclei. 

This is where gold, silver, titanium, uranium, lead, etc. on Earth came from. 

Formation of the Elements






• Primordial Elements (formed during the Big Bang)
• Hydrogen
• Helium
• Stellar Nucleosynthesis
• proton-proton reaction (T>10,000,000 K)
• triple-alpha reaction (T>100,000,000 K)
• carbon-helium fusion (T>200,000,000 K)
• carbon fusion (T>600,000,000)
• Fusion of heavier elements is possible, but repulsion of heavier nuclei becomes stronger requiring higher temperatures
  • More likely to produce heavier elements through the alpha process
  • Helium (alpha particles) can be produced when energy breaks a nucleus apart
• created all other elements
• fusion processes in active stars
• neutron capture to form intermediate mass elements above iron
• fusion during supernovae explosions to form the heaviest elements



• observational evidence for stellar nucleosynthesis
  • supernova energy emission
    light curve of type I supernova matches theoretical calculations of light emitted by radioactive decay of nickel-56 and cobalt-56

Types of Supernovae

Many supernovae are discovered every year
Latest Supernovae


There are two types of supernovae 
The light curve reveals the two different types


• Type I - binary system
• carbon-detonation supernova
• Type II -single massive star
• core-collapse supernova
More on this later ......  

Neutrinos from Supernovae

As the core of a massive star collapses,
it will consist entirely of simple elementary particles
• electrons, protons, neutrons, and photons
• the structure of nuclei in the core has been destroyed

The density continues to rise, and eventually the protons and electrons are crushed together

p + e^- -> n + neutrino

forming neutrons and neutrinos
neutronization
At this point the density of the core may be as high as
10¹²kg/m³
still, most of the neutrinos pass out of the core as if it were not there!

Supernova 1987A (type II)

In Large Magellanic Cloud (see image of Magellanic Clouds) 
160,000 light years from Earth 
First Supernova visible to naked eye since invention of telescope 

Exploding star was Sanduleak -69^o202 
Sanduleak -69^o202 was 20 M_SUN  
Main sequence star for 10⁷ years 

After passing through a Red Giant Phase, Sanduleak -69^o202 had shrunk and become a 

Blue Supergiant 


light curve differed from "standard" curve

Neutrinos from SN1987A

When core collapses into SN, it emits an enormous burst of neutrinos


What's a neutrino?



e^- + p -> n + neutrino 
(most of supernova energy carried away by neutrinos) 

Total luminostiy of neutrinos during first second 
10⁴⁶watts (L_SUN = 4 x 10²⁶ watts) 
This exceeds the luminosity of all the stars in all the galaxies in the part of the universe we can observe, momentarily 


10⁵⁸ neutrinos were emitted, going off in all directions about 160,000 years ago. 
Then - in 1987- 20 hours before the supernova was visible, the pulse of neutrinos passed through the earth: 500,000,000,000,000 = 5 x 10¹⁴ through every square meter . 


Very few interacted, but about one million people on Earth experienced a Neutrino interaction in their bodies. 
But they felt nothing


Two large detectors-giant water tanks, one in Japan and one on Lake Erie, saw 19 interactions! 

Type I Supernova - Binary Star System 


White Dwarf + companion

When companion expands in Red Giant phase, mass of White Dwarf increases and can exceed 1.4 M_SUN. 

Then core collapsed and carbon "burning" sets in -> rapid runaway burning -> explosion 
So the two types of supernovae are: 
• Type I - binary system
• carbon-detonation supernova
• White dwarf grows from accretion of material from companion, eventually exceeding the Chandrasekhar mass limit
• Type II - single massive star
• core-collapse supernova
• Massive star expends its fuel supply, ending with iron core, and collapses and explodes

Supernova Remnant

During Supernova explosion, much (or most) of the star is ejected into space with a small fraction left behind. 
(for example, the Crab Nebula - M1
Crab Nebula and pulsar photographed by Hubble Space Telescope ) 


(Example: 25 M_SUN -> 24 M_SUN ejected) 
-or it all could be ejected 

Corpse: Neutron star 

Recall during collapse protons and electron combined 
p + e- -> n + neutrino 
Corpse is a very dense star made of just neutrons. 

SN 1987A rings

NASA time lapse video of SN1987A rings from 1994 to 2009

NASA images of supernova remnants

Crab Nebula

One of best studied supernova remnants is the Crab Nebula

About 1800 pc (or about 5900 light-years) from Earth with an angular diameter about one-fifth of the moon

Explosion appeared in sky in 1054

So brilliant ancient Chinese and Middle Eastern astronomers reported its brightness exceeded that of Venus

Visible in broad daylight for nearly one month

Neutron Star

If M<3M_SUN , degenerate-neutron pressure will hold up weight of star. 


(if M>3M_SUN, Neutron star collapses into Black Hole.) 

Density of N-star 10¹⁴ g/cm³ (1 thimble full = 100 million tons) 

Diameter of l M_SUN N-star 30 km (20 miles) 


(Radius of white dwarf = 4200 km) 


note: conservation of angular momentum tells us the neutron star should be rapidly rotating (like spinning ice skater) 

Pulsars

A pulsating radio source produced by rapidly rotating N-star. 

1st pulsar discovered in 1968 by Jocelyn Bell, Cambridge U. (England) graduate student. 

Interstellar beacon sent out by extraterrestrial life on another star? Called source LGM 
(little green men ) N-STAR WAS EXPLANATION 

Typical pulsar period: 1/1000 seconds -> few seconds 
Example: Crab Pulsar -> 1/30 second 

Mechanism:  
N-star rotates with powerful magnetic field 
tilted relative to axis 
Charged particles are accelerated to poles and produce 2 beams of radiation. 

N-star rotates and beams sweep past our direction- we see pulses 

Only a very small object can rotate this fast (30/sec, for example) 

Now observing SN1987A for evidence of a pulsar

so far, no sign of the pulsar

The Cycle of Stellar Evolution

illustration


• Stars form when interstellar cloud collapses and fragments
• Stars create heaviest elements in cores, and spew them into interstellar medium
  • low-mass stars -> carbon, nitrogen, oxygen (life)
  • high-mass stars -> iron, silicon, other heavy elements (Earth, technology)
• Explosive dispersal of newly formed elements enriches medium, and compresses it into new star formation
  • recently formed stars have much more heavy elements than those formed long ago

Related Link

Supernova by HEASARC