CHAPTER10: DEATH OF STARS

Summary: Stars die by nova or supernova explosion.

Giant Stars
At the end of exhausting hydrogen fuel (90% of its life) by a main sequence star it becomes a giant. It may burn helium nuclei (>100 million degrees Kelvin) and spend 10% of its life. After hydrogen is exhausted in the core, nuclear reaction stops, Helium ashes begin to contract, generating heat that may trigger hydrogen burning in the inner wall of the outer shell of the giant. The inner shell burning of hydrogen pushes the outer shell of the star and the star dramatically increases its volume. Sun may become 10 to 100 times larger in diameter in this stage.
The star now has left the main sequence band and moved to cooler temperature and higher luminosity. ALDEBARAN, the red eye of the raging bull in Taurus constellation is such a giant star. Its diameter is 25 times larger than sun and its temperature is half the sun’s temperature. As the giant star expands, it increases the mass of he nuclei and eventual helium mass contraction that starts heating up the core. At 100 million degrees, it begins to fuse helium into carbon by nuclear reactions. Higher temperature increases the helium fusion. The result is a runaway explosion called helium flash for a few minutes that generate brightness equal to that of entire galaxy.

Massive star with more than 10 solar mass can ignite carbon burning at 1 billion degrees Kelvin in the core. Carbon fusion produces neon, neon fuses into oxygen. Oxygen fuses into silicon and silicon into iron as the final synthesis in the core of high solar mass star. Hydrogen fusion lasts 7 million years, oxygen fusion 6 months and silicon fusing into iron only a day in 25 solar mass star. Iron is most tightly bound atomic nuclei.

Significance of the H-R Diagram

H-R diagram is a panorama portraying how stars evolve
Star's position in diagram is a function of its mass, chemical composition, and current age
Important point is that stars evolve, i.e., they are born, age and eventually die
Stars of our Galaxy are not same age as Galaxy, since some are young, or middle-aged, or old

Summary of Stellar Evolution

Protostar contracts from interstellar medium
Burns hydrogen on main sequence
Contracts, initiates helium burning as in red giant
"Ash" of one nuclear-burning phase provides fuel for next burning phase
High-mass stars alternately contract/ignite new sources of thermonuclear burning
Shed outer layers and die
Low-mass stars as white dwarfs
High-mass stars as neutron stars
Extreme high-mass stars as black holes

Nucleosynthesis

Nucleosynthesis is a scheme for structural changes through stellar evolution
All stars convert hydrogen to helium
Smaller numbers convert helium into carbon
Massive stars convert in stages from hydrogen to helium to carbon to oxygen and even to iron
During supernova outburst by massive stars all elements beyond iron in periodic table are made in minutes.

Gravitation contraction inward in a star is resisted by radiation pressure outward generated by nuclear fusion.

Ultimately gravitational forces take over and a star inevitable dies.

In general, low-mass stars die relatively quietly, and high mass stars die in violent and gigantic explosion. To sort out the pictures of death scenarios, we divide star classes by
mass – the most important factor in star’s life.

1. LOW MASS RED DWARFS
2. MEDIUM MASS SUN LIKE STARS
3. HIGH MASS UPPER MAIN SEQUENCE STARS

Hydrogen fusion while at main sequence band lasts for a very long time (90% of star’s life). Most stars including the sun will probably become White Dwarfs - size of earth, extremely dense and intensely hot objects. The most massive stars explode leaving behind a massive neutron star or a blackhole.

Expansion into giants leaving the main sequence

 

Hydrogen fusion can raise the temperature to tens of millions but less than 100 million degrees K. This temperature is not sufficient for helium fusion for production higher mass nuclei. As the nuclear energy dies down in the core of hydrogen burning, gravitational pull contracts the core generating heat. When temperature rises, it starts H-burning in a shell just outside the helium core. At this time, stars like sun will become giants with 10 – 100 solar radius. Most massive stars like 25 solar mass will become 1000 times larger in diameter than the solar diameter. Such expansion of the star dramatically changes its position in the HR diagram.  

Degenerate Matter: Matter at extreme high densities is built by strong gravitational pull. Pressure of the degernerate electrons depends on speed of the electrons and NOT on temperature of the core. If the temperature reaches 100 million K, helium starts to fuse in the core. Helium fusion raises the temperature. This increases the helium fusion resulting in a RUNAWAY explosion called the helium flash. It is momentary in duration. Yet it produces energy in a few minutes, that is equivalent to more than energy present in the universe. Star gas in the interior becomes nondegenerate. Pressure temperature thermostat comes to play and star burns helium steadily. Stars less than 0.4 solar mass cannot start He fusion. Stars of more than 3 solar mass ignites helium undergoing helium fusion before the gas becomes degenerate.
At this point, sun-like stars and more massive stars burn helium in the core, and burn hydrogen by fusion in the inner wall of the shell. The star stops contraction of the core, the outer shell contracts, position of the star in HR diagram moves downward to left lower part of the diagram.
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REVISITING STARS

Post Main-Sequence Evolution

                    How long H-burning lasts depends on star's mass
High-mass stars <=> short main-sequence life
Low-mass stars <=> long main-sequence life
H-burning moves star up away from main sequence, increasing radius slightly
Eventually, all H in energy-generating core transformed to He, i.e., end of H-burning
Too cool to ignite He-burning, so must contract and heat up to 120,000,000 K

Helium "Burning"

 HELIUM FLASH
Helium flash expands (energy-generating) core allowing He-burning in regulated fashion
During star's life, "ashes" of one nuclear-burning phase provide fuel for next burning phase
Summary of thermonuclear burning by mass of star

MAIN-SEQUENCE MASS

THERMONUCLEAR BURNING PHASES

      Mstar <<Msun                                                                                                                  

  hydrogen                                        

       Mstar < Msun                                                                                                                                               

hydrogen and possibly helium                                         

 Mstar = Msun 

  hydrogen and helium

 Mstar > Msun 

 hydrogen, helium, carbon and
possibly oxygen, neon, and silicon

 Mstar >> Msun                                                                                                                

hydrogen, helium, carbon,
oxygen, neon, and silicon                                                 

 

PLANETARY NEBULA
Derives name from extended appearance around stars about to die, seen in small telescope - has nothing to do with planet properties.
Number in our Galaxy: about 30,000
Typical size: about 1 ly
Expansion velocity of shell: 10-30 km/s
Typical lifetime: 10,000 to 50,000 years
Some of the expelled matter is chemically evolved, particularly carbon.
Planetary nebula is observed surrounding giant stars. It consists of the expelled gas and dust clouds by hot giant stars. Many times an aging star may lose matter in a strong stellar wind. Most medium mass stars produce planetary nebula during their giant stage.

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Low-Mass Stars - Post-Giant Evolution

                    Star spends 10-25% of life as red giant in He-burning
After several hundred thousand years, low mass stars cease He-burning and expel hydrogen-rich envelope, a planetary nebula
Remnant of star contraction and heating up so that surface temperature is 100,000oK which radiates ultraviolet photons that are absorbed by surrounding gaseous shell

Red Giants

Red giants - second most prominent region in H-R diagram composed bright, cool stars

Red giants are luminous stars in spectral classes F, G, K, and M lying above main sequence in region that angles up toward bright, cool stars in upper right-hand corner

Despite being members of same luminosity class, red giants vary in luminosity by at least a factor of 100

These are 100 times more luminous than Sun on average

Surface temperature varies 3000 K to 7000 K

No relationship exists between mass and position on HR diagram for red-giant branch

Radii of giant stars do increase progressively upward toward upper right-hand corner of H-R diagram

High-Mass Star - Post-Main-Sequence Evolution

                    Hydrogen exhaustion
Gravitational contraction in core and expansion in outer layers
Gravitational potential energy converted to thermal energy and luminosity (surface)
Egravitational => Ethermal + Luminous energy
High-mass stars evolve across HR diagram as giant and supergiant branch
Time required < 1,000,000 to 50,000,000 years
11-50 Mass of sun star forms carbon-oxygen core producing neon and more oxygen
Oxygen-neon core contracts and heats until neon-burning reactions ignited
Cycle of contraction, heating, and ignition continues to core of iron-peak elements (26 protons & 30 neutrons)
Creating heavier elements takes up energy rather than releasing energy
Lighter elements burn in shells

Core grows until it exceeds Chandrasekhar limit 1.4 Mass of sun for white dwarf, then collapses
Conversion of gravitational potential energy to thermal energy during collapse
Iron-peak nuclei to decompose into helium nuclei that in turn decompose into neutrons
Core collapse produces degenerate-neutron sphere and Type II supernova outburst (Large Magellanic Cloud, 1987)

Bright and regular (Blue and Red) SupergiantsSupergiants - relatively rare stars, luminosity classes I and II Blue and red supergiants can be hundreds of thousands of times more luminous than SunBlue and red supergiant stars also do not possess definite relation between mass and position in H-R diagram, radii do increase toward upper right-hand corner. Although supergiants can be seen at tremendous distances because of their great luminosity, they appear to be very rare type of starFar more red-giant stars in our Galaxy than blue and red supergiants

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Death of Stars

Eventually, stars exhaust available nuclear fuel
Depending on mass of the main-sequence star, a star undergoes a particular evolution to its death

                              


Main-Sequence Mass

Death Sequence of Star

        0.1 < M < 0.8 Msun

spend entire life in H-burning and cool to white dwarf eventually

        0.8 < M < 11 Msun

 go through red-giant phase and end as white dwarfs (99% of all stars)

       11 < M < 50 Msun

undergo supernova outburst and end as neutron star
(1% or less of all stars)

       M > 50 Msun

undergo supernova outburst and end as black hole

 

White Dwarfs

Central star of a planetary nebula cools to become white dwarf
Gas in white dwarf is degenerate electron gas
Degenerate gas - contraction has reduced available energy states to approximately number of free electrons; pressure no longer controlled by temperature
Mass-radius relation - larger the mass, the smaller the radius up to about 1.4 Mass of sun as largest mass.
Chandrasekhar limit - no white dwarf can be stable if Mass of white dwarf > 1.4 Mass of sun 

The range:      

If Mass of white dwarf = 0.4 Mass of sun, then Radius of white dwarf = 10,000 km = 0.015 Radius of sun 
If Mass of white dwarf = 0.8 Mass of sun, then Radius of white dwarf = 7,000 km = 0.010 Radius of sun 
No correlation between spectral appearance and surface temperature or radius as seen in main sequence
Number of known white dwarfs is several thousand, some 400 studied spectroscopically
Estimated number of white dwarfs in our Galaxy is 35 billion
Black dwarf - when thermal energy radiated away from white dwarf, takes billions of years

White DwarfsWhite dwarfs - span spectral classes B, A, and F; composed of faint stars lying below main sequence

White dwarfs appear to be second most populous region in H-R diagram

White dwarfs are typically a few thousandths of luminosity of Sun

Surface temperature greater than that of Sun

White dwarf stars possess

Consequently, white dwarfs must have mean densities on order of millions of g/cm-cube
Suggests white dwarfs composed of matter in state unlike anything we possess on Earth

Supernova
Type II supernova
10-100 billion times greater than Luminisity of the sun (brightness comparable to entire galaxy)
Rapid rise to maximum brilliance followed by gradual decrease over several weeks to months.
Frequency of about 1 every 50 years per galaxy
During outburst, rapid evolution of chemical elements to fill out rest of periodic table
Examples
Crab nebula in Taurus constellation
Gum nebula in Vela constellation

Neutron star - sphere of degenerate neutrons produced during core collapse in high-mass star followed by supernova outburst

Properties:
Mass: 2-3 Mass of sun 
Radius: < 10 km
Density: 1014-1016 g/cubic cm 
Central temperature: 100,000,000,000 K
Pulsar - rapidly spinning neutron star, emits high-intensity bursts of radio radiation

TABLE


Main-Sequence Mass                       Death Sequence of Star

0.1 < M < 0.8 Msun

spend entire life in H-burning and cool to
white dwarf eventually

 

Eventually, stars exhaust available nuclear fuels and can no longer replace energy loss by luminosity

Dramatic changes follow leading to death of star

Very low-mass stars (0.1 < M < 0.8 Mass of sun) spend entire life in H-burning and cool to white dwarf eventually

Low-mass stars (0.8 < M < 11 Mass of sun) go through red-giant phase and end as white dwarfs (99% of all stars)

High-mass stars (11 < M < 50 Mass of sun) undergo supernova outburst and end as neutron star (1% or less of all stars)

Very high-mass stars (M > 50 Mass of sun) undergo supernova outburst and end as black hole

Low-Mass Stars - White Dwarfs

Central star of planetary nebula cools to become white dwarf

White dwarf is composed of degenerate-electron gas

Degenerate gas - contraction has reduced available energy states to approximately number of free electrons;
pressure not controlled by temperature, but by degenerate electrons

Mass-radius relation

Radius of white dwarf = 10,000 km = 0.015 radius of sun
Mass of white dwarf = 0.4 mass of the sun

     Chandrasekhar limit 1.4 Mass of sun

     Mwd = 0.8 Mass of sun    

     Rwd = 7,000 km = 0.010 Radius of sun

No correlation between spectral appearance and surface temperature or radius as for main sequence

Number of known white dwarfs is several thousand, some 400 studied spectroscopically

     Estimated number of white dwarfs in our Galaxy is 35 billion

     End state of old disk population of stars

     Black dwarf - when thermal energy radiated away for white dwarf, takes billions of years

High-Mass Stars - Supernova

     Even in 1930s, some astronomers thought stellar remnant of Type II supernova outburst too
massive to be white dwarf

We now know from stellar model calculations correlated with supernova observations that

High-mass stars (11 < M < 50 Mass of sun) undergo supernova outburst and end
as neutron star (1% or less of all stars)
Very high-mass stars (M > 50 Mass of sun) undergo supernova outburst and end as black hole


Hypernova (pl. hypernovae) refers to an exceptionally large star that collapses at the end of its lifespan—for example, a collapsar, or a large supernova. Up until the 1990s, it had a more specific meaning to refer to an explosion with an energy of over 100 supernovae (10 to the 46 joules). Such explosions were proposed to explain the exceptional brightnesses of gamma ray bursts. An extensive sky search found several apparent hypernova remnants, but the frequency was too low to support the hypothesis.[1] Today the term is used somewhat more prosaically to describe the supernovae of supermassive stars, the hypergiants, which have masses from 100 to 150 times that of the Sun.

High-Mass Stars - Neutron Star

Neutron star - sphere of degenerate neutrons produced during core collapse in high-mass star followed by supernova outburst

Neutron star predicted by Robert Oppenheimer in 1930s based on Chandrasekar’s studies of degenerate-electron gas structure for white dwarfs

Weight of overlying layers supported by pressure from degenerate neutrons rather than degenerate electrons as in white dwarf

Predicted properties

          Mass: 1.4-3 Mass of sun

          Radius < 10 km

 
Density: 10 to the 14-10 to the 16 g/cubic cm
Central temperature: 10 to the 10 K

Predicted object clearly so small that little chance of observing one unless extremely close to Solar System

Prediction forgotten until discovery of pulsar in 1968
Pulsar - Pulsating Radio Star

Pulsar - rapidly spinning neutron star, emits high-intensity bursts of radio radiation

1967 object found in Vulpecula emitting pulses of radio radiation with period of 1.337 seconds

Within months more found with period for radio pulses between 0.001 second and a few seconds

Constancy of time interval between pulses can be one part in 10 million (equivalent to gain or loss of 1 second/year for clock) 

Region size of Pulsar from evidence of emission

Emitting region cannot be larger than distance light can travel in time interval for pulses

Pulse duration of 0.001 s, emitting region < 300 km
White dwarfs have radii of 5000 to 10,000 km, thus smaller than white dwarf
Neutron star is only star small enough to be pulsar

Assume pulsar turns on everywhere at same time Earth

Pulsar Rotation and Distance

Conservation of angular momentum over lifetime means normal main-sequence rotation rates for massive stars become 1000 rotations per second for neutron stars

Only a small, extremely dense, extremely rigid mass can withstand forces of rotation that would tear object apart
Neutron star fits that description

Electrons in interstellar space affect velocities of radio waves

Longer wavelengths travel slower than shorter wavelengths
Different wavelengths bunched together in composite wave spread out along path to Earth
Short wavelengths arrive before long wavelengths
Delay time allows an estimate of distance of pulsar

Pulsars are relatively nearby objects confined in or near plane of our Galaxy, Typical distance is something like 3000 ly
Pulsar - Visible Objects

     Pulsars pulse not only in radio wavelengths, but some pulse also in

    Visible wavelengths, X-ray wavelengths, Gamma-ray wavelengths

     Several pulsars have been identified with visible object

    Hubble Space Telescope picture of isolated neutron star: Crab Nebula pulsar, Neutron Star

     Estimated properties, Surface temperature about 600,000 K

     Radius about 28 km, Magnetic field rotating with neutron star

     Magnetic/rotation scheme means that rotational energy is converted to electromagnetic energy

     Loss of rotational energy means neutron star slows rotation or interval between pulses grows longer

     Over 90 pulsars show a decrease in pulse rate

     Youngest neutron stars have shortest rotation period, while oldest ones have longest period

Average age about 2 million years; Oldest about 10 million years; Eventually rotation energy insufficient to power pulses in the pulsars.

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