http://www.highpoint.edu/~afuller/PHY-1050

• Read:– Death From The Skies Chapter 3: “The Stellar Fury of

Supernovae”– Death From The Skies Chapter 7: “The Death of the Sun”

• Pre-Lecture Quiz:– MasteringAstronomy Ch18 pre-lecture quiz due March 31– MasteringAstronomy Ch19 pre-lecture quiz due April 14

• Homework: – MasteringAstronomy Ch16 assignment due March 29– MasteringAstronomy Ch17 assignment due April 12– MasteringAstronomy Ch18 assignment due April 19– MasteringAstronomy Ch19 assignment due April 24

How does a star’s mass affect nuclear fusion?

• The mass of a main-sequence star determines its core pressure and temperature.

• Stars of higher mass have higher core temperature and more rapid fusion, making those stars both more luminous and shorter-lived.

• Stars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and longer lifetimes.

High-Mass Stars> 8MSun

Low-Mass Stars< 2MSun

Intermediate-Mass Stars

Brown Dwarfs

Star Clusters and Stellar Lives• Our knowledge of the life

stories of stars comes from comparing mathematical models of stars with observations.

• Star clusters are particularly useful because they contain stars of different mass that were born about the same time.

Life Cycle of a Low-Mass Star

Main Sequence Lifetimes and Stellar Masseshttp://www.highpoint.edu/~afuller/PHY-1050/textbook/17_MSLifetimeAndMass.htm

What happens when a star can no longer fuse hydrogen to helium in its core?

A. The core cools off.B. The core shrinks and heats up.C. The core expands and heats up.D. Helium fusion immediately begins.

What happens when a star can no longer fuse hydrogen to helium in its core?

A. The core cools off.B. The core shrinks and heats up.C. The core expands and heats up.D. Helium fusion immediately begins.

Life Track After Main Sequence

• Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over.

Red Giants: Broken Thermostat

• As the core contracts, H begins fusing to He in a shell around the core.

• Luminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting.

Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion.

Fusion of two helium nuclei doesn’t work, so helium fusion must combine three helium nuclei to make carbon.

What happens in a low-mass star when core temperature rises enough for helium fusion to begin? (Hint: Degeneracy pressure is the main form of pressure in the inert helium core.)

A. Helium fusion slowly starts.B. Hydrogen fusion stops.C. Helium fusion rises very sharply.

What happens in a low-mass star when core temperature rises enough for helium fusion to begin? (Hint: Degeneracy pressure is the main form of pressure in the inert helium core.)

A. Helium fusion slowly starts.B. Hydrogen fusion stops.C. Helium fusion rises very sharply.

Helium Flash, I

• The thermostat of a low-mass red giant is broken because degeneracy pressure supports the core against gravity instead of the energy released from nuclear fusion.

• Hydrogen continues to burn in a shell surrounding the helium core.

Helium Flash, II• Because hydrogen fusion

continues in outer shell, helium “ash” continues to get dumped onto helium core.

• As helium continues to pile up in the core, eventually helium fusion is triggered, causing the core temperature to rapidly rise.

• Helium fusion rate skyrockets until thermal pressure takes over and expands the core again.

A 5 Msun star with a helium core and a hydrogen-burning shell shortly after shell ignition.

Helium-burning stars neither shrink nor grow because core thermostat is temporarily fixed.

Life Track After Helium Flash

• Models show that a red giant should shrink and become less luminous after helium fusion begins in the core.

• The exact path of the life track depends on the star’s mass.

Life Track After Helium Flash• Observations of star

clusters agree with those models.

• Helium-burning stars are found on a horizontal branch on the H-R diagram.

• Combining models of stars of similar age but different mass helps us to age-date star clusters.

Life Track After Helium Flash

1 Msun Star 5 Msun Star

What happens when the star’s core runs out of helium?

A. The star explodes.B. Carbon fusion begins.C. The core cools off.D. Helium fuses in a shell around the core.

What happens when the star’s core runs out of helium?

A. The star explodes.B. Carbon fusion begins.C. The core cools off.D. Helium fuses in a shell around the core.

Double Shell Burning• After core helium fusion

stops, helium fuses into carbon in a shell around the carbon core, and hydrogen fuses to helium in a shell around the helium layer.

• This double shell–burning stage never reaches equilibrium—fusion rate periodically spikes upward in a series of thermal pulses.

• With each spike, convection dredges carbon up from core and transports it to surface.

A 5 Msun star well after all the helium in the core has been exhausted, with just a carbon-oxygen core.

Planetary Nebulae

• Double shell burning ends with a pulse that ejects the H and He into space as a planetary nebula.

• The core left behind becomes a white dwarf.• Despite the name, this phenomenon has

nothing immediate to do with planets

End of Fusion

• Fusion progresses no further in a low-mass star because the core temperature never grows hot enough for fusion of heavier elements (some helium fuses to carbon to make oxygen).

• All that remains is the exposed core of the star, called a white dwarf.

• Degeneracy pressure supports the white dwarf against gravity and it slowly cools.

• After several trillion years, the white dwarf will eventually reach temperatures around 10 K and become known as a black dwarf.

Understanding the Individual Stages of a Low-Mass Star’s Death Sequencehttp://www.highpoint.edu/~afuller/PHY-1050/textbook/17_DeathSeqStar.htm

Life Track of a Sun-like Star

Life Cycle of a High-Mass Star

Life Cycle of a High-Mass Star• Late life stages of high-mass stars

are similar to those of low-mass stars:– Hydrogen core fusion (main

sequence)– Hydrogen shell burning (supergiant)– Helium core fusion (supergiant)

• High-mass stars, however, have the ability to continue fusing elements well past helium.

• As such, the paths of high-mass stars on the H-R diagram are different from those of low-mass stars.

CNO Cycle

• High-mass main-sequence stars fuse H to He at a higher rate using carbon, nitrogen, and oxygen as catalysts.

• Greater core temperature enables hydrogen nuclei to overcome greater repulsion.

High-mass stars make the elements necessary for life

High core temperatures allow helium to fuse with heavier elements.

Big Bang made 75% H, 25% He. Stars make everything else.

Helium fusion can make carbon in low-mass stars. (It can also make beryllium and oxygen in what is known as the triple-alpha cycle.)

Insert image, PeriodicTable2.jpg.

CNO cycle can change carbon into nitrogen and oxygen.

Helium capture builds carbon into oxygen, neon, magnesium, and other elements.

Advanced Nuclear Burning

Core temperatures in stars with >8 MSun allow fusion of elements as heavy as iron.

Advanced reactions in stars make elements like Si, S, Ca, Fe.

Insert image, PeriodicTable5.jpg

Evidence for helium capture

Higher abundances of elements with even numbers of protons.

Multiple Shell Burning

Advanced nuclear burning proceeds in a series of nested shells.

Time Frames

Because C, O, and Si burning produce nuclei with masses progressively closer to Fe, less and less energy is generated per gram of fuel. As a result, the time scale for each succeeding reaction becomes shorter.

The Death Sequence of a High-Mass Starhttp://www.highpoint.edu/~afuller/PHY-1050/textbook/IF_17_12_HighMassDeathSeq.htm

The Party Ends With Iron

Iron is a dead end for fusion because nuclear reactions involving iron do not release energy. (This is because iron has lowest mass per nuclear particle.)

The Party Ends With Iron

• Iron builds up in core until degeneracy pressure can no longer resist gravity.

• To make matters worse, fusion of iron will require—not release—energy.

• The core then suddenly collapses, creating a supernova explosion.

Supernova Step 1: Photodisintegration

• At the billion-degree temperatures now present in the core, the photons possess enough energy to destroy heavy nuclei, a process known as photodisintegration.

• This destroys heavy elements created in each stage of fusion.

• This process requires energy, so thermal energy is removed from the gas that would otherwise have resulted in the pressure necessary to support the star’s core.

Supernova Step 2: Creation of Neutrinos

• Free electrons that had assisted in supporting the star through degeneracy pressure now collide with the protons produced through photodisintegration.

• The result of this is that a massive amount of the star’s mass is converted into neutrons and neutrinos.

• Neutrons collapse to the center, forming a neutron star.

• Neutrinos escape to space mostly uninhibited.

Supernova Step 3: Core Collapse• Through the photodisintegration of iron,

combined with the creation of neutrons and neutrinos, most of the core’s support in the form of electron degeneracy pressure is suddenly gone and the core begins to collapse extremely rapidly.

• The inner core collapses so fast, it decouples from the outer core, completely separating from it, causing the outer core to go into free-fall.

• During the collapse, speeds can reach almost 70,000 km/s (0.25 c), and within about one second a volume the size of Earth has been compressed to a diameter of 100 km.

• This process takes roughly a quarter of a second.

Supernova Step 3: Suspended Shells

• Since “word” that the core has collapsed propagates through the star at a much smaller speed, there is not enough time for the outer layers to immediately learn about what has happened.

• The outer layers, including the O, C, and He shells, as well as the outer envelope, are left in a precarious position of being almost suspended above the catastrophically collapsing core.

Supernova Step 4: Neutron Degeneracy• The inner core continues to collapse

until it reaches a point where neutron degeneracy is strong enough to resist gravity and support the core.

• The result is that the inner core rebounds somewhat, sending pressure waves outward into the in-falling material from the outer core.

• This “core bounce” takes only 20 milliseconds to occur and is known as a “prompt hydrodynamic explosion.”

Supernova Step 5: Shock Wave Propagation

• The pressure waves speed up and become full shock waves that work their way toward the surface.

• If the remainder of the iron core is less than roughly 1.2 Msun, the shock waves “snowplow” the H-rich envelope and the remainder of the nuclear-processed matter in front of it.

• If the remainder of the core is more massive than 1.2 Msun, then the shock wave stalls. Neutrinos are blocked by this stalled shock wave. Eventually the build-up of neutrinos pushes the shock wave back into motion, releasing roughly 1047 J. (The sun produces 1045 J of energy over its entire lifetime on the main sequence.)

Supernova Step 6: The Remnant• Energy released by the collapse of the

core drives the star’s outer layers into space.

• If the initial mass of the star on the main sequence is not too large (< 25 Msun), the remnant inner core will stabilize and become a neutron star, supported by degenerate neutron pressure.

• However, if the initial stellar mass is much larger, even the pressure of neutron degeneracy cannot support the remnant against the pull of gravity. The final collapse will produce a black hole.

• The Crab Nebula is the remnant of the supernova seen in A.D. 1054.

Supernova Time Frame

Energy and neutrons released in supernova explosion enable elements heavier than iron to form, including gold and uranium.

Insert figure, PeriodicTable6.jpg

A Star’s Mass Determines Its Destiny

The Role of Mass

• A star’s mass determines its entire life story because it determines its core temperature.

• High-mass stars with >8 Msun have short lives, eventually becoming hot enough to make iron, and end in supernova explosions.

• Low-mass stars with <2 Msun have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs.

• Intermediate-mass stars can make elements heavier than carbon but end as white dwarfs.

Low-Mass Star Summary

1. Main sequence: H fuses to He in core. 2. Red giant: H fuses to He in shell around He core.3. Helium core burning: He fuses to C in core while H

fuses to He in shell.4. Double shell burning: H and He both fuse in shells.5. Planetary nebula leaves white dwarf behind.

Reasons for Life Stages

• Core shrinks and heats until it’s hot enough for fusion. • Nuclei with larger charge require higher temperature for

fusion.• Core thermostat is broken while core is not hot enough for

fusion (shell burning).• Core fusion can’t happen if degeneracy pressure keeps core

from shrinking.

High-Mass Star Summary

1. Main sequence: H fuses to He in core. 2. Red supergiant: H fuses to He in shell around He core.3. Helium core burning: He fuses to C in core while H

fuses to He in shell.4. Multiple shell burning: Many elements fuse in shells.5. Supernova leaves neutron star behind.