20. Stellar Death Dredge-ups bring red-giant material to the surface Low-mass stars undergo two distinct red-giant stages Dredge-ups bring red-giant material to the surface Low -mass stars die gently as planetary nebulae Low -mass stars end as white dwarfs High-mass stars synthesize heavy elements High-mass stars die violently as supernovae Supernova 1987A Supernovae produce abundant neutrinos Binary white dwarfs can become supernovae Detection of supernova remnants

Low-Mass Stars: Three Red Giant Phases Low-mass definition < ~ 4 MSun during main-sequence lifetime Red giant phases Initiation of shell hydrogen fusion Red giant branch on the H-R diagram Initiation of core helium fusion Horizontal branch of the H-R diagram Initiation of shell helium fusion Asymptotic giant branch of the H-R diagram

The Sun’s Post-Main-Sequence Fate

Interior of an Old Low-Mass AGB Star

Stellar Evolution In a Globular Cluster

Dredge-Ups Mix Red Giant Material Main-sequence lifetime The core remains completely separate There is no exchange of matter with overlying regions Decreasing H Increasing He in the core Overlying regions retain cosmic chemical proportions ~ 74 % H ~ 25% He ~ 1% “metals” Red giant phases Three possible stages Stage 1 dredge-up After core H fusion ends Stage 2 dredge-up After core He fusion ends Stage 3 dredge-up After shell He fusion begins Only if MStar > 2 MSun One possible result A carbon star Abundant CO ejected into space Same isotopes of C & O that are in human bodies

Low-Mass Stars Die Gently He-shell flashes produce thermal pulses Caused by runaway core He fusion in AGB stars Cyclical process at decreasing time intervals 313,000 years 295,000 years 251,000 years 231,000 years All materials outside the core may be ejected ~ 40% of mass lost from a 1.0 MSun star > 40% of mass lost from a >1.0 MSun star Hot but dead CO core exposed At the center of an expanding shell of gas Velocities of ~ 10 km . sec-1 to ~ 30 km . sec-1 Velocities of ~ 22,000 mph to ~ 66,000 mph

A Carbon Star & Its CO Shell

Thermal Pulses of an 0.7 MSun AGB Star

One Example of a Planetary Nebula

The Helix Nebula: 140 pc From Earth

Making An Elongated Planetary Nebula

Low-Mass Stars End As White Dwarfs UV radiation ionizes the expanding gas shell This glows in what we see as a planetary nebula Name given because they look somewhat like planets No suggestion that they have, had, or will form planets This gas eventually dissipates into interstellar space No further nuclear fusion occurs Supported by degenerate electron pressure About the same diameter as Earth ~ 8,000 miles It gradually becomes dimmer Eventually it will become too dim to detect

The Chandrasekhar Limit White dwarf interiors Initially supported by thermal pressure Ionized C & O atoms A sea of electrons As the white dwarf cools, particles get closer Pauli exclusion principle comes into play Electrons arrange in orderly rows, columns & layers Effectively become one huge crystal White dwarf diameters The mass-radius relationship The larger the mass, the smaller the diameter The diameter remains the same as a white dwarf cools Maximum mass degenerate electron pressure can support ~ 1.4 MSun After loss of overlying gas layers This white dwarf upper mass limit is the Chandrasekhar limit

Evolution From Giants To White Dwarfs

White Dwarf “Cooling Curves”

High-Mass Stars Make Heavy Elements High-mass definition > ~ 4 MSun as a ZAMS star Synthesis of heavier elements High-mass stars have very strong gravity Increased internal pressure & temperature Increased rate of core H-fusion into He Increased rate of collapse once core H-fusion ends Core pressure & temperature sufficient to fuse C The CO core exceeds the Chandrasekhar limit Degenerate electron pressure cannot support the mass The CO core contracts & heats Core temperature > ~ 6.0 . 108 K C fusion into O, Ne, Na & Mg begins

Synthesis of Even Heavier Elements Very-high-mass definition > ~ 8 MSun as a ZAMS star Synthesis of still heavier elements End of core-C fusion Core temperature > ~ 1.0 . 109 K Ne fusion into O & Mg begins Percentage of O & Mg increases in the core End of core-Ne fusion Core temperature > ~ 1.5 . 109 K O fusion into S begins End of core-O fusion Core temperature > ~ 2.7 . 109 K Si fusion into S & Fe begins Additional layers of shell fusion begin

The Interior of an Old High-Mass Star

Consequence of Multiple Shell Fusion Core changes Core diameter decreases with each step Ultimately about the same diameter as Earth ~ 8,000 miles Rate of core fusion increases with each step Energy changes Each successive fusion step produces less energy All elements heavier than iron require energy input Core fusion cannot produce elements heavier than iron All heavier elements must be produced by other processes

Evolutionary Stages of a 25-MSun Star

High-Mass Stars Die As Supernovae Basic physical processes All thermonuclear fusion ceases The core collapses It is too massive for degenerate electron pressure to support The collapse rebounds Luminosity increases by a factor of 108 As bright as an entire galaxy > 99% of energy is in the form of neutrinos Matter is ejected at supersonic speeds Powerful compression wave moves outward Appearance Extremely bright light where a dim star was located Supernova remnant Wide variety of shapes & sizes

Supernova: The First 20 Milliseconds

Supernova 1987A Important details Basic physical processes Located in the Large Magellanic Cloud A companion to the Milky Way ~ 50,000 parsecs from Earth Discovered on 23 February 1987 Near a huge H II region called the Tarantula Nebula Visible without a telescope First naked-eye supernova since 1604 Basic physical processes Primary producer of visible light Shock wave energy < 20 days Radioactive decay of cobalt, nickel & titanium > 20 days Dimmed gradually after radioisotopes were gone > 80 days Luminosity only 10% of a normal supernova

Unusual Feature of SN 1987A Relatively low-mass red supergiant Outer gaseous layers held strongly by gravity Considerable energy required to disperse these gases Significantly reduced luminosity Unusual supernova remnant shape Hourglass shape Outer rings Ionized gas from earlier gentle ejection Central ring Shock wave energizing other gases

Supernova 1987A: Three-Ring Circus

White Dwarfs Can Become Supernovae Observed characteristics No spectral lines of H or He These gases are gone The progenitor star must be a white dwarf Strong spectral line of Si II Basic physical processes White dwarf in a close-binary setting Overcontact situation Companion star fills Roche lobe The white dwarf may exceed the Chandrasekhar limit Degenerate electron pressure cannot support the mass Core collapse begins, raising temperature & pressure Unrestrained core C-fusion begins The white dwarf blows apart

The Four Supernova Types Type Ia Type Ib Type Ic Type II No H or He lines Strong Si II line No H lines Strong He I line Strong H lines

Type Ia & II Supernova Light Curves

Gum Nebula: A Supernova Remnant

Pathways of Stellar Evolution

Important Concepts Death of low-mass stars Death of high-mass stars ZAMS mass < 4 MSun Red giant phases Start of shell H fusion Start of core He fusion Start of shell He fusion No elements heavier than C & O Gentle death Dead core becomes a white dwarf Expelled gases become planetary neb. Death of high-mass stars ZAMS mass > 4 MSun Red supergiant phases No elements heavier than Fe Catastrophic death Dead core a neutron star or black hole Supernova remnant Elements heavier than Fe produced Pathways of stellar evolution Low-mass stars Produce planetary nebulae End as white dwarfs High-mass stars Produce supernovae End as neutron stars or black holes