Our Sun’s Story …and that of heavy stars mass of a star determines its core pressure and temperature: our sun’s low mass  cooler core and slower fusion rate lower internal temperature, and external (yellow) smaller luminosity longer lifetime (10 B y) high mass stars (> 8 msun) have higher core temperature more luminous and higher external temperature (blue) more rapid fusion shorter-lived (30 M y)

Intermediate-Mass Stars High-Mass Stars > 8 MSun Intermediate-Mass Stars Low-Mass Stars < 2 MSun Brown Dwarfs famous Herztsprung- Russell diagram

as long as it has hydrogen in its core to fuse into helium sun is stable… as long as it has hydrogen in its core to fuse into helium Main-seq_lifetime_and_mass.swf

Thought Question What happens when a star no longer has enough hydrogen in its core to fuse to helium? i.e. after 10 B years A. Core cools off B. Core shrinks and heats up C. Core expands and heats up D. Helium fusion immediately begins

Thought Question What happens when a star can no longer fuse hydrogen to helium in its core? A. Core cools off B. Core shrinks and heats up C. Core expands and heats up D. Helium fusion immediately begins

Life Track after Main Sequence Observations of stars in clusters, all born at the same time, show that a star becomes larger, redder, and more luminous after fusing all the H in its core

as inert He core contracts, H in a shell around the He core begins burning luminosity increases 1,000 x too hot for life on Earth radius grows 100 x, out to Earth increased fusion rate in the H shell does not stop He core from contracting H shell burns for ~1 B years

luminosity of a sun? same as for any black body… L  Area  T4 Stefan-Boltzman law even though T down  2, A up (100)2  for red giant,

Helium fusion does not begin until heated by collapse requires 100 MK since charge (+2)2 leads to 4 x greater repulsion than with 2 protons Fusion of 2 helium nuclei doesn’t work (8Be unstable), helium fusion must combine 3 He nuclei to make carbon

Thought Question What happens in a low-mass star when core temperature rises enough for helium fusion to begin? A. Helium fusion slowly starts up B. Hydrogen fusion stops C. Helium fusion (triple alpha) starts very sharply Hint: this is a strong reaction (no neutrinos) once the temperature is hot enough to overcome Coulomb barrier

Thought Question What happens in a low-mass star when core temperature rises enough for helium fusion to begin? A. Helium fusion slowly starts up B. Hydrogen fusion stops C. Helium fusion rises very sharply

Helium Flash Core temperature rises rapidly when helium fusion begins Helium fusion rate skyrockets until thermal pressure takes over and expands core again

Helium burning stars neither shrink nor grow, Helium burning stars neither shrink nor grow, core He burns to C for 100 M years, then expand again in a second red giant phase

Thought Question 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 to C in a shell around a heavier carbon core

Thought Question 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 used up, He fuses into carbon in a shell around the inert carbon core H fuses to He in a shell around the fusing helium layer 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

Our Sun’s Dregs: a Planetary Nebula after few M years, double-shell burning ends in a pulse, ejecting H, He, C out into space a planetary nebula (but nothing to do with planets) white dwarf, carbon core left behind …two example pix from Hubble

for our sun, C is the end of the fusion trail fusion progresses no further in a low-mass star mass too small for gravity to collapse it further, and heat it up even more electron degeneracy pressure supports white dwarf against gravity (e-’s approach speed c if m > 1.4 msolar ) temperature never grows hot enough (400 M K) for fusion to heavier elements e.g. for He to fuse with C to make oxygen

Life stages of a low-mass star like the Sun Death_sequence_of_sun.swf

Life Path of a Sun-Like Star

How different are life stages of high-mass (e.g. 25 m๏) star? similar to those of low-mass stars, like our sun, but each is faster Hydrogen core fusion, ~ M years not pp, but much faster CNO cycle, higher luminosity making N and O as well as He becomes a red supergiant when core H exhausted Hydrogen shell burning around a He core Helium core fusion to carbon, lasting ~ 100,000 years Carbon burning (0.6 B ºK) for ~ 100 years

What are the life stages of a high-mass star? CNO Cycle High-mass fuse H to He at a higher rate using carbon catalyst, CNO cycle Greater core temperature heavy nuclei overcome greater Coulomb repulsion

How do high-mass stars make the elements necessary for life?

Big Bang made 75% H, 25% He – stars make everything else

3 Helium fusion makes carbon in low-mass stars

CNO cycle changes C into N and O

Helium Capture by O and Ne higher core temperatures from successive gravitational collapses gives helium the energy to thwart ever stronger Coulomb barriers (zZ) of heavier elements

Advanced Nuclear Burning Core temperatures in stars with >8MSun allow fusion of elements as heavy as iron

Multiple Shell Burning Advanced nuclear burning proceeds in a series of nested shells

Iron = dead end for fusion nuclear reactions of iron release no energy Fe has lowest mass per nucleon

signature of helium capture nucleosysthesis: highest abundances are elements with even numbers of protons

Core then suddenly collapses, creating supernova explosion Iron builds up in core until degeneracy pressure can no longer resist gravity Core then suddenly collapses, creating supernova explosion Death_seq_of_high-mass_star.swf Download a good supernova explosion movie from the Chandra Science Center (chandra.harvard.edu)

Energy and neutrons released in supernova explosion enable elements heavier than iron to form, including Au and U

What causes a supernova collapse? Core degeneracy pressure disappears electrons combine with protons, making neutrons and neutrinos kT + me + mp > mn kT + 0.5 + 938.3 > 939.6 MeV Ethermal = kT @ 10 BK = 1 MeV, k = Boltzman’s constant Neutrons collapse to the center, forming a much smaller (~10km~Boston) neutron star (me/mn ~ 1/2000) … then collapsing to a black hole if  12 msun

Supernova Remnant energy released by core collapse drives outer layers into space Crab Nebula the remnant of supernova of AD 1054 …and its neutron star

our picture of a pulsar (neutron star) during collapse… angular momentum conserved big spin up magnetic fields pinched very strong but in chaos of explosion, magnetic  rotational axis beams of radiation escape along magnetic axis “lighthouse” beam sweeps periodically past earth… spinning so fast it can only be from a compact source, r ~ 10 km

Supernova 1987A closest supernova in the last four centuries