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 ratelower internal temperature, and external (yellow)smaller luminositylonger lifetime (10 B y)

high mass stars (> 8 msun) have higher core temperaturemore luminous and higher external temperature (blue)more rapid fusion

shorter-lived (30 M y)

High-Mass Stars

> 8 MSun

Low-Mass Stars< 2 MSun

Intermediate-Mass Stars

Brown Dwarfs

famous Herztsprung- Russell diagram

sun is stable…

as long as it has hydrogen in its core to fuse into helium

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 offB. Core shrinks and heats upC. Core expands and heats upD. Helium fusion immediately begins

Thought Question

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

A. Core cools offB. Core shrinks and heats upC. Core expands and heats upD. 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 Earthincreased 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 upB. Hydrogen fusion stopsC. 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 upB. Hydrogen fusion stopsC. 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, 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 explodesB. Carbon fusion beginsC. The core cools offD. 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 explodesB. Carbon fusion beginsC. The core cools offD. Helium fuses in a shell around the core

Double Shell BurningAfter core helium used up,

He fuses into carbon in a shell around the inert carbon coreH 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

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

CNO CycleHigh-mass fuse H to He at a higher rate using carbon catalyst, CNO cycle

Greater core temperature heavy nuclei overcome greater Coulomb repulsion

What are the life stages of a high-mass star?

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 capturenucleosysthesis:

highest abundances are elements with

even numbers of protons

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

Core then suddenly collapses, creating supernova explosion

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 disappearselectrons 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