Physical Universe Chapter 17

Mass of a main-sequence star determines:

its core pressure and temperature

Stars of higher mass have: (3)

• higher core temperature and
  more rapid fusion

• this makes those stars both more luminous and shorter-lived.

• High-Mass Stars : > 8M Sun

Stars of lower mass have: (3)

• cooler cores and slower fusion rates

• this gives them smaller luminosities and longer lifetimes

• Low-Mass Stars < 2M Sun

Life Track after Main Sequence: (3)

• depends on stars mass

• Low-mass: red giant → planetary nebula → white dwarf

• High-mass: red super giant → supernova → black hole/neutron star

Red Giants: Broken Thermostat (3)

• during contraction, H fuses into He in a shell around the core

• luminosity increases as the increasing fusion within the shell doesn’t stop contraction

• overall increases temperature

Helium fusion does not begin right away because: (2)

• helium fusion requires higher temperature, as a larger electric charge leads to greater repulsion

• two helium nuclei cannot fuse, rather three fuse to make carbon

The thermostat of a low-mass red giant is broken because:

• the core is supported by degeneracy pressure

Core temperature rises rapidly when: 

helium fusion begins.

Helium fusion rate skyrockets until thermal pressure takes over and expands the core again. (Helium Flash)

Helium-burning stars neither shrink nor grow because:

• core thermostat is temporarily fixed

Life Track after Helium Flash: (2)

• models show that a red giant should shrink and become less luminous after helium fusion begins

• Helium burning stars are found on a horizontal branch on the HR Diagram

After core helium fusion stops: (2)

• the helium fuses into carbon around the now carbon core

• hydrogen fuses into helium in a shell around the helium layer

Double-shell burning stage  (3)

• stage where a lower mass star has a carbon core and two active fusion shells, outer hydrogen and inner helium

• equilibrium is never reaches, fusion rate periodically spikes in a series of thermal pulses

• with each spike, convection transports carbon from the core to the surface

Double shell burning ends with: (2)

• a pulse that ejects the hydrogen and helium into space as a planetary nebula

• the core left behind becomes a white dwarf

Fusion progresses no further in a low-mass star
because: (2)

• core temp never gets hot enough for fusion of heavier elements

• degeneracy pressure supports the now white dwarf star

Life Track of a Sun-like Star:

sub giant/red giant core → helium burning core → double shell burning core

Fate of Earth: (2)

• eventually, the suns luminosity will rise to 1000x its current level, making it too hot to sustain life on Earth

• the sun’s radius will grow to be near the current radius of earth’s orbit

CNO Cycle (2)

• high mass, main sequence stars begin fusing hydrogen to helium at a faster rate using carbon, nitrogen, and oxygen as catalysts

• the higher core temp allows hydrogen nuclei to overcome repulsion

Life Stages of High-Mass Stars: (2)

• 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)

Helium Capture

• higher core temperatures allow helium to
  fuse with heavier elements

Advanced Nuclear Burning

• core temperatures in stars with > 8M Sun allow fusion of elements as heavy as iron.

Multiple Shell Burning (2)

• advanced nuclear burning proceeds in a series of nested shells.

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

End of High-Mass star: (2)

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

• The core then suddenly collapses, creating a supernova explosion

Supernova Explosion (2)

• Core degeneracy pressure ceases because electrons combine with protons, making neutrons and neutrinos

• Neutrons collapse to the center, causing a supernova a forming a neutron star

Supernova remnant (2)

• energy released by the collapse of the core drives the star’s outer layers into space

• Ex. The Crab Nebula

Supernova 1987A

• The closest supernova in the last four centuries was seen in 1987

Intermediate mass stars:

• can fuse elements heavier than carbon but end as white dwarf

Low-Mass star summary: (5)

• Main Sequence: H fuses to He in core

• Red giant: H fuses to He in shell around core 

• Helium core burning: H fuses to C in core, H fuses to He in shell around core 

• Double shell burning: H and He both fuse in shell

• Nebula leaves behind white dwarf 

High-Mass star Summary: (5)

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.

Life stage reasons: (4)

• Core shrinks and heats until it’s hot enough for fusion.

• Nuclei with larger charge require higher temperature for fusion (ex. helium requires higher than hydrogen)

• Core thermostat is broken while core is not hot enough for fusion (shell burning).

• Core fusion can’t happen if degeneracy pressure prevents core from shrinking

Mass Exchange

• stars with close companions can exchange
  mass, altering the usual life tracks of stars

White dwarf (3)

• the remaining cores of dead stars.

• electron degeneracy pressure supports them against the crush of gravity

• they cool off and grow dimmer with time

White dwarf: Sizes (2)

• white dwarfs with same mass as Sun are
  about same size as Earth

• Higher-mass white dwarfs are smaller

The White Dwarf Limit (4)

• aka “Chandrasekhar limit”

• quantum mechanics say that electrons must move faster when condensed in a small space (degeneracy pressure)

• when a white dwarf’s mass approaches 1.4M Sun, it gets smaller and more dense, meaning its inner electrons move faster, approaching the speed of light

• since nothing can move faster than light, the Chandrasekhar limit is 1.4M sun

White dwarf in a close binary system:

• the smaller star in the system gains mass from its companion until the mass-losing star becomes a white dwarf

Accretion Disks (3)

• the mass falling towards a white dwarf from its binary companion has some angular momentum

• this matter therefore orbits the white dwarf in an accretion disk

• friction between the orbiting rings of matter in the disk transfers angular momentum outward, causing the disk to heat up and glow

Nova (3)

• when fusion begins suddenly and explosively, a nova forms

• the nova star system will temporarily appear much brighter, the explosion drives the accreted matter into space

• the temp of that matter eventually becomes hot enough for fusion

Massive star supernova

• when the iron core of a massive star reaches the white dwarf limit, it collapses into a neutron star and causes an explosion

White Dwarf Supernova

• carbon fusion suddenly begins when a white dwarf star in a close binary system reaches the limit, causing an explosion

Telling supernova apart: 

• using a light curve, showing how luminosity changes with time

Nova vs. Supernova (3)

• supernova are much more luminous

• Nova: hydrogen to helium fusion of the layer of accreted matter leaves a white dwarf behind

• Supernova: complete explosion of a white dwarf, leaving nothing behind
  

Massive star vs white dwarf (2)

• the light curves and spectra are different

• ex. exploding white dwarfs don’t have hydrogen absorption lines