Stellar Birth and Life

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Stellar Birth and Life

• 2950
• Dr Bryce

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Class notices

• Remember homework is due every Friday at 5pm
• Second class exam on October 22nd

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C
B
Which star is the hottest?
D
Luminosity
A
Temperature
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C
B
Which star is the most luminous?
D
Luminosity
A
Temperature
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C
B
Which star has the largest radius?
D
Luminosity
A
Temperature
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Stellar Birth

• Star formation
• Where do stars form?
• Why do stars?
• How long does it take?

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Orion Nebula
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Star-Forming Clouds

• Stars form in dark clouds of dusty gas in
  interstellar space
• The gas between the stars is called the
  interstellar medium

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The Interstellar Medium

• The gas and dust between stars
• Space is not empty
• We will return to this topic when we discuss the
  Milky Way

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Composition of Clouds

• We can determine the composition of interstellar
  gas from its absorption lines in the spectra of
  stars
• 70 H, 28 He, 2 heavier elements in our region
  of Milky Way

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Molecular Clouds

• Most of the matter in star-forming clouds is in
  the form of molecules (H2, CO,)
• These molecular clouds have a temperature of
  10-30 K and a density of about 300 molecules per
  cubic cm

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Molecular Clouds

• Most of what we know about molecular clouds comes
  from observing the emission lines of carbon
  monoxide (CO)

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Interstellar Dust

• Tiny solid particles of interstellar dust block
  our view of stars on the other side of a cloud
• Particles are elements like C, O, Si, and Fe

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Interstellar Reddening

• Stars viewed through the edges of the cloud look
  redder because dust blocks (shorter-wavelength)
  blue light more effectively than
  (longer-wavelength) red light

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Observing Newborn Stars

• Visible light from a newborn star is often
  trapped within the dark, dusty gas clouds where
  the star formed

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Observing Newborn Stars

• Observing the infrared light from a cloud can
  reveal the newborn star embedded inside it

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Glowing Dust Grains

• Dust grains that absorb visible light heat up and
  emit infrared light of even longer wavelength

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Glowing Dust Grains

• Long-wavelength infrared light is brightest from
  regions where many stars are currently forming

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Gravity versus Pressure

• Gravity can create stars only if it can overcome
  the force of thermal pressure in a cloud
• Emission lines from molecules in a cloud can
  prevent a pressure buildup by converting thermal
  energy into infrared and radio photons

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Mass of a Star-Forming Cloud

• A typical molecular cloud (T 30 K, n 300
  particles/cm3) must contain at least a few
  hundred solar masses for gravity to overcome
  pressure

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Resistance to Gravity

• A cloud must have even more mass to begin
  contracting if there are additional forces
  opposing gravity
• Both magnetic fields and turbulent gas motions
  increase resistance to gravity

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Fragmentation of a Cloud

• Gravity within a contracting gas cloud becomes
  stronger as the gas becomes denser
• Gravity can therefore overcome pressure in
  smaller pieces of the cloud, causing it to break
  apart into multiple fragments, each of which may
  go on to form a star

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Fragmentation of a Cloud

• This simulation begins with a turbulent cloud
  containing 50 solar masses of gas

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Fragmentation of a Cloud

• The random motions of different sections of the
  cloud cause it to become lumpy

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Fragmentation of a Cloud

• Each lump of the cloud in which gravity can
  overcome pressure can go on to become a star
• A large cloud can make a whole cluster of stars

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The First Stars

• Elements like carbon and oxygen had not yet been
  made when the first stars formed
• Without CO molecules to provide cooling, the
  clouds that formed the first stars had to be
  considerably warmer than todays molecular clouds
• The first stars must therefore have been more
  massive than most of todays stars, for gravity
  to overcome pressure

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Simulation of the First Star

• Simulations of early star formation suggest the
  first molecular clouds never cooled below 100 K,
  making stars of 100MSun

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Trapping of Thermal Energy

• As contraction packs the molecules and dust
  particles of a cloud fragment closer together, it
  becomes harder for infrared and radio photons to
  escape
• Thermal energy then begins to build up inside,
  increasing the internal pressure
• Contraction slows down, and the center of the
  cloud fragment becomes a protostar

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Growth of a Protostar

• Matter from the cloud continues to fall onto the
  protostar until either the protostar or a
  neighboring star blows the surrounding gas away

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Conservation of Angular Momentum

• The rotation speed of the cloud from which a star
  forms increases as the cloud contracts

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Flattening

• Collisions between particles in the cloud cause
  it to flatten into a disk

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Formation of Jets

• Rotation also causes jets of matter to shoot out
  along the rotation axis

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Jets are observed coming from the centers of
disks around protostars
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From Protostar to Main Sequence

• Protostar looks starlike after the surrounding
  gas is blown away, but its thermal energy comes
  from gravitational contraction, not fusion
• Contraction must continue until the core becomes
  hot enough for nuclear fusion
• Contraction stops when the energy released by
  core fusion balances energy radiated from the
  surfacethe star is now a main-sequence star

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Birth Stages on a Life Track

• Life track illustrates stars surface temperature
  and luminosity at different moments in time

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Assembly of a Protostar

• Luminosity and temperature grow as matter
  collects into a protostar

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Convective Contraction

• Surface temperature remains near 3,000 K while
  convection is main energy transport mechanism

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Radiative Contraction
Luminosity remains nearly constant during late
stages of contraction, while radiation is
transporting energy through star
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Self-Sustaining Fusion

• Core temperature continues to rise until star
  arrives on the main sequence

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Life Tracks for Different Masses

• Models show that Sun required about 30 million
  years to go from protostar to main sequence
• Higher-mass stars form faster
• Lower-mass stars form more slowly

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Stellar Mass and 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

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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

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Broken Thermostat

• As the core contracts, H begins fusing to He in a
  shell around the core
• Luminosity increases because the core thermostat
  is brokenthe increasing fusion rate in the shell
  does not stop the core from contracting

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• Helium fusion does not begin right away because
  it requires higher temperatures than hydrogen
  fusionlarger charge leads to greater repulsion
• Fusion of two helium nuclei doesnt work, so
  helium fusion must combine three He nuclei to
  make carbon

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Helium Flash

• Thermostat is broken in low-mass red giant
  because degeneracy pressure supports core
• Core temperature rises rapidly when helium fusion
  begins
• Helium fusion rate skyrockets until thermal
  pressure takes over and expands core again

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Helium burning stars neither shrink nor grow
because core thermostat is temporarily fixed.
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Life Track after Helium Flash

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

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Life Track after Helium Flash

• Observations of star clusters agree with those
  models
• Helium-burning stars are found in a horizontal
  branch on the H-R diagram

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Double Shell Burning

• After core helium fusion stops, He fuses into
  carbon in a shell around the carbon core, and H
  fuses to He in a shell around the helium layer
• This double-shell burning stage never reaches
  equilibriumfusion rate periodically spikes
  upward in a series of thermal pulses
• With each spike, convection dredges carbon up
  from core and transports it to surface

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Low temperature stars
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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

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Planetary Nebulae
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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 He
  fuses to C to make oxygen)
• Degeneracy pressure supports the white dwarf
  against gravity

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Life Track of a Sun-Like Star
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Earths Fate

• Suns luminosity will rise to 1,000 times its
  current leveltoo hot for life on Earth

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Earths Fate

• Suns radius will grow to near current radius of
  Earths orbit

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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 H nuclei to
  overcome greater repulsion

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Life Stages of High-Mass Stars

• 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)

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Helium Capture

• High core temperatures allow helium to fuse with
  heavier elements

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Evidence for helium capture Higher abundances
of elements with even numbers of protons
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Advanced Nuclear Burning

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

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Advanced reactions in stars make elements like
Si, S, Ca, Fe
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Multiple Shell Burning

• Advanced nuclear burning proceeds in a series of
  nested shells

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Iron is dead end for fusion because nuclear
reactions involving iron do not release
energy (Fe has lowest mass per nuclear particle)
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Supernova Explosion

• Core degeneracy pressure goes away because
  electrons combine with protons, making neutrons
  and neutrinos
• Neutrons collapse to the center, forming a
  neutron star

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Supernova Remnant

• Energy released by collapse of core drives outer
  layers into space
• The Crab Nebula is the remnant of the supernova
  seen in A.D. 1054

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Supernova 1987A

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

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Rings around Supernova 1987A

• The supernovas flash of light caused rings of
  gas around the supernova to glow

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Impact of Debris with Rings

• More recent observations are showing the inner
  ring light up as debris crashes into it

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Role of Mass

• A stars mass determines its entire life story
  because it determines its core temperature
• High-mass stars with 8MSun have short lives,
  eventually becoming hot enough to make iron, and
  end in supernova explosions
• Low-mass stars with 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

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• Low-Mass Star Summary
• Main Sequence H fuses to He in core
• Red Giant H fuses to He in shell around He core
• Helium Core Burning
• He fuses to C in core while H fuses to He in
  shell
• Double Shell Burning
• H and He both fuse in shells
• 5. Planetary Nebula leaves white dwarf behind

Not to scale!
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• Reasons for Life Stages
• Core shrinks and heats until its 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 cant happen if degeneracy pressure
  keeps core from shrinking

Not to scale!
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• Life Stages of High-Mass Star
• Main Sequence H fuses to He in core
• Red Supergiant H fuses to He in shell around He
  core
• Helium Core Burning
• He fuses to C in core while H fuses to He in
  shell
• Multiple Shell Burning
• Many elements fuse in shells
• 5. Supernova leaves neutron star behind

Not to scale!
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Thought Question

• The binary star Algol consists of a 3.7 MSun main
  sequence star and a 0.8 MSun subgiant star.
• Whats strange about this pairing?
• How did it come about?

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Stars in Algol are close enough that matter can
flow from subgiant onto main-sequence star
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• Star that is now a subgiant was originally more
  massive
• As it reached the end of its life and started to
  grow, it began to transfer mass to its companion
  (mass exchange)
• Now the companion star is more massive