Stellar Astrophysics:
Course Review
Noah Kurinsky
Astronomy 15, Fall 2013
Outline
• First Half Topics – Quick Review
• Recent Topics
• Review Session Discussion
• Final: December 13th, comprehensive
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Previously, On Astrophysics Review…
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Celestial Mechanics
Astronomical Distance
Temperature and Radiation
Spectral Lines
Binary Systems/Kinematics
Stellar Interiors
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Highlights: Celestial Mechanics
• Newton’s Law of
Gravitation
– Inverse square nature
• Kepler’s Laws
– Elliptical Orbits
– Specifically, the third law
𝑃2 ∝ 𝑎3
• Virial Theorem:
𝑈 = −2 𝐾
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Highlights: Astronomical Distance
• Distance Modulus
• Astronomical Distance
Ladder
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–
–
–
–
Parallax
Spectra
MS Fitting
Variable Stars
Type 1a SN
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Highlights: Temperature and Radiation
• Blackbody Radiation and the Planck function
– Wien’s displacement law: 𝜆 ∝ 1/𝑇
– Stefan-Boltzmann equation: 𝐿 ∝ 𝑅2 𝑇 4
• Spectral Color
• Energy State v Temperature:
– Boltzmann’s Equation
– Maxwell-Boltzmann Distribution
– Saha Equation
• Five temperatures in Astrophysics
– Effective, Excitation, Ionization, Color, Kinetic
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Highlights: Spectral Types
• Defining Main Sequence
Stars
• Spectral Types: OBAFGKM
– Over Beers, All Friends Get
Kind Mumbles
– Oh Be A Fine
Girl/Guy/Giraffe, Kiss Me
– Oh Blimey, A Fox Got Ken’s
Milkshake
• Spectral Types on the HR
Diagram
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Highlights: Spectral Lines
• Spectral Line Broadening:
– Thermal, surface gravity,
velocity dispersion
• Spectral line versus
atomic transition
– 𝐸 = ℎ𝜈
• Spectral Lines and
Composition
– Most lines can only be
produced by one transition
in one element
– Example: Balmer Lines in
Hydrogen
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Highlights: Binary Systems
• Relative masses of
visual binaries
• Importance and
relevance of binary
systems to the galaxy
• Kinematic properties of
binaries
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Highlights: Stellar Interiors
• Stellar Equations of State
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–
–
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Ideal Gas Law
Hydrostatic Equilibrium
Heat Transfer Within Stars
Radiation Pressure
• How does a star support itself?
• Energy Transport in a Star
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–
–
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Convection
Conduction
Radiation
Neutrinos
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Recently Covered
• Stellar Nucleosynthesis
• Star Formation and the ISM
• Main Sequence Evolution
– Isochrones
– Mass relations
• Post Main Sequence Evolutionary Stages
• Stellar End Stages
– White Dwarves and Planetary Nebulae
– Neutron Stars and Pulsars
– Supernovae of Various Types
• Close Binaries and Planetary Systems
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Nucleosynthesis
• Primordial – elements fused from hydrogen in the
first minutes of the universe, including D, He, Li,
Be, B
• Stellar – H burning on the main sequence, He, Li,
and C burning in the giant phase
• Explosive – Elements heavier than Fe56 formed
during energetic events such as supernovae
• Cosmic Rays – high energy bombardment of
nuclei by free neutrons creates heavier elements
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Hydrogen Burning Cycles
• PP Chain
– 4H -> 1 He
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Hydrogen Burning Cycles
• PP Chain
– 4H -> 1 He
– I, II, and III
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Hydrogen Burning Cycles
• PP Chain
– 4H -> 1 He
– I, II, and III
• CNO Cycle
– Many different
pathways, this is just the
cold CNO I pathway
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Other Burning Stages
• Helium Burning
– Triple Alpha Process
– Alpha “Ladder”
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Other Burning Stages
• Helium Burning
– Triple Alpha Process
– Alpha “Ladder”
• Heavy Elements
(“Metals”)
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–
–
–
–
Li - > 2 He
C ->
Neon ->
O -> S, Si, P
Si - > Fe, Ni
• Slow Neutron Capture
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The Iron Limit
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Explosive Nucleosynthesis
• Neutron Capture
– R processes
• Proton capture
• Photo-disintegration
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The Cycle of Stellar Evolution
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The Space Between Stars
• The vacuum of space is filled with an
interstellar medium (ISM) composed of gas
and dust
– Primarily hydrogen (HI and H2), but cold enough
for complex molecules (e.g. PAHs) to form
• We observe the ISM mainly through extinction
• We can trace its distribution through the
distribution of hydrogen, which produces the
hyperfine 21-cm line
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ISM Extinction
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Where in the ISM do Stars Form?
• Stars form within Giant
Molecular Clouds
(GMCs)
– 𝑇 ∼ 15𝐾
– 𝑀 ∼ 105 − 106 𝑀⊙
– Primarily atomic
hydrogen
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Cloud Collapse
• From our stellar models, we know that collapse
will occur when 2𝐾 < 𝑈
• This allows us to derive a critical mass or radius at
which a given cloud will collapse to a dense
protostar, called the Jeans mass and length
𝑀𝐽 ∝
𝑅𝐽 ∝
3 −1
𝑇 2 𝜌0 2
1
𝑇 2
𝜌0
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Cloud Collapse
• These criteria neglect rotation and magnetic fields, as
well as external pressures; stars can form
spontaneously through loss of equilibrium, or can be
forced to form from ISM shocks
• Once collapse is set into motion, density and
temperature also determine fragmentation and free
fall timescale
1
−2
𝑡𝑓𝑓 ∝ 𝜌0
• Fragmentation is due to the fact that the entire cloud
may exceed the Jeans limit, but it cannot form one star
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Pre-Main-Sequence Evolution
• GMC collapses into a
protostar
• Protostar follows the
Hayashi track in the HR
diagram, prior to beginning
any nucleosynthesis
• Stage achieves early
burning of deuterium,
which is not abundant
enough to significantly
affect evolution
• MS begins with stable
hydrogen burning
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Main Sequence Stars
• A main sequence star burns primarily
Hydrogen
• Vast majority of a star’s lifetime is spent on
the main sequence
• Because the same few processes control all
stars, their mass and luminosity, and therefore
HR diagram position, are highly correlated
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HR Diagram Trends
• Main Sequence Lifetime
• Mass Luminosity
Relation
• Color-Temperature
Relation
• Morphologies along
main sequence
• Cluster Aging
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Aging Clusters w/ Isochrones
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Post Main Sequence Tracks
• Main sequence position
versus initial mass
• Horizontal giant tracks
• Notice the dredge up
phases and burning
phases
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Red Giants
• A red giant forms as the
hydrogen burning in the
core of a main
sequence star stars to
decrease; the core
contracts, releasing
radiation which
expands the envelope
• Eventually, the core can
sustain helium burning
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Low Mass Post Main Sequence
• Low mass stars may
never achieve more
than minor helium
burning, if that
• Minor TP-AGB phase
• Only one or two dredge
up phases
• Falls to White Dwarf
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Intermediate Mass Post Main
Sequence
• Intermediate mass stars
can sustain more
Helium burning, but
may have little more
fusion in their cores
• Also form a planetary
nebula and white dwarf,
may blow away the PN
if large enough
• More drastic AGB phase
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Stellar Pulsation: Variable Stars
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Cepheid variables
RR Lyrae
Novae
AGB Stars
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Stellar Remnants
• When a star dies, it forms a different product
depending on its mass
– Very Low mass: Brown Dwarf or Dwarf Star
– Low-Intermediate Mass: White Dwarf, Planetary
Nebula
– Intermediate-High Mass: Neutron star, small
supernova
– High-Very High Mass: Black Hole, massive corecollapse supernova
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Planetary Nebulae
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White Dwarves
• Chandrasekhar Limit:
1.44 Solar Masses
• Supported by electron
degeneracy pressure
• Mainly composed of
Carbon and Oxygen
• Very high surface
temperature due to
compact size
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Neutron Stars
• More massive than a white
dwarf, composed only of
neutrons and supported by
neutron degeneracy
pressure
• Very high rates of rotation,
and strong magnetic fields
– Pulsars
• Usually situated within a
supernova remnant
• Lower mass limit for
formation around 3 solar
masses
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Black Holes
• Immensely compact,
internal state unclear
• Event horizon
(Schwarzschild radius)
beyond which light may
not escape
• Can exist at any mass,
provided enough
external pressure
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Supernovae
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GRBs and Cosmic Rays
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Other Topics
• Close Binary Systems
– Relation to Type 1a SN
• Planetary Systems
– Detecting Exoplanets
• Relation of various evolutionary stages to each
other
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Review Session Logistics
• Noah Proposing Tuesday, around 3pm
• Most likely in STC, up for debate
• Will be interactive, may order food if everyone
is interested
• Please come; it is only worthwhile if you are
all there, and I have to prepare for it
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Good Luck!
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