# Astrophysics & Cosmology

Astrophysics is physics done at a distance. Everything we know about a star
or a galaxy arrives as light, so the subject begins by learning to read that
light — its brightness, its colour, its spectrum — and ends by reconstructing
the history of the whole universe from it.


_Figure 001 — The Hubble flow: galaxies recede with a velocity proportional to their
distance, the signature of an expanding universe.
_

The founding observation is that distant galaxies are almost all receding,
faster the farther away they lie. That linear velocity–distance
law, _v = H₀ d_, is what an expanding space looks like from
the inside, and running it backwards points to a hot, dense beginning.


Between the light we collect and the cosmos we infer sits the physics of
**stars**. A star is a self-gravitating ball of plasma held up
by the pressure of its own nuclear fire, and its whole life is the slow
contest between gravity pulling in and fusion pushing out.


_Figure 002 — A star's life traced on the H–R diagram: main sequence, giant branch, and
the collapse to a white dwarf.
_

Plot luminosity against temperature and stars fall into a pattern — the
Hertzsprung–Russell diagram. Most sit on the main sequence burning hydrogen;
when the core is spent they swell into giants and then shed their envelopes,
ending as white dwarfs, neutron stars, or black holes.


The densest of those remnants make the most extreme laboratories in nature.
When two of them orbit closely, general relativity bleeds the orbit of energy
as **gravitational waves**, and the pair spirals together toward
a merger that detectors on Earth can hear.


_Figure 003 — A compact binary inspiral: the orbit decays as it radiates gravitational
waves, ending in a merger.
_

Gravity also bends the paths of light. A foreground mass acts as a lens,
warping the images of whatever lies behind it into arcs and rings — a direct
test of relativity and one of the sharpest ways we have to weigh dark
matter we cannot otherwise see.


_Figure 004 — Gravitational lensing: a mass bends background light into arcs, a probe of
unseen matter.
_

Stitched together, these threads become a single narrative. From the Big
Bang and the cooling that released the cosmic microwave background, through
the first stars and the growth of galaxies, the universe has a datable
history — and modern surveys measure it with startling precision.


_Figure 005 — The cosmic timeline: Big Bang, the CMB at recombination, and the later
growth of structure.
_

The course follows Carroll & Ostlie, Ryden, and Maoz, grounding each
idea in the observations — Planck's map of the early universe, LIGO's
chirps — that turned cosmology into a measured science.


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## Contents

### 1. Orientation

1. [The Sun and the Life of Stars](/astrophysics-cosmology/orientation/the-sun-and-stars)
2. [Cataclysmic Events and the Final States of Stars](/astrophysics-cosmology/orientation/stellar-death-final-states)
3. [Galaxies, Cosmology, and the Evolving Universe](/astrophysics-cosmology/orientation/galaxies-and-cosmology)

### 2. Observational Foundations

1. [Magnitudes, Fluxes, and the Distance Modulus](/astrophysics-cosmology/observational-foundations/magnitudes-fluxes-and-the-distance-modulus)
2. [Stellar Spectra and Spectral Classification](/astrophysics-cosmology/observational-foundations/stellar-spectra-and-spectral-classification)
3. [Telescopes and Detectors Across the Spectrum](/astrophysics-cosmology/observational-foundations/telescopes-and-detectors-across-the-spectrum)
4. [The Cosmic Distance Ladder](/astrophysics-cosmology/observational-foundations/the-cosmic-distance-ladder)

### 3. Radiation and Matter

1. [Blackbody Radiation and Specific Intensity](/astrophysics-cosmology/radiation-and-matter/blackbody-radiation-and-specific-intensity)
2. [Radiative Transfer and the Transfer Equation](/astrophysics-cosmology/radiation-and-matter/radiative-transfer-and-the-transfer-equation)
3. [Spectral-Line Formation and Broadening](/astrophysics-cosmology/radiation-and-matter/spectral-line-formation-and-broadening)
4. [Opacity Sources and the Rosseland Mean](/astrophysics-cosmology/radiation-and-matter/opacity-and-the-rosseland-mean)

### 4. Stellar Structure

1. [Hydrostatic Equilibrium and the Virial Theorem](/astrophysics-cosmology/stellar-structure/hydrostatic-equilibrium-and-the-virial-theorem)
2. [The Equations of Stellar Structure](/astrophysics-cosmology/stellar-structure/the-equations-of-stellar-structure)
3. [The Equation of State and Polytropes](/astrophysics-cosmology/stellar-structure/the-equation-of-state-and-polytropes)
4. [The Standard Solar Model](/astrophysics-cosmology/stellar-structure/the-standard-solar-model)

### 5. Nuclear Astrophysics

1. [Thermonuclear Reaction Rates and the Gamow Peak](/astrophysics-cosmology/nuclear-astrophysics/thermonuclear-reaction-rates-and-the-gamow-peak)
2. [Hydrogen Burning: pp Chains and the CNO Cycle](/astrophysics-cosmology/nuclear-astrophysics/hydrogen-burning-pp-chains-and-cno)
3. [Helium Burning and the Triple-Alpha Process](/astrophysics-cosmology/nuclear-astrophysics/helium-burning-and-the-triple-alpha-process)
4. [Advanced Burning, the Iron Peak, and the s/r Processes](/astrophysics-cosmology/nuclear-astrophysics/advanced-burning-and-neutron-capture-nucleosynthesis)

### 6. The Interstellar Medium

1. [The Phases of the Interstellar Medium](/astrophysics-cosmology/ism-and-star-formation/phases-of-the-interstellar-medium)
2. [Molecular Clouds and Gravitational Collapse](/astrophysics-cosmology/ism-and-star-formation/molecular-clouds-and-gravitational-collapse)
3. [Protostars and Pre-Main-Sequence Evolution](/astrophysics-cosmology/ism-and-star-formation/protostars-and-the-pre-main-sequence)

### 7. Stellar Evolution

1. [The Main Sequence and Its Structure](/astrophysics-cosmology/stellar-evolution/the-main-sequence-and-its-structure)
2. [Post-Main-Sequence Evolution of Low-Mass Stars](/astrophysics-cosmology/stellar-evolution/post-main-sequence-low-mass-evolution)
3. [The Evolution of Massive Stars](/astrophysics-cosmology/stellar-evolution/the-evolution-of-massive-stars)
4. [Stellar Pulsation and the Instability Strip](/astrophysics-cosmology/stellar-evolution/stellar-pulsation-and-the-instability-strip)

### 8. Stellar Death and Compact Remnants

1. [White Dwarfs and the Chandrasekhar Limit](/astrophysics-cosmology/stellar-death-and-compact-remnants/white-dwarfs-and-the-chandrasekhar-limit)
2. [Core-Collapse Supernovae](/astrophysics-cosmology/stellar-death-and-compact-remnants/core-collapse-supernovae)
3. [Thermonuclear Supernovae](/astrophysics-cosmology/stellar-death-and-compact-remnants/thermonuclear-supernovae-type-ia)
4. [Neutron Stars and Pulsars](/astrophysics-cosmology/stellar-death-and-compact-remnants/neutron-stars-and-pulsars)
5. [Black Holes, Schwarzschild and Kerr](/astrophysics-cosmology/stellar-death-and-compact-remnants/black-holes-schwarzschild-and-kerr)

### 9. Binaries and Gravitational Waves

1. [Binary Systems and Mass Transfer](/astrophysics-cosmology/binaries-and-gravitational-waves/binary-systems-and-mass-transfer)
2. [Accreting Compact Objects](/astrophysics-cosmology/binaries-and-gravitational-waves/accreting-compact-objects)
3. [Gravitational Waves from Inspiraling Binaries](/astrophysics-cosmology/binaries-and-gravitational-waves/gravitational-waves-from-inspiraling-binaries)
4. [Multimessenger Astronomy and Gamma-Ray Bursts](/astrophysics-cosmology/binaries-and-gravitational-waves/multimessenger-astronomy-and-gamma-ray-bursts)

### 10. Galaxies and Dark Matter

1. [The Milky Way Galaxy](/astrophysics-cosmology/galaxies/the-milky-way)
2. [Galaxy Morphology and Classification](/astrophysics-cosmology/galaxies/galaxy-morphology-and-classification)
3. [Galaxy Rotation Curves and Dark Matter](/astrophysics-cosmology/galaxies/galaxy-rotation-curves-and-dark-matter)
4. [Active Galactic Nuclei](/astrophysics-cosmology/galaxies/active-galactic-nuclei-and-supermassive-black-holes)
5. [Galaxy Clusters and Large-Scale Structure](/astrophysics-cosmology/galaxies/galaxy-clusters-and-large-scale-structure)

### 11. Cosmic Expansion and Dynamics

1. [The Expanding Universe and Hubble's Law](/astrophysics-cosmology/cosmology-expansion-and-dynamics/the-expanding-universe-and-hubbles-law)
2. [The FRW Metric and Cosmological Redshift](/astrophysics-cosmology/cosmology-expansion-and-dynamics/the-frw-metric-and-cosmological-redshift)
3. [The Friedmann Equations and Cosmic Dynamics](/astrophysics-cosmology/cosmology-expansion-and-dynamics/the-friedmann-equations-and-cosmic-dynamics)
4. [Cosmological Models and Distances](/astrophysics-cosmology/cosmology-expansion-and-dynamics/cosmological-models-and-distances)
5. [Dark Energy and the Accelerating Universe](/astrophysics-cosmology/cosmology-expansion-and-dynamics/dark-energy-and-the-accelerating-universe)

### 12. The Hot Big Bang

1. [The Thermal History of the Universe](/astrophysics-cosmology/the-hot-big-bang/the-thermal-history-of-the-universe)
2. [Big Bang Nucleosynthesis](/astrophysics-cosmology/the-hot-big-bang/big-bang-nucleosynthesis)
3. [Recombination and the Cosmic Microwave Background](/astrophysics-cosmology/the-hot-big-bang/recombination-and-the-cosmic-microwave-background)
4. [CMB Anisotropies and Cosmological Parameters](/astrophysics-cosmology/the-hot-big-bang/cmb-anisotropies-and-cosmological-parameters)
5. [Cosmic Inflation](/astrophysics-cosmology/the-hot-big-bang/cosmic-inflation)
6. [Structure Formation and the Growth of Perturbations](/astrophysics-cosmology/the-hot-big-bang/structure-formation-and-the-growth-of-perturbations)
7. [Dark Matter, Dark Energy, and Open Questions](/astrophysics-cosmology/the-hot-big-bang/dark-matter-dark-energy-and-open-questions)