Physics of the Cosmos

Real astrophysics and cosmology for talented high schoolers.

Online from anywhere · or in-person in Princeton, NJ

A World Through the Lens of Astrophysics and Cosmology

This course explores the universe at its grandest scales, one of the four Modern Physics electives in the Physics Lyceum: High School curriculum. It follows the Deep Physicsmodel that runs through the whole SoTS Physics Lyceum.

The course runs on theory, problem-solving sessions, and homework, taking a physics-first approach to understanding astronomical phenomena.

1. Scales, and the Inverse-Square Sky

Where every inference starts. Cosmic distances, times, masses and densities, with the units built for them: astronomical unit, light-year, parsec, solar mass, solar luminosity. Angular size. Flux, luminosity, and the inverse-square law that separates how bright a thing looks from how bright it is. Order-of-magnitude estimation, used as a working tool rather than an occasional exercise.

2. Light: Our Principal Messenger

Almost everything we know arrived as photons. The electromagnetic spectrum and photon energy. Thermal radiation: the blackbody spectrum, Wien’s displacement law, the Stefan-Boltzmann law, and the temperature they hand you. Atomic spectra in emission and in absorption, and the composition they reveal. Spectral classification. The Doppler shift and radial velocity.

3. How Astronomers Actually Observe

The instrument is part of the physics. Telescopes: aperture and light gathering, diffraction and angular resolution, why radio telescopes are enormous and why some telescopes go to space. Detectors and data: pixels, exposure, photon statistics, signal-to-noise. Reading a real image or spectrum, and telling measurement, noise, and interpretation apart.

4. Gravity, Orbits, and Masses

How to weigh something you cannot touch. Newtonian gravity, circular motion, and Kepler’s laws. Masses measured from orbits, including binary stars. Escape velocity and gravitational potential energy. The virial theorem, and the characteristic dynamical times that come with it.

5. Stars: Structure, Energy, and the HR Diagram

Distance first, then everything else. The cosmic distance ladder: parallax, standard candles such as Cepheid variables and Type Ia supernovae, and the distance modulus. Colour and surface temperature. The relation between luminosity, radius and temperature. The Hertzsprung-Russell diagram. Hydrostatic equilibrium: pressure against gravity, and the central pressure and temperature it implies for the Sun. Energy transport by radiation and convection. Nuclear binding energy, the proton-proton chain and the CNO cycle, and the lifetime a star’s fuel supply buys it.

6. Star Formation, Stellar Death, and Extreme Physics

Where the elements come from. Interstellar gas and dust, molecular clouds, extinction and reddening. Star formation as gravity beating pressure, and the collapse that follows. How mass decides a star’s fate. Cluster ages from the main-sequence turnoff. Nucleosynthesis and supernovae. White dwarfs and electron degeneracy. Neutron stars and pulsars. Black holes, the Schwarzschild radius, and accretion.

7. The Milky Way, Galaxies, and Exoplanets

Structure on the largest scales, and planets around other stars. The Milky Way: disk, bulge, halo, stellar populations. Galactic rotation, and the evidence for dark matter. Spiral, elliptical and irregular galaxies, clusters, and large-scale structure. Quasars, active galactic nuclei, supermassive black holes and their jets. Exoplanets by radial velocity and by transit, and the selection effects that come with each method. Comparative planetology alongside them: what makes a planet rocky or gaseous, and why a world holds on to an atmosphere or loses it.

8. Cosmology, and the New Astrophysics

How we know the universe has a history. Redshift and Hubble’s law. The scale factor, the Friedmann equation qualitatively, and the age of the universe. The thermal history: early hot plasma, primordial nucleosynthesis, recombination, and the cosmic microwave background. Dark matter, dark energy, cosmic inflation, and structure formation, with a line drawn between what is observed and what is assumed. Multi-messenger astronomy: neutrinos, gravitational waves, cosmic rays, and pulsars used as clocks.

The specific topics, and the depth given to each, may shift depending on class priorities and the dynamics of the cohort.

How Astrophysics Is Taught in This Course

The course is physics-first rather than a tour of astronomical objects. Every topic starts from something that can be observed from Earth and asks what physics lets us infer from it. Order-of-magnitude estimation is a standing method rather than occasional enrichment, and real data appears roughly every other week.

Texts and Requirements

Texts. The course relies on several university astrophysics texts rather than a single primary text, with Arnab Rai Choudhuri's Astrophysics for Physicists (Cambridge University Press) as the main theoretical backbone.

Prerequisites. Mechanics of Motion or an equivalent course elsewhere, plus solid algebra and trigonometry. Basic energy, momentum, and waves, and familiarity with atoms, are helpful rather than required. Special Relativity is not a prerequisite: the relativistic topics this course reaches, compact objects and cosmic expansion, are treated descriptively.

No calculus. The course is quantitatively serious but algebraically accessible. A few relations are written the way physicists write them, a star’s structure as dP/dr = −ρGM(r)/r² and the expanding universe as a scale factor a(t), and what those say in words is explained where they appear. A student comfortable with algebra can follow every one of them.

Schedule, Tuition, and Enrollment

Schedule, tuition, and enrollment
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