ASTR 201: Astronomy for Science Majors
Spring 2026
Welcome to ASTR 201
This is not a “tour of the cosmos” course. ASTR 201 is a quantitative introduction to astrophysics where you’ll learn to think like an astronomer: extracting physical insight from limited observations using mathematics and physics.
Astronomers can’t visit stars or hold galaxies in a lab. Instead, we measure light — its brightness, color, spectrum, and timing — and from these constrained observations, we infer masses, temperatures, distances, ages, and compositions. This course teaches you how we know what we know about the universe.
You’ll develop skills in dimensional analysis, scaling relations, and order-of-magnitude reasoning that apply far beyond astronomy. By the end, you’ll be able to estimate the mass of a black hole from orbital data, derive the temperature of a star from its color, and understand why stellar lifetimes span millions to trillions of years.
Instructor
Module 4 readings are posted. We are closing the course with galaxies, dark matter, expansion, the CMB, and the origin of the first nuclei.
📄 Midterm 2 materials are available on the Exams page for review, including the formula sheet, exam, and solutions.
What You’ll Explore
Module 1: Foundations We can’t visit stars or study them in a lab — so how do we learn anything real about them? This module builds the core toolkit: gravity and orbits (how motion encodes mass), plus the physics of light (how information survives a trip across space).
Module 2: Stellar Properties We start by learning to measure what looks unmeasurable: distances to stars, and spectra that reveal temperature, composition, and motion. Then a pattern snaps into focus: plot temperature against luminosity and stars fall into distinct regions. The H–R diagram isn’t just classification — it’s the first clue that stars change over time.
Module 3: Stellar Structure & Evolution What’s happening inside a star is a balancing act: gravity pulling in, pressure pushing out, and nuclear fusion rewriting the periodic table. Stars fuse hydrogen into helium and (in later stages) build heavier nuclei; their deaths spread those elements into space. The finale depends on mass — ending as a white dwarf, a neutron star, or a black hole.
Module 4: Galaxies & Cosmology We zoom out from stars to galaxies and the universe itself. Galaxies become evolving ecosystems of gas, dust, stars, and feedback; galaxy and cluster motions reveal unseen mass; redshift, deep fields, the cosmic microwave background, and light-element abundances reveal an expanding universe with a hot early history.
How to Succeed
- Show your reasoning. Steps, units, and assumptions matter more than final answers.
- Use feedback. Homework and grade memos prepare you for exams.
- Ask early. Office hours and the discussion board exist for a reason.
- Office hours: Fridays 11 am–1 pm (and by appointment) in Physics 239
- Canvas discussion: Post questions others might share
- SDSU Astronomy Help Room: Free tutoring (schedule on Canvas)