Astronomy Notes

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As a testament to the value of this material, numerous copies of this material (in various stages of revision) are found all over the web. Entering ``strobel astronomy'' in any of the internet search engines will bring up a lengthy list of some of the copies out there. If you find an old copy, please let the website manager know of the official Astronomy Notes website at

These documents were created by Nick Strobel for the introductory astronomy courses he teaches. They are copyrighted by Nick Strobel. This website is offered to the net as a resource in astronomy education. See my copyright notice for fair use practices. You can purchase a hardcopy version of this website! Select the Purchase Book link for ordering information. This website is continually updated but links to pages are STABLE.

Currently these notes cover: a brief overview of astronomy's place in the scientific endeavor, the philosophy of science and the scientific method, astronomy that can be done without a telescope, a history of astronomy and science, Newton's law of gravity and applications to orbits, Einstein's Relativity theories, electromagnetic radiation, telescopes, all the objects of the solar system, solar system formation, determining properties of the stars, the Sun, fusion reactions, stellar structure, stellar evolution, the interstellar medium, the structure of the Milky Way galaxy, extra-galactic astronomy including active galaxies and quasars, cosmology, and extra-terrestrial life. This site also has pages giving angular momentum examples, a quick mathematics review, improving study skills, astronomy tables, and astronomy terms.

Links to pages in this website are STABLE and won't break. Although, this site is not as flashy as others, the website structure is the most stable astronomy website anywhere on the web. Links to pages within this website from other external sites have worked since 2001 (that's "forever" in terms of the internet) while the content on the pages have continually updated. Pages are added to the structure for entirely new material and topics while "old" pages are updated, so links to the older pages will still work even as their content is updated. If you know of another website that has been around since 2001 (or longer) with a stable structure that has enabled links from external sites to pages within the website to still work as content has been updated, please let me know.

All of the line drawings were done with Create on my old NeXT machine or with Freehand on a wintel laptop and Macintosh or Adobe Illustrator at home. The line art images on the screen are GIF and PNG images. If you have comments about these notes, please email me.

Contents (chapters + descriptions):

  1. Astronomy as a Science and a Sense of Scale. I introduce astronomy's place in science, and give a sense of the size and time scales involved. Also discussion of the scientific method and how astrology is not a science and what makes astronomy a science.

    A separate section about the Science-Religion interface and interaction is available on this site. It is not part of the regular textbook. I take a middle road between the fundamentalists on both sides of the "debate"/dialogue.

  2. Method for Finding Scientific Truth. Borrowing from Pine's book ``Science and the Human Prospect'' I discuss the scientific method, correlations, problem of induction, positivism, levels of testimony, empiricism, models correspondence with reality, peer review process, and misinformation and disinformation.

    Pseudoscience vs. science article. Borrowing from Carl Sagan's "The Demon-Haunted World", I take up the subject of UFOs as alien spacecraft. This article is not part of the regular textbook. Other documents on Astronomy Notes about fake science and news: "The Seven Warning Signs of Bogus Science" and "Fake or Real? How to Self-Check the News and Get the Facts" (from NPR's All Tech Considered: original link).

  3. Astronomy Without a Telescope. I discuss the celestial sphere, motions of the Sun (solar and sidereal days, time zones, equation of time, and seasons), motions of the Moon (phases and eclipses, including my own pictures of some solar eclipses), and planetary motions. Update: additional diagrams and animations for describing phases of the moon.

  4. History of Astronomy. I focus on the rise of modern science in Europe, from the ancient Greeks to Kepler.

  5. Newton's Law of Gravity. Newton's laws of motion and his law of gravity are discussed. Applications of those laws (esp. gravity) are covered (e.g., measuring the masses of planets and stars, orbital motion, interplanetary trips, tides, etc.). Update: additional diagram for orbits section.

  6. Einstein's Relativity. I discuss Einstein's Special Relativity and General Relativity theories. The concepts of spacetime and gravity as a warping of spacetime are introduced along with observational proofs of his theories, including the search for gravity waves with LIGO. Update: LIGO/Virgo discoveries.

  7. Electromagnetic Radiation (Light). General properties of light, definition of frequency, spectrum, temperature. Light production: Continuous (thermal) spectra, emission lines, absorption lines and the Bohr model for the atom. Doppler Effect and why spectral lines must be used to measure the doppler shifts. Updates: added "How do you do that?" box on figuring energies of photons for hops in an atom. Also links to interactives about types of spectra.

  8. Telescopes. Covers refractors, reflectors, radio telescopes, light-gathering power, resolving power, interferometers, magnification, and atmospheric distortion such as seeing, reddening, and extinction. Also a section with tips on buying a telescope. Updates: added section about buying personal telescope and updated material about new large research telescopes in the near future.

  9. Planetary Science. This chapter is an introduction to planetary science. I discuss the techniques astronomers use to find out about the planets, their atmospheres (what determines if an atmosphere sticks around; behavior of gases; what determines the surface temperature; atmosphere layers; the transport of energy; effects of clouds, mountains, and oceans; weather vs. climate and climate change agents with feedbacks; and appearance), their magnetic fields (the magnetic dynamo theory), and their interiors including the geological forces at work reshaping their surfaces. In a separate section I focus on a comparison between the atmospheres of Earth, Venus, and Mars and why they are now so radically different from each other (greenhouse effect, carbon cycle, runaway refrigerator, runaway greenhouse, etc.) Mars discussion now includes proofs for liquid water in past and sub-surface water ice. The Earth discussion now includes the role of plate tectonics in the carbon cycle, evidence for human contribution to the atmospheric carbon dioxide and to the observed global temperature rise. There are links to two flowcharts: a Earth-Venus-Mars comparison and a flowchart of the calculations involved in determining if an atmosphere sticks around for billions of years. I end the chapter with a discussion of the major moons in the solar system and ring systems. Updates: weather vs. climate section, magnetic fields, earthquake resources, climate change discussion resources, jovian moons, rings, Mars, and fixing broken links to external websites (never-ending task because other websites do not have stable structures).

    Beautiful Planet photo album of nature photography has imagery of mountains, lakes, streams, waterfalls, large trees, flowers, aurorae, other landscape images and some images of insects and frogs. Most images are from the western United States but some are also from eastern Australia and the aurorae are from Fairbanks, Alaska. National park/monument photo sets include: Crater Lake, Bryce Canyon, Grand Canyon, Zion, Grand Teton, Yellowstone, Devils Tower, Glacier, Colorado, Arches, North Cascades, and Death Valley. There are also photo sets for Canyon X of the Antelope Canyon complex and Oak Creek Canyon. The rest of the album are from various beautiful places in the western United States and eastern Australia.

    Answers to Global Warming Skeptics is a separate section about the climate change debate going on among the general public. It is not part of the regular textbook. Also, is a short "How I Know" PDF document with embedded links explaining why I accept the conclusion that Earth's climate is changing and that humans play a roleā€”just one sheet of paper needed to print. After a wet winter in 2016-17, the California Water Future article explains why water conservation is still needed.

  10. Solar System Fluff. The basics of meteorites, asteroids, and comets are introduced and how they can tell us the ``when'' and the ``how'' of the formation of the solar system. At the end is an exploration of the other planetary systems. Updates: Rosetta mission to Comet 67P/Churyumov-Gerasimenko, New Horizons at Pluto, and exoplanets.

  11. Determining Star Properties. Notes for the properties of stars and how we determine them. Things like distances to stars, their masses, radii, composition and speeds. Also HR diagram, spectral types, and spectroscopic parallax. The dangers of selection effects and biased samples are also discussed with the application of finding what a typical star is like. Update: tweak to Inverse Square Law section.

  12. The Sun and Stellar Structure. This chapter covers: The Sun, interiors of stars, and nuclear fusion, neutrinos, the solar neutrino problem, and helioseismology. The concept of hydrostatic equilibrium is used to explain the mass-luminosity relation and the reason for the mass cut-off at the high and low ends. Updates: 2017 solar eclipse pictures, fixed broken links to external websites and added additional resources.

  13. Lives and Deaths of Stars. This chapter covers: stellar evolution (all nine stages) and stellar remnants (white dwarfs, neutron stars, black holes). Updates: additional material and diagrams in stellar nucleosynthesis section, LIGO/Virgo results about black holes, and fixed broken links to external websites.

  14. The Interstellar Medium and the Milky Way. This chapter covers: the dust and gas between the stars and how we use the 21-cm line radiation to map the Galaxy. Also, the structure of the Milky Way Galaxy, our place in it, and how we determine these things. The rotation curve and the existence of the dark matter halo, stellar populations, and the galactic center are also discussed. Updates: fixed broken links to external websites and updated content in Cepheids and central supermassive black hole sections.

  15. Other Galaxies and Active Galaxies. This chapter covers: the characteristics of other normal galaxies, active galaxies, and finding distances to other galaxies (this includes the distance-scale ladder). Also, large-scale structure is covered (galaxy clusters and collisions and superclusters). Updates: fixed broken links to external websites and updated material about dark matter in galaxies, origins of galaxies, galaxy collisions & mergers, large scale structure (superclusters), supercomputer simulations of galaxy motions & evolution, imaging M87's supermassive black hole with the Event Horizon Telescope, and the "Steps to the Hubble Constant" page.

  16. Cosmology. This chapter covers cosmology: the study of the nature, origin, and evolution of the universe as a whole. The distance-scale ladder topic is dealt with in the Steps to the Hubble Constant document. I discuss Olbers' Paradox, the cosmic microwave background radiation, the fate of the universe (open or closed), dark matter, dark energy, inflation, and the cosmological constant. Updates: fixed broken links to external websites and updated material about cosmic microwave background radiation from Planck mission, observations of first galaxies, dark matter, temperature power spectrum (also added graph from 2018 final Planck data release), BICEP2 discussion, dark energy, and tension with Hubble Constant measurements.

  17. Life Beyond the Earth. This chapter covers: life zones (habitable zones), types of stars to focus on in the search for suitable planets, characteristics of life, evolution by natural selection, working definitions of life, the kind of planet where we think life would likely arise, bio-markers in exoplanet spectra, and finally the frequencies we use in the Search for Extra-Terrestrial Intelligence (S.E.T.I.). Updates: exoplanet discoveries, habitable zones (including new figure about optimistic and conservative boundaries), life in liquid methane, biomarkers throughout a planet's history, Drake Equation with Kepler results, added section about Fermi's Question (Paradox), and fixed broken links to external websites.


  1. Angular Momentum in Astronomy. I define angular momentum and give several examples of angular momentum in astronomy: Kepler's second law of orbital motion, Earth-Moon system, rapidly spinning neutron stars, accretion disk in a binary system, and a collapsing galactic cloud.

  2. Quick Mathematics Review. Here's a quick run through of some basic mathematics: working with fractions and percentages, exponents, roots, powers of ten, working with really BIG or really small numbers---scientific notation and the metric system. I assume that the reader has had this stuff before, so the quick run through will be sufficient to jog the dormant memory.

  3. Tables. Astronomy constants, physical constants, planets (orbital properties, physical characteristics, atmospheres), 100 nearest stars, and 100 brightest stars as seen from the Earth.

  4. Glossary. Definitions of astronomy terms used in this website.

  5. Bakersfield Night Sky. A bimonthly (twice per month) look at the latest astronomy news and what's happening up in the sky that you can see without a telescope. This is not in the hardcopy and not even on the Astronomy Notes site BUT it is a major part of my outreach to the world on astronomy topics that it deserves a link here on the Astronomy Notes homepage. Often, a recent astronomy result is discussed on the Bakersfield Night Sky site before it makes it into the appropriate chapter in the Astronomy Notes site.

From the Student Guide:

  1. Study Skills: Great Expectations, Textbook ``study reading'', homework, exams, and writing (not typing) lecture notes. College is not high school---greater expectations of the student! Also, some tips to improve your study skills so that you study more efficiently and take exams with better results. Although the homework and exam tips are addressed to my own students, most of these tips will also apply to students at other schools. New page added about why it is better to WRITE your lecture notes instead of type them.
  2. Astronomy Careers. A brief overview of a career in astronomy research. It covers the likes and attitudes of research astronomers, need for formal writing ability, where astronomers work, and expected pay scale. Brief enough to fit on a single sheet of paper, back-to-back.
  3. Why College? Education Pays! Data from the Bureau of Labor Statistics on median salary and unemployment rates for different levels of educational attainment (associate degree, bachelors degree, masters, etc.). Updated annually once all of the income tax forms have been compiled.
Astronomy beyond this website:

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This page last updated: July 19, 2021
(note individual pages in this website may have been updated after this date)

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel