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 now purchase a hardcopy version of this website! Select the Purchase Book link for ordering information.

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.

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


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  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.

    A new separate section about the teaching of Intelligent Design in our public schools is now available on this site. It is not part of the regular textbook.

  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.

  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), and planetary motions.

  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.).

  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.

  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.

  8. Telescopes. Covers refractors, reflectors, radio telescopes, light-gathering power, resolving power, interferometers, magnification, and atmospheric distortion such as seeing, reddening, and extinction.

  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, the transport of energy, and appearance), their magnetic fields (the magnetic dynamo theory), and their interiors. 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, etc.) There are links to three graphical descriptions of the concepts covered: the Earth-Venus-Mars comparison (a flowchart), the greenhouse effect and the UV dissociation of water, 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.

    Two new additions: "Earth beautiful" scenery photography section and a zoom-in movie to the Mars Exploration Rover locations. Yosemite pictures were recently added to the "Earth beautiful" albums. Also, the Titan (Saturn's large moon) section has been updated with Cassini-Huygens results.

  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. The other planetary systems has been greatly expanded and updated!

  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.

    Two new illustrations and accompanying text added to illustrate how luminosity, temperature, and size depend on each other. Also, a couple of points in the star masses section have been clarified (see the "caution" notes).

  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.

  13. Lives and Deaths of Stars. This chapter covers: stellar evolution (all nine stages) and stellar remnants (white dwarfs, neutron stars, black holes).

  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.

  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).

  16. Cosmology. This chapter covers cosmology: the study of the nature, origin, and evolution of the universe as a whole. The distance-scale 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, inflation, and the cosmological constant. This chapter has been greatly updated!

  17. Life Beyond the Earth. This chapter covers: lifezones, types of stars to focus on in the search for suitable planets, basic definitions of life, the kind of planet where we think life would likely arise, and finally the frequencies we use in the Search for Extra-Terrestrial Intelligence (S.E.T.I.).


  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.

From the Student Guide:

  1. Study Skills: Great Expectations, Textbook ``study reading'', homework and exams. 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.
Astronomy beyond this website:

This page last updated: April 21, 2006
(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