Astronomy Cheat Sheet 2026
The 30 highest-yield Astronomy facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.
- What is stellar interferometry used to measure? → The angular diameters of distant stars
- Which moon of Saturn has a thick nitrogen-rich atmosphere and liquid methane lakes? → Titan
- Which planet is tilted so extremely that it essentially rotates on its side? → Uranus
- What is the Hill sphere of a planet? → The region within which the planet's gravity dominates over the Sun's gravity
- The Big Bang theory predicts that in the early universe, matter was: → Extremely hot and dense
- What does a photometer measure in astronomical observations? → The brightness or flux of celestial objects
- According to Kepler's Third Law, how does a planet's orbital period relate to its distance from the Sun? → The square of the period is proportional to the cube of the semi-major axis
- A supernova of Type Ia is valuable to cosmologists because it: → Has a consistent peak brightness usable as a distance marker
- Where in the galaxy are new stars most actively forming? → Dense molecular clouds and nebulae
- What is the approximate age of the universe according to current estimates? → 13.8 billion years
- Which of Kepler's Laws states that planets sweep out equal areas in equal times? → Second Law (Law of Equal Areas)
- Which planet has the strongest magnetic field of any planet in the Solar System? → Jupiter
- What is the difference between sidereal and synodic orbital periods? → Sidereal is relative to distant stars; synodic is relative to Earth-Sun alignment
- Which planet has the fastest rotation, completing a full spin in under 10 hours? → Jupiter
- The asteroid belt lies mainly between the orbits of which two planets? → Mars and Jupiter
- What primarily causes the seasons on Earth? → The tilt of Earth's rotational axis
- What is the eccentricity of a perfectly circular orbit? → 0
- Which term describes a region of space where gravity is so strong that nothing, not even light, can escape? → Black hole
- A star's color is primarily an indicator of its what? → Surface temperature
- What color are the hottest stars? → Blue
- What are the two largest satellite galaxies of the Milky Way? → Large and Small Magellanic Clouds
- What is NASA's Artemis program designed to accomplish? → Return humans to the Moon
- What is the main-sequence turnoff point of a star cluster used to estimate? → The cluster's age
- What is a light-year a measure of? → Distance
- What is an active galactic nucleus (AGN)? → An extremely luminous core powered by a supermassive black hole accreting matter
- What do astronomers study using the 21-cm hydrogen line in radio astronomy? → The distribution and motion of neutral hydrogen gas in the galaxy
- What is the tail of a comet always directed away from? → The Sun
- Absolute magnitude describes a star's brightness as seen from what standard distance? → 10 parsecs
- What is the unit used to measure the light-gathering power of a telescope related to its mirror diameter? → Aperture
- The era when the universe cooled enough for electrons and protons to form neutral atoms, releasing the CMB, is called: → Recombination
Turn these facts into recall: