Science
Astronomy
Light, gravity, planets, stars, galaxies and the universe, and how astronomers know.
- 10 chapters
- 30 lessons
- 30 visual guides
- All free
Chapters
1
Studying the Universe
Explore studying the universe through explanations, original examples, concept flashcards, and section-linked practice.
3 lessons
- 1.1Observations, Models, and EvidenceAstronomy studies distant objects mostly through signals that reach us. Separate what an instrument records from the model used to explain it.Visual guide
- 1.2Mapping the SkyThe sky appears to turn because Earth rotates. Sky coordinates and horizon directions help locate an object without implying that every visible star is equally distant.Visual guide
- 1.3Astronomical Units and ScalesA light-year measures distance, not time. Large distance units help describe scales, but changing units does not change the physical distance.Visual guide
2
Light as Evidence
Explore light as evidence through explanations, original examples, concept flashcards, and section-linked practice.
3 lessons
- 2.1Electromagnetic RadiationLight carries information through its wavelength and intensity. Visible light is only a small part of the signals astronomers can measure.Visual guide
- 2.2Spectroscopy and MotionSpectral features are fingerprints interpreted with physics. Temperature, motion, and intervening material can affect the signal, so one feature should not carry every conclusion.Visual guide
- 2.3Luminosity, Flux, and BrightnessBrightness depends on both the source and the journey its light takes. A bright-looking object is not necessarily the most powerful emitter.Visual guide
3
Observing Tools and Data
Explore observing tools and data through explanations, original examples, concept flashcards, and section-linked practice.
3 lessons
- 3.1Telescopes and ResolutionA telescope collects and separates signals; magnification alone is not its most useful measure. Atmosphere and instrument design limit what it can reveal.Visual guide
- 3.2Observing Across WavelengthsDifferent wavelengths reveal different processes. No one telescope band supplies a complete picture of every object.Visual guide
- 3.3Calibration, Noise, and ReproducibilityAn image needs calibration before it becomes trustworthy evidence. Colors may represent chosen wavelength bands rather than what an unaided eye would see.Visual guide
4
Gravity and Planetary Systems
Explore gravity and planetary systems through explanations, original examples, concept flashcards, and section-linked practice.
3 lessons
- 4.1Gravity and Orbital MotionGravity changes motion, while inertia keeps objects moving. An orbit is continuous free fall rather than a place where gravity stops.Visual guide
- 4.2Kepler’s LawsKepler's laws describe idealized orbital relationships. Check the assumptions and units before using them for a particular system.Visual guide
- 4.3Planetary FormationSolar-System formation is reconstructed from several clues. A model explains broad patterns while allowing later collisions and migration to change individual histories.Visual guide
5
Our Solar System
Explore our solar system through explanations, original examples, concept flashcards, and section-linked practice.
3 lessons
- 5.1Rocky Planets and Their SurfacesRocky worlds share some ingredients but have different histories. Atmosphere, interior, impacts, and energy balance help explain their contrasting surfaces.Visual guide
- 5.2Giant Planets, Moons, and Small BodiesOuter planets, moons, and small bodies form varied systems. A planet's category does not tell you everything about each moon or ring around it.Visual guide
- 5.3Moon Phases, Eclipses, Tides, and SeasonsPhases, eclipses, and tides involve geometry and gravity, but they are not the same phenomenon. Draw the positions before naming the result.Visual guide
6
Exoplanets and Astrobiology
Explore exoplanets and astrobiology through explanations, original examples, concept flashcards, and section-linked practice.
3 lessons
- 6.1Detecting ExoplanetsAn exoplanet is a planet outside our Solar System. Detection methods infer its presence through particular signals and have different biases.Visual guide
- 6.2Inferring Exoplanet PropertiesMeasurements constrain a planet's properties without revealing every detail. A radius and mass can suggest composition, but several interiors may fit similar values.Visual guide
- 6.3Habitability and Evidence of LifeHabitable does not mean inhabited. Conditions allowing liquid water are one part of a broader investigation, not proof that a planet has life.Visual guide
7
The Sun and Stellar Properties
Explore the sun and stellar properties through explanations, original examples, concept flashcards, and section-linked practice.
3 lessons
- 7.1How the Sun WorksThe Sun is powered by nuclear fusion in its interior. Its visible surface and atmosphere reveal different conditions, and activity affects surrounding space.Visual guide
- 7.2Classifying StarsStars can be classified by spectra and temperature, then compared by luminosity. A diagram position is evidence about properties, not a physical place in space.Visual guide
- 7.3Measuring Stellar DistancesStellar distances need several methods at different scales. A variable star is useful only when its relevant physical relationship has been calibrated.Visual guide
8
Stellar Lives and Endings
Explore stellar lives and endings through explanations, original examples, concept flashcards, and section-linked practice.
3 lessons
- 8.1Star FormationStars form when regions of gas collapse and heat. A forming star and a mature hydrogen-fusing star are different stages.Visual guide
- 8.2Sun-Like Stars and White DwarfsA Sun-like star's late stages involve changing fuel conditions and an expanding outer region. Its remnant is not a black hole or an ordinary small star.Visual guide
- 8.3Massive Stars and Stellar RemnantsMassive-star endings depend on mass and history. Supernovae, neutron stars, and black holes are connected, but not every star follows the same route.Visual guide
9
Galaxies and Extreme Objects
Explore galaxies and extreme objects through explanations, original examples, concept flashcards, and section-linked practice.
3 lessons
- 9.1Black Holes and Multimessenger AstronomyA black hole is not a cosmic vacuum cleaner. Its effects follow gravity, and astronomers infer its properties through the surrounding environment and other signals.Visual guide
- 9.2The Milky WayThe Milky Way contains several components with different properties. The bright band in our sky is our inside view, not the outline of a separate nearby galaxy.Visual guide
- 9.3Galaxy Types and InteractionsGalaxies change through star formation and interactions. Their appearance is useful evidence, but shape alone does not reveal every step in their history.Visual guide
10
Cosmology and Investigation
Explore cosmology and investigation through explanations, original examples, concept flashcards, and section-linked practice.
3 lessons
- 10.1Dark Matter and Large-Scale StructureDark matter is inferred from gravitational evidence. Naming an inferred component is different from identifying the particles that constitute it.Visual guide
- 10.2Expansion and Cosmic HistoryThe expanding-universe model explains several observations together. Expansion does not require Earth to sit at the center of an explosion into empty space.Visual guide
- 10.3Planning an Observing InvestigationAn astronomy investigation needs a measurable question and limits. A useful conclusion explains what the data support without turning an attractive image into proof of everything.Visual guide