Astronomy

Solar system

Planets, Kepler, gravity, escape speed.

Basics

Planet types

Terrestrial (Mercury–Mars): rocky, dense. Gas giants (Jupiter, Saturn): H/He. Ice giants (Uranus, Neptune). Dwarf planets (e.g. Pluto) have not cleared their neighborhood, by the IAU definition.

Kepler's laws

1: Orbits are ellipses with the Sun at one focus. 2: Equal areas in equal times (faster at perihelion). 3: Period squared is proportional to semi-major axis cubed.

Formulas

Newton’s gravity

F = G M m / r²

Attraction between masses. Starting point for planets, moons, and tides.

Symbols

G gravitational constant
r distance between centers

Kepler III (Newtonian)

T² = 4π² a³ / (G (M+m))

In the solar system M≫m so T² ∝ a³. Used to weigh bodies.

Symbols

T orbital period
a semi-major axis

Escape speed

v_esc = √(2 G M / r)

Speed needed to go to infinity from that radius. Also hints whether an atmosphere can stay.

Surface gravity

g = G M / R²

g at the surface. Larger mass or smaller radius raises g.

Symbols

R planetary radius

Key table

AU	1 AU ≈ 1.496×10¹¹ m (Earth–Sun mean)
Light-year	1 ly ≈ 9.46×10¹⁵ m
Parsec	1 pc ≈ 3.26 ly

In this field

Stars

HR diagram, fusion, lifetimes, magnitude.

Galaxies and cosmology

Galaxies, Hubble, redshift, black holes.

Observing basics

Celestial sphere, coordinates, telescopes, spectra.

Exoplanets and life

Detection, habitable zone, a SETI sketch.

The Sun and space weather

Solar structure, spots, flares and CMEs.

Moon, tides, and eclipses

Lunar phases, tides, and solar and lunar eclipses.

Magnitudes and spectra

Apparent and absolute magnitude, distance modulus, colour and spectral type, Wien’s law.

Galaxies and Hubble

Galaxy shapes, rotation curves, the Hubble–Lemaître law, and a standard-candle sketch.

Telescopes and light

Aperture, diffraction limit, collecting area, seeing versus space telescopes.

Stellar death

Red giants, supernovae, and sketches of white dwarfs, neutron stars, and black holes.