Key facts & rules

• Solar system: Sun (star) → 8 planets → moons, asteroids, comets.
• Planets orbit the Sun due to gravitational attraction — centripetal force provided by gravity.
• Geostationary orbit: period = 24 h, above equator, always above same point — used for communications.
• Low Earth orbit: period ~90 min, lower altitude — used for weather satellites, ISS.
• Life cycle of a star: nebula → protostar → main sequence (most of its life) → red giant/supergiant → white dwarf (small star) OR supernova → neutron star or black hole (large star).
• Red-shift: galaxies moving away show longer wavelength light — evidence for expanding universe.
• Big Bang theory: universe began ~13.8 billion years ago from a single point; has been expanding since.
• 1 light-year: distance light travels in 1 year ≈ 9.46 × 10¹⁵ m.