Space & Astronomy
Stars, solar-system scale, Moon phases, telescopes and the electromagnetic universe.
Light and the electromagnetic spectrum
Visible light is only a narrow portion of the electromagnetic spectrum, which also includes radio waves, infrared, ultraviolet, X-rays and gamma rays.
GuideWhy Earth has seasons
Earth's seasons are caused mainly by the tilt of its rotation axis, not by large changes in distance from the Sun.
GuideHow stars form
Stars form when dense regions inside large clouds of gas and dust collapse under gravity and heat until nuclear fusion begins.
GuideThe life cycle of a star
Stars change as nuclear fuel and internal structure evolve, with different outcomes for low-mass and high-mass stars.
GuideUnderstanding the scale of the solar system
The solar system is so large that familiar maps compress distances enormously.
GuideWhy the Moon has phases
Moon phases occur because we see different portions of the Moon's sunlit half as it orbits Earth.
GuideWhy telescopes observe different wavelengths
Different parts of the electromagnetic spectrum reveal different physical processes, so astronomy uses many kinds of telescopes.
GuideHow modern clocks keep time
Clocks count regular physical oscillations, from pendulums and quartz crystals to atomic transitions.