Astronomy
What Is Astronomy?
Astronomy is the scientific study of celestial objects — stars, planets, galaxies, and the universe itself. It is one of the oldest sciences, practiced by every civilization since prehistory.
Our Solar System
Eight planets orbit the Sun, plus dwarf planets, moons, asteroids, and comets:
- Inner planets (rocky): Mercury, Venus, Earth, Mars
- Outer planets (gas/ice giants): Jupiter, Saturn, Uranus, Neptune
- Other objects: Pluto (dwarf planet), the asteroid belt, Kuiper Belt, Oort Cloud
Stellar Evolution
Stars are born in nebulae, live on the main sequence fusing hydrogen into helium, and die in dramatic ways depending on their mass:
- Low-mass stars → red giant → white dwarf
- High-mass stars → supergiant → supernova → neutron star or black hole
Galaxies & the Large-Scale Universe
- Milky Way — our barred spiral galaxy, containing 100–400 billion stars
- Galaxy types: spiral, elliptical, irregular
- Observable universe: ~93 billion light-years in diameter, containing ~2 trillion galaxies
Key Concepts
- Light-year — the distance light travels in one year (~9.46 trillion km)
- Redshift — light stretched by the expansion of space; evidence for the Big Bang
- Dark matter & dark energy — together make up ~95% of the universe's total energy content
- Exoplanets — planets orbiting other stars; thousands confirmed since the 1990s
Major Milestones
- 1543 — Copernicus places the Sun at the center (heliocentrism)
- 1609 — Galileo's telescope observations
- 1929 — Hubble discovers the expanding universe
- 1969 — Apollo 11 lands on the Moon
- 1990 — Hubble Space Telescope launched
- 2015 — First detection of gravitational waves