Astronomy
The study of everything beyond the atmosphere.
Astronomy studies objects and phenomena beyond Earth's atmosphere. Its central difficulty is that nothing can be handled or experimented on — almost everything known comes from analysing light that arrived on its own.
Scale
| Unit | Equals | Used for |
|---|---|---|
| Astronomical unit (AU) | ~150 million km, Earth to Sun | Within the solar system |
| Light-year | ~9.46 trillion km | Stars and nearby galaxies |
| Parsec | 3.26 light-years | Professional distance work |
Because light takes time to travel, looking outward is looking backward. The Sun is seen as it was eight minutes ago, the nearest star four years ago, distant galaxies billions of years ago. A telescope is a time machine in the only direction available.
Stellar life cycles
Stars form in collapsing clouds of gas and dust, and spend most of their lives fusing hydrogen into helium on the main sequence. What happens after depends entirely on mass: a Sun-like star swells to a red giant then sheds its layers and leaves a white dwarf; a much more massive star fuses heavier elements, collapses, and explodes as a supernova, leaving a neutron star or a black hole.
Every element heavier than helium was made inside stars or in their deaths, so the statement that we are made of stellar material is literal chemistry rather than sentiment.
Measuring distance
Parallax — the apparent shift of a nearby star against the background as Earth orbits — works for closer stars. Beyond that, standard candles such as Cepheid variables and type Ia supernovae have known intrinsic brightness, so observed dimness gives distance. Each method calibrates the next, forming the cosmic distance ladder.