Life, Earth and Universe

Cosmology

study of the universe as a whole

Astronomy

study of space and celestial objects

Astrophysics

the application of physics to the study of space and celestial objects

Scientific Method

science must be empirically testable, and models must be falsifiable and make testable predictions

Positivism

knowledge must be based on empirical, observable phenomena

Logical Positivism

scientific theories must be empirically testable and based on observational data

Falsifiability

the property of a scientific hypothesis that allows for the possibility of evidence that could contradict it

Cosmological Principle

at sufficiently large scales and on average, the universe can be assumed to be homogeneous (statistical similarity between locations = irrelevant where you are) and isotropic (statistical similarity between directions = see the same in all directions)

Perfect Fluid Approximation

at sufficiently large scales and on average, cosmic content is modelled as a perfect fluid = barotropic medium (pressure depends only on density), in cosmology assumed compressible (to allow for the expansion or contraction of space); galaxies (and clusters) are treated as point-like masses in the fluid

1920 “Great Debate”

Key Questions: what is the true scale of the universe, and what are the mysterious “spiral nebulae” seen in the sky?

Harlow Shapely: “Milky Way is the entire universe, spiral nebulae are small, nearby clouds.”

Heber Curtis: “Milky Way is just one of many galaxies. Spiral nebulae are huge, faraway “island universes”.”

Parallax

the apparent shift in an object's position when you view it from two different points

Stellar Parallax

apparent shift of a nearby star’s position against background stars, as Earth moves around the Sun; works only for relatively nearby stars

Cosmological Units

Astronomical Unit (AU): average distance from the earth to the sun

Parsec (pc): the distance at which the mean radius of the earth’s orbit subtends and angle of one arc second (1/3600th degree)

Light Year (ly): the distance light travels in a year

Standard Candles

an astronomical object with a known luminosity; used in the Cosmic Distance Ladder

Luminosity

the true power output of a source (unit: L)

Apparent Brightness / Flux

the power measured per unit area [m^2] in distance r [m] from the point source; follows inverse-square law

Main Sequence Star

stable, long lived stars that still fuse hydrogen (like the sun)

White Dwarf Stars

no fusion, just stable core of smaller stars

Giant / Supergiant Stars

evolved stars with expanded envelopes, burning helium and heavier elements up to iron group

Hertzsprung-Russel Diagram