Astronomy

Galaxies and cosmology

Galaxies, Hubble, redshift, black holes.

Basics

The Milky Way

A barred spiral. The Sun sits in the disk about 8 kpc from the center. A dark-matter halo keeps the rotation curve flat. A supermassive black hole (Sgr A*) sits at the center.

Expanding universe

More distant galaxies show larger redshift (Hubble–Lemaître). The CMB is leftover light from the early universe. Rough contents: dark energy, dark matter, ordinary matter.

Formulas

Hubble–Lemaître

v = H₀ d

Recession speed proportional to distance. H₀ is around 70 km/s/Mpc, with a well-known tension.

Symbols

H₀ Hubble constant
d proper distance

Redshift

z = (λ_obs − λ_emit) / λ_emit

Cosmological z is expansion of space. Nearby objects also have Doppler shifts.

Symbols

z redshift

Schwarzschild radius

R_s = 2 G M / c²

Scale of the event horizon. About 3 km for a solar mass.

Key table

CMB	≈ 2.725 K blackbody
Age (approx.)	about 13.8 billion years
Local Group	Milky Way, Andromeda (M31), Triangulum (M33), and dwarfs

In this field

Solar system

Planets, Kepler, gravity, escape speed.

Stars

HR diagram, fusion, lifetimes, magnitude.

Observing basics

Celestial sphere, coordinates, telescopes, spectra.

Exoplanets and life

Detection, habitable zone, a SETI sketch.

The Sun and space weather

Solar structure, spots, flares and CMEs.

Moon, tides, and eclipses

Lunar phases, tides, and solar and lunar eclipses.

Magnitudes and spectra

Apparent and absolute magnitude, distance modulus, colour and spectral type, Wien’s law.

Galaxies and Hubble

Galaxy shapes, rotation curves, the Hubble–Lemaître law, and a standard-candle sketch.

Telescopes and light

Aperture, diffraction limit, collecting area, seeing versus space telescopes.

Stellar death

Red giants, supernovae, and sketches of white dwarfs, neutron stars, and black holes.