Astronomy

Galaxies and Hubble

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

Basics

Shapes

Hubble’s tuning-fork sketch splits ellipticals, spirals (barred or not), and irregulars. Ellipticals are gas-poor with older stars; spirals form stars in arms. The Milky Way is a barred spiral. Shape is a snapshot, not a path from elliptical into spiral. Mergers can make ellipticals and irregulars.

Rotation curves

In spirals, orbital speed in the disk often stays flat far out instead of falling. Stars and gas we see do not supply enough gravity, so a dark-matter halo is added. Modified gravity is studied too; the standard sketch is a halo. The Milky Way shows the same pattern. Do not count mass by light alone.

Hubble–Lemaître

Nearer galaxies show smaller redshifts. v ≈ H₀ d is the linear sketch. Photometric ladders and the CMB give slightly different H₀ — the “Hubble tension”. In the far universe one reads a changing scale factor, not a simple speed. Recession is expanding space, not galaxies tunnelling through the void.

The distance ladder

Parallax (Gaia) measures nearby stars; Cepheid period–luminosity and the near-standard peak of Type Ia supernovae extend the ruler. Each rung passes calibration error to the next. Gravitational-wave “standard sirens” test another path. Numbers are revised; in class, keep the backbone of the methods.

Formulas

Hubble–Lemaître (nearby)

v ≈ H₀ d

Low speed and redshift. H₀ in km s⁻¹ Mpc⁻¹.

Symbols

v recession-speed sketch
H₀ Hubble constant (today)
d distance (near proper distance)

Redshift

z = (λ_obs − λ_em) / λ_em

In cosmology, 1+z = 1/a at emission.

Symbols

λ_obs observed wavelength
λ_em emitted wavelength

Circular speed

v = √(G M(<r) / r)

A flat curve suggests M(<r) grows with r.

Symbols

M(<r) mass interior to r

Key table

H₀ ballpark	around 70 km s⁻¹ Mpc⁻¹; methods differ by a few km
Local Group	Milky Way, Andromeda (M31), Triangulum (M33), …
Dark matter	almost no light, gravity only; the particle is still unconfirmed in the lab