Variable Stars & The Milky Way

Our galaxy is a barred spiral containing ~200 billion stars. From inside, we see it as a band across the night sky. Direct measurement is difficult because dust blocks visible light toward the center; infrared and radio observations reveal much more.

The Milky Way has five main components: the galactic disk (thin, rotating, containing spiral arms, gas, dust, and young stars), the galactic bulge (older stars surrounding the center), the galactic nucleus (a supermassive black hole), the galactic halo (sparse very old stars and dark matter, extending spherically to ~150,000 light-years), and ~150 globular clusters distributed in the halo. The disk is ~100,000 light-years (~30 kpc) across; the Sun sits ~8 kpc from the center and orbits the galaxy every ~225 million years.

Distance within the galaxy is measured using pulsating variable stars: RR Lyrae stars (periods 0.5–1 day, all roughly the same luminosity) and Cepheids (periods 1–100 days, with a tight period-luminosity relation discovered by Henrietta Leavitt in 1908). Luminosity plus apparent brightness gives distance — good to ~25 Mpc. These build on parallax as the next rung of the cosmic distance ladder.

Spiral arms are not solid structures rotating with the disk — differential rotation would wind them up in a few rotations. Instead they are density waves: traffic jams of compressed gas through which stars and clouds move. The compression triggers star formation, and the bright young blue stars highlight the wave.

Flat galactic rotation curves imply the galaxy contains far more mass than is visible in stars and gas. The excess — dark matter — dominates the total mass.

At the center sits Sagittarius A*, a supermassive black hole of ~4 × 10⁶ M☉, inferred from the rapid Keplerian orbits of nearby stars (and imaged directly by the Event Horizon Telescope in 2022).