Variable Stars, Novae, and Supernovae

Variable Stars

Stars whose brightness changes over time:

• Intrinsic variables - Brightness changes due to physical changes in the star (pulsation, eruptions).
• Extrinsic variables - Brightness changes due to eclipses or rotation (eclipsing binaries, rotating spotted stars).

Pulsating Variables

• Cepheids - Regular pulsation with period related to luminosity. Used as "standard candles" to measure cosmic distances.
• RR Lyrae stars - Short-period pulsators common in globular clusters.
• Mira variables - Long-period red giants with large amplitude variations.

Novae

A nova is a sudden brightening (up to 10,000x) of a binary system containing a white dwarf. Material transferred from the companion builds up on the white dwarf's surface until it undergoes runaway fusion. The star isn't destroyed and can recur.

Supernovae

Stellar explosions that can outshine entire galaxies:

• Type Ia - White dwarf exceeds critical mass, completely disrupted. Used as standard candles for cosmology.
• Type II - Core collapse of massive star, leaving neutron star or black hole.