• Hydrogen shell burning: Fuse helium in core, fuse hydrogen in surrounding shell
• White dwarf: end state of low mass star, supported by electron degeneracy pressure
• Planetary nebula: White dwarf ionizes the shrugged-off outer layers of former red giant
• High mass star evolution: Onion layer of shells down to iron core; ends with supernova
• Cluster evolution: Can date cluster by looking at main sequence turnoff

• Giant branch: Upper right in HR diagram (high luminosity, low temperature), where stars go as they expand and their surface cools
• Horizontal branch: Star stably burning helium in core moves away from giant branch back towards main sequence
• Nova: Flare due to white dwarf burning hydrogen dumped onto surface; does not disrupt star
• Supernova: Type 1a is white dwarf blowing up after exceeding mass limit, type 2 is core-collapse of massive star followed by rebound shock

Researchers using a fleet of five NASA satellites have discovered that explosions of magnetic energy a third of the way to the moon power substorms that cause sudden brightenings and rapid movements of the aurora borealis, called the Northern Lights.

The culprit turns out to be magnetic reconnection, a common process that occurs throughout the universe when stressed magnetic field lines suddenly snap to a new shape, like a rubber band that's been stretched too far. "We discovered what makes the Northern Lights dance," said Dr. Vassilis Angelopoulos of the University of California, Los Angeles.