In Greek myth, Zeus punished king Ixion by binding him for eternity to what?

In Greek mythology, Ixion (meaning 'strong native') was king of the Lapiths, the most ancient tribe of Thessaly. Ixion married Dia, a daughter of Deioneus (or Eioneus), and promised his father-in-law a valuable present. However, he did not pay the bride price, so Deioneus stole some of Ixion's horses in retaliation. Ixion concealed his resentment and invited his father-in-law to a feast at Larissa. When Deioneus arrived, Ixion pushed him into a bed of burning coals and wood.

This act alone would warrant Ixion a terrible punishment, but Zeus took pity on Ixion and brought him to Olympus and introduced him at the table of the gods. Instead of being grateful, Ixion grew lustful for Hera, Zeus's wife, a further violation of guest–host relations.

Ixion was expelled from Olympus and blasted with a thunderbolt. Zeus ordered Hermes to bind Ixion to a winged fiery wheel that was always spinning. Therefore, Ixion was bound to a burning solar wheel for all eternity, at first spinning across the heavens, but in later myth transferred to Tartarus. Only when Orpheus played his lyre during his trip to the Underworld to rescue Eurydice did it stop for a while.