Their brother, Phaethon, died after attempting to drive his father's chariot (the sun) across the sky. He was unable to control the horses and fell to his death (according to most accounts, Zeus struck his chariot with a thunderbolt to save the Earth from being set afire). The Heliades grieved for four months and the gods turned them into poplar trees and their tears into amber. According to some sources, their tears (amber) fell into the river Eridanos, in which Phaethon had fallen.
According to Hyginus, the Heliades were turned to poplar trees because they yoked the chariot for their brother without their father Helios' permission". (From Wikipedia)