Late Stages of Stellar Evolution
At the end of their lives, stars go through a sequence of red giant phases accompanied by intense stellar mass loss. This results in atomic and molecular circumstellar envelopes enshrouding the central asymptotic giant branch (AGB) stars and leading to bright infrared sources. Eventually the star becomes hot enough to ionize the envelope, starting the evolution of a planetary nebula (PN). Furthermore, the rapid transition stage between AGB and PN phase is crucial for our understanding of the properties of planetary nebula. Objects in this stage are called proto-planetary nebulae (PPNe) and are very rare.