Protoplanetary nebula

The Egg Nebula, a protoplanetary nebula in Cygnus

A protoplanetary nebula (PPN) is an astronomical object which is formed as a star evolves into a planetary nebula. As a star at the end of its life sheds its outer layers, they will initially form a PPN which emits strongly in infrared radiation, and forms a reflection nebula.

Protoplanetary nebulae last for a few thousand years. As more material is lost from a star's surface, deeper and deeper layers are exposed which are successively hotter. When the star's surface temperature reaches about 30,000 K, there is enough ultraviolet radiation to ionise the emitted gases, and the PPN becomes an emission nebula and a fully fledged planetary nebula (PN).

In some cases, a PPN may never become a PN, as the ejected gases may expand and disperse before the star begins to emit ionising radiation. In this case, the object is sometimes referred to as a 'lazy planetary nebula'.

References

Volk K.M., Kwok S. (1989), Evolution of protoplanetary nebulae, Astrophysical Journal, v. 342, p. 345-363.