Curious minds: the biggest mysteries about space answered

Dr James De Buizer is senior scientist at the Stratospheric Observatory For Infrared Astronomy Science Center. He explains the difference...

A star-forming nebula is created by very bright young stars which are heating and ionising the leftover cloud of dust and gas from which they formed. A planetary nebula is formed at the end of the life of a star, after it has bloated into a red giant and run out of fuel. The central material from the red giant collapses to form a white dwarf star, and the heat and ionisation from this star blow the outer layers of material outwards.

Since some star-forming nebulae and planetary nebulae can look quite similar, astronomers must use a variety of clues to help differentiate them. The most obvious one is to see if you can identify a white dwarf at the centre of the nebula, in which case it is a planetary nebula. Also, the chemistry of the gas contained in the nebulae will be different, with planetary nebulae showing more highly processed chemicals that are forged in the interiors of the progenitor red giant stars.

The local environment of the nebulae can also be a clue; star-forming nebulae are typically found to be associated with clusters of young stars and found within dense molecular clouds. These types of diagnostics require astronomers to use data from multiple facilities and to look at multiple wavelengths to be sure they know what type of nebula they are dealing with.