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The Chemical Evolution of FGSge

``Old stars don't just fade away, some come back another day.'' But what happens when they do? A new study by Simon Jeffery has challenged the canonical view of one `born-again' star.

Although most white dwarfs do indeed fade away to become stellar cinders, gigantic crystals of carbon and oxygen, a few have just enough hydrogen and helium fuel left on their surface to be re-ignited. When the nuclear reactions restart, the white dwarf expands by a factor of over 1,000,000,000,000 in volume to become a yellow supergiant in an event so rare it has only been seen to occur three times in the last century. One of these, FGSagittae, has expanded over an interval of about 100 years. According to a series of publications from the 1960's to the 1990's, it has begun to dredge up very rare nuclei which could only have been produced by light elements capturing neutrons deep in the stellar interior in the distant past. Observers have claimed that the surface abundances of these `s-process' elements (including strontium, barium, zirconium, europium, lanthanum, neodymium, praseodymium and samarium) have increased as much as a thousand-fold. Such claims are directly contradicted by theory which argues that the dredge-up of processed material can only occur a considerable time after such a star has completed its expansion (see Figure 7). Simon Jeffery and collaborator Detlef Schönberner (Astrophysikalisches Institut Potsdam) have completed a comprehensive review of data covering over half a century of observations and, by reconstructing the results of many different authors, have shown that the chemical composition of the surface of FGSge probably has not changed at all in the last fifty years -- as theory suggests.