As quoted above the answer is written in the WD past….
Like old, retired people, WDs are considered to have concluded the (thermonuclearly) active period of their life, and their evolution is described as a pure cooling process, during which these stars become cooler and fainter as function of time. Our discovery demonstrates that this is not true for all WDs: depending on their past history, some of these stars can still have an unexpected reservoir of energy that allows them to age more slowly than their normal sisters. Turning back to the similarity with the human experience, particularly critical experiences occurred during the life of a human being can have drained all the energy and people reach their retirement completely exhausted. Some lucky people, instead, can have skipped such damaging experiences and reach their retirement with a residual extra-pocket of energy that allows them to age more slowly.
In the case of stars, the critical experience is the so-called “third dredge-up”, a mixing process (occurring during the evolutional stage preceding the WD phase) where most of the hydrogen in the envelope of the proto-WD is burned and the star arrives to the WD stage totally exhausted. However, because of a lighter mass, some star can skip this critical event and arrives to the WD stage with a residual hydrogen envelope, which is very thin (10-4 solar masses), but thick enough to allow stable thermonuclear burning providing the star with an extra-energy production that delays its ageing. Thanks to a slightly lower mass, the vast majority of the stars in M13 were able to skip this dramatic experience and preserve a thin hydrogen envelope.
“By combining the WD slow cooling models with the standard models of stellar evolution, we were able to exactly reproduce the properties of the stellar populations observed in M3 and M13: thus, this discovery offered us the opportunity to probe the causal connection linking different phenomena occurring in the final evolutionary stages of low-mass stars”, adds Prof. Maurizio Salaris from John Moores University in Liverpool (UK), a top expert of stellar evolution and WD, co-author of the paper.
This discovery has direct consequences for how astronomers measure the age of stars in the Milky Way. The evolution of WDs has previously been modelled as a predictable cooling process. This relatively straightforward relationship between age and temperature has led astronomers to use the WD cooling rate as a natural clock to determine the age of star clusters. However, WDs with active hydrogen burning could make these age estimates inaccurate by as much as 1 billion years.
“Our discovery changes the definition of white dwarfs as we currently teach it to students, and it opens a new perspective on the way these stars get old. We are now investigating other clusters similar to M13 to further constrain the conditions that drive stars to maintain the thin hydrogen envelope that allows them to age slowly”, concludes Ferraro.