The Goldilocks effect.
The plot shows nearby stars measured with the Hipparcos
satellite, arranged from hot to cool (right to left) and very
bright to very dim (up to down). "Hot stars" are too young, to
be useful : they tell us little about the history of Helium production.
The "Evolved stars" have been producing lots of Helium --- making
them of little use as fossil records of the Helium in the Universe
at the time such stars were born. Very cool stars (lower right) have
very complex spectra making the prediction of their brightness still
too difficult to be reliable. The
K dwarfs utilised in this study are neither too cool, nor too
hot, but "just right".
