  The stellar abundance ratio of Mg/Fe is an important tool in diagnostics of
galaxy evolution. In order to make reliable measurements of the Mg abundance of
stars, it is necessary to have accurate values for the oscillator strength
(f-value) of each of the observable transitions. In metal-poor stars the Mg I
3p-4s triplet around 5175 AA (Fraunhofer's so-called b lines) are the most
prominent magnesium lines. The lines also appear as strong features in the
solar spectrum. We present new and improved experimental oscillator strengths
for the optical Mg I 3p-4s triplet, along with experimental radiative lifetimes
for six terms in Mg I. With these data we discuss the implications on previous
and future abundance analyses of metal-poor stars. The oscillator strengths
have been determined by combining radiative lifetimes with branching fractions,
where the radiative lifetimes are measured using the laser induced fluorescence
technique and the branching fractions are determined using intensity calibrated
Fourier Transform (FT) spectra. The FT spectra are also used for determining
new accurate laboratory wavelengths for the 3p-4s transitions. The f-values of
the Mg I 3p-4s lines have been determined with an absolute uncertainty of 9 %,
giving an uncertainty of +-0.04 dex in the log gf values. Compared to values
previously used in abundance analyses of metal-poor stars, rescaling to the new
values implies an increase of typically 0.04 dex in the magnesium abundance.
