The stellar population of the Milky Way bulge is thoroughly studied , with a plethora of measurements from virtually the full suite of instruments available to astronomers .
It is thus perhaps surprising that alongside well-established results lies some substantial uncertainty in its star-formation history .
Cosmological models predict the bulge to host the Galaxy ’ s oldest stars for [ Fe/H ] \lesssim - 1 , and this is demonstrated by RR Lyrae stars and globular cluster observations .
There is consensus that bulge stars with [ Fe/H ] \lesssim 0 are older than t \approx 10 Gyr .
However , at super-solar metallicity , there is a substantial unresolved discrepancy .
Data from spectroscopic measurements of the main-sequence turnoff and subgiant branch , the abundances of asymptotic giant branch stars , the period distribution of Mira variables , the chemistry and central-star masses of planetary nebulae , all suggest a substantial intermediate-age population ( t \approx 3 Gyr ) .
This is in conflict with predictions from cosmologically-motivated chemical evolution models and photometric studies of the main-sequence turnoff region , which both suggest virtually no stars younger than t \approx 8 Gyr .
A possible resolution to this conflict is enhanced helium-enrichment , as this would shift nearly all of the age estimates in the direction of decreasing discrepancy .
Enhanced helium-enrichment is also arguably suggested by measurements of the red giant branch bump and the R-parameter .