The Controversial Star-Formation History and Helium Enrichment of the
  Milky Way Bulge
  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]$\lesssim0$ are older than $t \approx10$ 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.
