A massive open cluster hiding in full sight
  Obscuration and confusion conspire to limit our knowledge of the inner Milky
Way. Even at moderate distances, the identification of stellar systems becomes
compounded by the extremely high density of background sources. Here we provide
a very revealing example of these complications by unveiling a large, massive,
young cluster in the Sagittarius arm that has escaped detection until now
despite containing more than 30 stars brighter than $G=13$. By combining Gaia
DR2 astrometry, Gaia and 2MASS photometry and optical spectroscopy, we find
that the new cluster, which we name Valparaiso~1, located at $\sim2.3\:$kpc, is
about 75~Ma old and includes a large complement of evolved stars, among which
we highlight the 4~d classical Cepheid CM~Sct and an M-type giant that probably
represents the first detection of an AGB star in a Galactic young open cluster.
Although strong differential reddening renders accurate parameter determination
unfeasible with the current dataset, direct comparison to clusters of similar
age suggests that Valparaiso~1 was born as one of the most massive clusters in
the Solar Neighbourhood, with an initial mass close to $10^{4}\:$M$_{\odot}$.
