V838 Monocerotis is an unusual variable star that underwent a sudden outburst in 2002 .
Unlike a classical nova , which quickly evolves to high temperatures , V838 Mon remained an extremely cool , luminous supergiant throughout its eruption .
It continues to illuminate a spectacular series of light echoes , as the outburst light is scattered from nearby circumstellar dust .
V838 Mon has an unresolved B3 V companion star .

During a program of spectroscopic monitoring of V838 Mon , we serendipitously discovered that a neighboring 16th-mag star is also of type B .
We then carried out a spectroscopic survey of other stars in the vicinity , revealing two more B-type stars , all within 45 ^ { \prime \prime } of V838 Mon .
We have determined the distance to this sparse , young cluster , based on spectral classification and photometric main-sequence fitting of the three B stars .
The cluster distance is found to be 6.2 \pm 1.2 kpc , in excellent agreement with the geometric distance to V838 Mon of 5.9 kpc obtained from Hubble Space Telescope polarimetry of the light echoes .
An upper limit to the age of the cluster is about 25 Myr , and its reddening is E ( B - V ) = 0.85 .

The absolute luminosity of V838 Mon during its outburst , based on our distance measurement , was very similar to that of M31 RV , an object in the bulge of M31 that was also a cool supergiant throughout its eruption in 1988 .
However , there is no young population at the site of M31 RV .

Using our distance determination , we show that the B3 V companion of V838 Mon is sufficient to account for the entire luminosity of the variable star measured on sky-survey photographs before its outburst .
The B3 star is currently , however , about 1 mag fainter than before the eruption , suggesting that it is now suffering extinction due to dust ejected from V838 Mon .
These results indicate that , whatever the nature of the progenitor object , it was not of high luminosity .
Nor does it appear possible to form a nova-like cataclysmic binary system within the young age of the V838 Mon cluster .
These considerations appear to leave stellar-collision or -merger scenarios as one of the remaining viable explanations for the outbursts of V838 Mon and M31 RV .