Following the outburst of the unusual variable star V838 Monocerotis in 2002 , a spectacular light echo appeared .
A light echo provides the possibility of direct geometric distance determination , because it should contain a ring of highly linearly polarized light at a linear radius of ct , where t is the time since the outburst .
We present imaging polarimetry of the V838Â Mon light echo , obtained in 2002 and 2005 with the Advanced Camera for Surveys onboard the Hubble Space Telescope , which confirms the presence of the highly polarized ring .
Based on detailed modeling that takes into account the outburst light curve , the paraboloidal echo geometry , and the physics of dust scattering and polarization , we find a distance of 6.1 \pm 0.6 Â kpc .
The error is dominated by the systematic uncertainty in the scattering angle of maximum linear polarization , taken to be \theta _ { max } = 90 ^ { \circ } \pm 5 ^ { \circ } .
The polarimetric distance agrees remarkably well with a distance of 6.2 \pm 1.5 Â kpc obtained from the entirely independent method of main-sequence fitting to a sparse star cluster associated with V838Â Mon .
At this distance , V838Â Mon at maximum light had M _ { V } \simeq - 9.8 , making it temporarily one of the most luminous stars in the Local Group .
Our validation of the polarimetric method offers promise for measurement of extragalactic distances using supernova light echoes .