V838 Monocerotis - Light Echo

Light Echo

Rapidly brightening objects like novae and supernovae are known to produce a phenomenon known as light echo. The light that travels directly from the object arrives first. If there are clouds of interstellar matter around the star, some light is reflected from the clouds. Because of the longer path, the reflected light arrives later, producing a vision of expanding rings of light around the erupted object.

In the case of V838 Monocerotis, the light echo produced was unprecedented and is well documented in images taken by the Hubble Space Telescope. While the photos appear to depict an expanding spherical shell of debris, they are actually formed by the illumination of an ever-expanding ellipsoid with the progenitor star at one focus and the observer at the other. Hence, despite appearances, the structures in these photos are actually concave toward the viewer. In other words, the light is reflecting dust that is mostly 'behind' the star, not in 'front' of it.

It is not yet clear if the surrounding nebulosity is associated with the star itself. If that is the case, they may have been produced by the star in earlier eruptions which would rule out several models that are based on single catastrophic events. However, there is strong evidence that the V838 Monocerotis system is very young and still embedded in the nebula from which it formed.

Interestingly, the eruption initially emitted at shorter wavelengths (i.e. was bluer), which can be seen in the light echo: the outer border is bluish in the Hubble images.