The initial light curve resembled that of a nova, an eruption that occurs when enough hydrogen gas accumulates on the surface of a white dwarf from its close binary companion.

V838 Monocerotis reached a maximum visual magnitude of 6.75 on February 6, 2002, after which it started to dim rapidly, as expected.

[16] At the currently accepted distance of 6,100 pc, the measured angular diameter in late 2004 (1.83 mas) corresponded to a radius of 1,200±150 R☉, but by 2014, it shrunk to 750±200 R☉, around 70 solar radii larger than Antares.

In fact, some astronomers argue that the spectrum of the star initially resembled that of L-type brown dwarfs.

The star, designated M31-RV, reached the absolute bolometric magnitude of −9.95 at maximum (corresponding to a luminosity of 0.75 million L☉) before dimming beyond detectability.

Combined with the apparent magnitude measured from pre-eruption photographs, it was thought to be an underluminous F-type dwarf, which posed a considerable enigma.

Munari et al. (2005) suggested that the progenitor star was a very massive supergiant with an initial mass of about 65 M☉,[9] but this has been contested.

[10] Based on the photometric parallax of the companion, Munari et al. calculated a greater distance, 36,000 light years (10 kpc).

[9] Rapidly brightening objects like novae and supernovae are known to produce a phenomenon known as light echo.

[2][21] 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.

By March 2003 the size of the light echo in the sky was twice the angular diameter of Jupiter and was continuing to grow.

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.

[14] 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.

[14] Based on further observations of stars similar to V838 Monocerotis, such as V1309 Scorpii, astronomers have reached the conclusion that this is the most likely scenario.

Stellar birth is less active in outer galactic regions, and it is not clear how such a massive star can form there.

As the planet penetrated deeper into the atmosphere, friction would become stronger and kinetic energy would be released into the star more rapidly.