page_title: V838 Monocerotis
section_title: Progenitor star
hierarchical_section_title: V838 Monocerotis / Progenitor star
caption_attribution_description: English: Location of V838 Mon in the Milky Way Galaxy Español: Ubicación de v838 Mon en la vía láctea.
context_page_description: V838 Monocerotis is a cataclysmic variable - or outburst - star in the constellation Monoceros about 20,000 light years from the Sun. The previously unknown star was observed in early 2002 experiencing a major outburst, and was possibly one of the largest known stars for a short period following the outburst. Originally believed to be a typical nova eruption, it was then identified as something completely different. The reason for the outburst is still uncertain, but several conjectures have been put forward, including an eruption related to stellar death processes and a merger of a binary star or planets.
  The remnant is evolving rapidly. By 2009 its temperature had increased a bit to 3,270 K and its luminosity was 15,000 times solar, but its radius had decreased to 380 times that of the Sun although the ejecta continues to expand. The opaque ejected dust cloud has completely engulfed a B-type companion.
context_section_description: Some details are emerging on the nature of the star that experienced the outburst. Based on an incorrect interpretation of the light echo the eruption generated, the distance of the star was first estimated to be 1,900 to 2,900 light years. 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.
  More accurate measurements gave a much larger distance, 20,000 light years (6 kpc). It appears that the star was considerably more massive and luminous than the Sun. The star probably has a mass of from 5 to 10 times solar (M☉). It was apparently either a B1.5V star with a B3V companion, or an A0.5V with a B4V companion. In the latter case it would have had a luminosity around 550 L☉ (being 0.43 times as luminous as its companion), and in the former case it would have been more luminous (about 1.9 times as luminous as its companion). The star may have originally had a radius roughly 5 R☉ and its temperature would have been that of a B-type star (more than 10,000K but less than 30,000K). Munari et al. (2005) suggested that the progenitor star was a very massive supergiant with an initial mass of about 65 M☉, but this has been contested. There seems to be agreement that the star system is relatively young.
  Munari et al. conclude that the system may be only about 4 million years old.
  The spectrum of V838 Monocerotis reveals a companion, a hot blue B-type main sequence star probably not very different from the progenitor star. It is also possible that the progenitor was slightly less massive than the companion and only just entering the main sequence.
  Based on the photometric parallax of the companion, Munari et al. calculate a greater distance, 36,000 light years (10 kpc).
caption: Location of V838 Monocerotis within the Milky Way galaxy.
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