M33'S variable A: A hypergiant star more than 35 years in eruption

Roberta M. Humphreys, Terry J. Jones, Elisha Polomski, Michael Koppelman, Andrew Helton, Kristen McQuinn, Robert D. Gehrz, C. E. Woodward, R. Mark Wagner, Karl Gordon, Joannah Hinz, S. P. Willner

Research output: Contribution to journalArticlepeer-review

37 Scopus citations


Variable A in M33 is a member of a rare class of highly luminous, evolved stars near the upper luminosity boundary that show sudden and dramatic shifts in apparent temperature due to the formation of optically thick winds in high mass loss episodes. Recent optical and infrared spectroscopy and imaging reveal that its "eruption," begun in ∼ 1950, has ended, having lasted 7ap; 45yr. Our current observations show major changes in its wind from a cool, dense envelope to a much warmer state surrounded by low-density gas with rare emission lines of Ca II, [Ca II], and K I. Its spectral energy distribution has unexpectedly changed, especially at the long wavelengths, with a significant decrease in its apparent flux, while the star remains optically obscured. We conclude that much of its radiation is now escaping out of our line of sight. We attribute this to the changing structure and distribution of its circumstellar ejecta, corresponding to the altered state of its wind as the star recovers from a high mass loss event.

Original languageEnglish (US)
Pages (from-to)2105-2113
Number of pages9
JournalAstronomical Journal
Issue number4
StatePublished - Apr 2006


  • Stars: individual (M33 Var A)
  • Stars: winds, outflows
  • Supergiants

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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