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A Mystery Star Wrapped in a Stingray

SAO 244567 is nestled in the heart of the Stingray Nebula, a tiny planetary nebula. Planetary nebulae are formed when an intermediate mass (roughly 0.6–10 solar masses) star enters the last stages of stellar evolution.

Observations of the Stingray Nebula in 1989 led astronomers to conclude that SAO 244567 was a post-AGB star, since it had already produced a planetary nebula.

But spectra taken in 1971 and analyzed in 1995 indicated the star was still evolving at the time, meaning it would have made the transition from giant to post-AGB in twenty years, which is far too fast for any known evolutionary models.  Further, the 1971 observations yielded an effective temperature of 21,000 K for the star, but in 2002 different observations found the star to be at 60,000 K. Spectral observations in between those times support the star growing steadily hotter, though it appears lately (since 2006) to be cooling again slightly. That is a massive change in temperature in roughly thirty years.

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