Stingray Nebula

Lore & Background

Prior to the discovery of the nebula, its central star was known as He 3-1357, which Karl Gordon Henize classified as an A- or B-type Hα emission-line star in 1976. It was observed in 1971 to be post-asymptotic giant branch B1 or B2 supergiant. Planetary nebula emission lines were identified in this star in 1989 by the International Ultraviolet Explorer. As the nebula would be newly formed and very small, ground-based observations were not able to resolve it; so Matt Bobrowsky observed it with the Hubble Space Telescope, discovering the nebula, which he named the 'Stingray Nebula'.

In 1993, M. Parthasarathy et al. looked at the history of measurements of the brightness of the central star, and concluded that it was fading in the optical region of the spectrum. It was given its variable star designation, V839 Arae, in 1997. In 1995 the central planetary nebula nucleus was observed as a DA white dwarf, having seemingly faded by a factor of three between 1987 and 1995. The white dwarf has an estimated mass of 0.6 M☉ and luminosity of 3,000 L☉ and has an observed companion star separated by 0.3 arcsecond. The mass of the nebula is estimated as 0.015 M☉. In 1998 Bobrowsky et al. described how the Hubble Space Telescope observations revealed a 17th-magnitude companion to the Stingray's 15th-magnitude central star.

The central star is unusual in that it has brightened and faded over a period of 20 years. Its temperature went up by 40,000 °C. An explanation for this is that it has undergone a helium flash. In January 2021, NASA discovered that the nebula had been fading since the 1990s, when it reached its peak brightness. Previously photoionized, the positive ions of the nebula have been recombining with the electrons.

Reader's Guide

The Stingray Nebula is significant as the youngest-known planetary nebula, having appeared only in the 1980s, allowing astronomers to observe a nebula's evolution in real time. The image of the nebula shows how the older outer shells of gas are acting as a collimator for the more recent gas outflow from the central star—an important observation, as this process has not been well understood. Its central star, SAO 244567, is fast-evolving and unusual in that it has brightened and faded over a period of 20 years, with its temperature going up by 40,000 °C, possibly due to a helium flash. The nebula's fading since the 1990s, with positive ions recombining with electrons, was noted by NASA in January 2021. As Martín A. Guerrero stated, 'This is very, very dramatic, and very weird. What we're witnessing is a nebula's evolution in real time. In a span of years, we see variations in the nebula. We have not seen that before with the clarity we get with this view.' The legacy of the Stingray Nebula lies in its role as a unique laboratory for studying planetary nebula formation and evolution, particularly the collimation of outflows by older gas shells.

Did You Know?

• The Stingray Nebula is the youngest-known planetary nebula, having appeared in the 1980s.
• Its central star, SAO 244567, increased in temperature by 40,000 °C over 20 years.
• The nebula is only about one tenth the size of most other known planetary nebulae.
• In January 2021, NASA reported that the nebula had been fading since the 1990s.