A Dying Star Fades Away Before Hubble’s Very Eyes
By: Ethan Siegel
Tags: Astronomy, Space, Hubble, Nebula, Stingray

All stars, even our Sun, will someday eventually die.

After burning on the main sequence for billions of years, the Sun will expand into a red giant, switch to helium burning, move to the asymptotic branch, and then eject its outer layers. As the core contracts, it heats up, illuminating the gas in a planetary nebula. Over about 20,000 years, that nebula will fade away, eventually becoming invisible. (WIKIMEDIA COMMONS USER SZCZUREQ)

Upon exhausting their core’s nuclear fuel, Sun-like stars die in a predictable fashion.

Near the end of a Sun-like star’s life, it begins to blow off its outer layers into the depths of space, forming a protoplanetary nebula like the Egg Nebula, seen here. Its outer layers have not yet been heated to sufficient temperatures by the central, contracting star to create a true planetary nebula just yet. (NASA AND THE HUBBLE HERITAGE TEAM (STSCI / AURA), HUBBLE SPACE TELESCOPE / ACS)

The core contracts, forming white dwarfs, which heats and illuminates the blown-off outer layers, creating planetary nebulae.

This Hubble Space Telescope image of the Helix Nebula shows a typical planetary nebula/white dwarf combination: the result of a Sun-like star reaching the end of its life. The central white dwarf is much fainter than a standard star, but is very hot and emits ionizing radiation. The illuminated nebula is made of ejecta from the star’s outer layers, and is illuminated by the central stellar remnant. (NASA, ESA, AND C.R. O’DELL (VANDERBILT UNIVERSITY))

These nebulous remnants persist for ~20,000 years, experiencing slow, gradual changes.

After 20 years of Hubble observations, however, the Stingray Nebula appears doubly special.

This animation shows how significant the fading of the Stingray Nebula has been since 1996. Note the background star, just to the upper left of the central, fading white dwarf, which remains constant over time, which confirms that the nebula itself is dimming significantly. (NASA, ESA, B. BALICK (UNIVERSITY OF WASHINGTON), M. GUERRERO (INSTITUTO DE ASTROFÍSICA DE ANDALUCÍA), AND G. RAMOS-LARIOS (UNIVERSIDAD DE GUADALAJARA))

First, it’s faded away tremendously, becoming far less luminous.

Normally, a planetary nebula will appear similar to the Cat’s Eye Nebula, shown here. A central core of expanding gas is lit up brightly by the central white dwarf, while the diffuse outer regions continue to expand, illuminated far more faintly. This is in contrast to the Stingray Nebula, which appears to be contracting. (NORDIC OPTICAL TELESCOPE AND ROMANO CORRADI / WIKIMEDIA COMMONS / CC BY-SA 3.0)

Second, the shells of gas are contracting and diffusing, appearing less crisp.

The Dumbbell Nebula, as imaged here through an 8" amateur telescope, was the first planetary nebula ever discovered: by Charles Messier in 1764. The shells of gas are slowly expanding and their definition remains constant over time, typical for a planetary nebula. The Stingray Nebula, somehow, is different. (MIKE DURKIN; MADMIKED/FLICKR)

These changes are unprecedented, but different elemental signatures reveal clues.

This image from NASA’s Chandra X-ray Observatory shows the location of different elements in the Cassiopeia A supernova remnant including silicon (red), sulfur (yellow), calcium (green) and iron (purple). Each element reveals its own particular spectral signature and set of photometric emissions, enabling us to map out the location of various elements in all sorts of stellar remnants and nebulae. (NASA/CXC/SAO)

Nitrogen and hydrogen emissions substantially decreased, but oxygen emissions plummeted almost a thousandfold.

This 2016 image from the Hubble Space Telescope, of the Stingray Nebula, brings out all the details in the nebula to the best of the image’s ability, revealing a much fainter and less sharply defined nebula than earlier images. The central star has cooled significantly from its peak of 60,000 K, which it rose to from the 1970s until about ~2000. The temperature has been dropping ever since. (ESA/HUBBLE & NASA)

This is driven by the central star’s temperature changes: rising from ~22,000 K to ~60,000 K previously, and now dropping rapidly.

This image from ESO’s Very Large Telescope shows the glowing green planetary nebula IC 1295 surrounding a dim and dying star located about 3300 light-years away. The green color arises from emission line transitions in the ionized gas surrounding the dim, dying star. Typically, green colors only appear from doubly ionized oxygen, requiring temperatures of ~50,000 K or above. (ESO / FORS INSTRUMENT)

At 50,000 K, oxygen loses two electrons, getting doubly ionized, emitting a brilliant green glow.

The Asymptotic Giant Branch star, LL Pegasi, is shown with its ejecta, along with a cutaway of its core. Surrounding the carbon-oxygen core is a shell of helium, which can fuse at the interface of the carbon-oxygen core. In the remnant powering the Stingray nebula, even though the outer hydrogen and helium has been mostly ejected, a transient helium-burning shell likely heated this remnant extremely recently, which now fades away. (ALMA (ESO/NAOJ/NRAO) / HYOSUN KIM ET AL. (MAIN); NOAO (INSET))

This hints at a recent burst of fusion: where helium in a shell around the core ignited, illuminating the surroundings.

Initially, the Stingray Nebula, Hen 3–1357, exhibited bright blue shells near its center, as this 1996 image shows. It was touted as perhaps the youngest planetary nebula on record. Given its recent fading and dimming, that conclusion may be wildly incorrect. (NASA, ESA, B. BALICK (UNIVERSITY OF WASHINGTON), M. GUERRERO (INSTITUTO DE ASTROFÍSICA DE ANDALUCÍA), AND G. RAMOS-LARIOS (UNIVERSIDAD DE GUADALAJARA))

With that burst over, the nebula fades as the central engine cools.

The Stingray Nebula has faded dramatically, as this 2016 image shows as compared to earlier ones. It has dimmed in brightness and changed in shape, with the decreased oxygen emissions comprising the most notable change. The nebula no longer ‘pops’ against the bright background of empty space. (NASA, ESA, B. BALICK (UNIVERSITY OF WASHINGTON), M. GUERRERO (INSTITUTO DE ASTROFÍSICA DE ANDALUCÍA), AND G. RAMOS-LARIOS (UNIVERSIDAD DE GUADALAJARA))

Additionally, the gas contracts instead of expanding: something never previously observed.

The Medusa Nebula, shown here, is faint, diffuse, and shows a complex structure indicative of its old age. Planetary Nebulae only persist for about 10,000 to 20,000 years, and this one is apparently nearing the end of its life. As the gas becomes neutral or too diffuse to shine and the central white dwarf cools, the nebula fades away entirely. (JSCHULMAN555 / WIKIMEDIA COMMONS / MT. LEMMON SKYCENTER)

This planetary nebula could disappear entirely — a first — perhaps in merely 20–30 years.