Leaving so soon? Unusual planetary nebula fades mere decades after it arrived
IMAGE: TWO IMAGES OF THE STINGRAY NEBULA, LOCATED IN THE DIRECTION OF THE SOUTHERN CONSTELLATION ARA -- OR THE ALTAR -- CAPTURED 20 YEARS APART BY NASA'S HUBBLE SPACE TELESCOPE. THE... view more
CREDIT: NASA/ESA/BRUCE BALICK/MARTÍN GUERRERO/GERARDO RAMOS-LARIOS
Stars are rather patient. They can live for billions of years, and they typically make slow transitions -- sometimes over many millions of years -- between the different stages of their lives.
So when a previously typical star's behavior rapidly changes in a few decades, astronomers take note and get to work.
Such is the case with a star known as SAO 244567, which lies at the center of Hen 3-1357, commonly known as the Stingray Nebula. The Stingray Nebula is a planetary nebula -- an expanse of material sloughed off from a star as it enters a new phase of old age and then heated by that same star into colorful displays that can last for up to a million years.
The tiny Stingray Nebula unexpectedly appeared in the 1980s and was first imaged by scientists in the 1990s using NASA's Hubble Space Telescope. It is by far the youngest planetary nebula in our sky. A team of astronomers recently analyzed a more recent image of the nebula, taken in 2016 by Hubble, and found something unexpected: As they report in a paper accepted to the Astrophysical Journal, the Stingray Nebula has faded significantly and changed shape over the course of just 20 years.
If dimming continues at current rates, in 20 or 30 years the Stingray Nebula will be barely perceptible, and was likely already fading when Hubble obtained the first clear images of it in 1996, according to lead author Bruce Balick, an emeritus professor of astronomy at UW.
"This is an unprecedented departure from typical behavior for a planetary nebula," said Balick. "Over time, we would expect it to imperceptibly brighten and expand, which could easily go unnoticed in a century or more. But here we're seeing the Stingray nebula fade significantly in an incredibly compressed time frame of just 20 years. Moreover, its brightest inner structure has contracted -- not expanded -- as the nebula fades."
Planetary nebulae form after most stars, including stars like our own sun, swell into red giants as they exhaust hydrogen fuel. At the end of the red giant phase, the star then expels large amounts of its outer material as it gradually -- over the course of a million years -- transforms into a small, compact white dwarf. The sloughed-off material expands outward for several thousand years while the star heats the material, which eventually becomes ionized and glows.
Balick and his co-authors, Martín Guerrero at the Institute of Astrophysics of Andalusia in Spain and Gerardo Ramos-Larios at the University of Guadalajara in Mexico, compared Hubble images of the Stingray Nebula taken in 1996 and 2016. Hen 3-1357 changed shape markedly over 20 years, losing the sharp, sloping edges that gave the Stingray Nebula its name. Its colors have faded overall and once-prominent blue expanses of gas near its center are largely gone.
"In a planetary nebula, the star is really the center of all the activity," said Balick. "The material around it is directly responsive to the energy from its parent star."
The team analyzed light spectra from Hen 3-1357 emitted by chemical elements in the nebula. Emission levels of hydrogen, nitrogen, sulfur and oxygen all dropped between 1996 and 2016, particularly oxygen, which dropped by a factor of 900. The resulting fade in color and the nebula's change in shape are likely connected to the cooling of its parent star -- from a peak of about 107,500 degrees Fahrenheit in 2002 to just under 90,000 degrees Fahrenheit in 2015 -- which means it is giving off less ultraviolet ionizing radiation that heats the expelled gas and makes it glow.
"Like a doused forest fire, the smoke wanes more slowly than the flames that created it," said Balick. "Even so, we were amazed when the Hubble images revealed how quickly the nebula was fading. It took a month of work to believe it."
Astronomers have yet to understand why SAO 244567 made the Stingray Nebula light up and then fade almost as quickly. One theory, posited by a team led by Nicole Reindl at the University of Potsdam, is that the star underwent a brief burst of fresh helium fusion around its core, which stirred up its outer layers and caused its surface to both shrink and heat.
If so, then as its outer layers settle back down, the star may return to a more typical transition from red giant to white dwarf. Only future observations of the star and its nebula can confirm this.
"Unfortunately, the best tool to follow future changes in the Stingray Nebula, the Hubble Space Telescope, is near the end of its life as well," said Balick. "We can hope, but the odds aren't good for Hubble's survival as its three remaining gyroscopes start to fail. It's a good race to the finish."
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The Hubble Space Telescope is an international partnership between NASA and the European Space Agency, or ESA, and managed by NASA's Goddard Space Flight Center in Maryland. The Space Telescope Science Institute is responsible for Hubble science operations. The research was also funded by the European Union and the National Council of Science and Technology in Mexico.
Hubble captures unprecedented fading of
Stingray nebula
IMAGE: THIS IMAGE COMPARES TWO DRASTICALLY DIFFERENT PORTRAITS OF THE STINGRAY NEBULA CAPTURED BY NASA'S HUBBLE SPACE TELESCOPE 20 YEARS APART. THE IMAGE ON THE LEFT, TAKEN WITH THE WIDE FIELD... view more
CREDIT: CREDITS: NASA, ESA, B. BALICK (UNIVERSITY OF WASHINGTON), M. GUERRERO (INSTITUTO DE ASTROFÍSICA DE ANDALUCÍA), AND G. RAMOS-LARIOS (UNIVERSIDAD DE GUADALAJARA)
Astronomers have caught a rare look at a rapidly fading shroud of gas around an aging star. Archival data from NASA's Hubble Space Telescope reveal that the nebula Hen 3-1357, nicknamed the Stingray nebula, has faded precipitously over just the past two decades. Witnessing such a swift rate of change in a planetary nebula is exceeding rare, say researchers.
Images captured by Hubble in 2016, when compared to Hubble images taken in 1996, show a nebula that has drastically dimmed in brightness and changed shape. Bright, blue, fluorescent tendrils and filaments of gas toward the center of the nebula have all but disappeared, and the wavy edges that earned this nebula its aquatic-themed name are virtually gone. The young nebula no longer pops against the black velvet background of the vast universe.
"This is very, very dramatic, and very weird," said team member Martín A. Guerrero of the Instituto de Astrofísica de Andalucía in Granada, Spain. "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."
Researchers discovered unprecedented changes in the light emitted by glowing nitrogen, hydrogen, and oxygen being blasted off by the dying star at the center of the nebula. The oxygen emission, in particular, dropped in brightness by a factor of nearly 1,000 between 1996 and 2016.
"Changes in nebulae have been seen before, but what we have here are changes in the fundamental structure of the nebula," said Bruce Balick of the University of Washington, Seattle, leader of the new research. "In most studies, the nebula usually gets bigger. Here, it's fundamentally changing its shape and getting fainter, and doing so on an unprecedented time scale. Moreover, to our surprise, it's not growing any larger. Indeed, the once-bright inner elliptical ring seems to be shrinking as it fades."
Ground-based observations of other planetary nebulae have shown hints of changes in brightness over time, but those speculations haven't been confirmed until now. Only Hubble can resolve the changes in structure in this tiny nebula. The new paper examines every image of the Stingray nebula from Hubble's archives.
"Because of Hubble's optical stability, we are very, very confident that this nebula is changing in brightness with time," added Guerrero. "This is something that can only be confirmed with Hubble's visual acuity."
The researchers note the nebula's rapid changes are a response to its central star, SAO 244567, expanding due to a temperature drop, and in turn emitting less ionizing radiation.
A 2016 study by Nicole Reindl, now of the University of Potsdam, Germany, and a team of international researchers, also using Hubble data, noted the star at the center of the Stingray nebula, SAO 244567, is special in its own right.
Observations from 1971 to 2002 showed the temperature of the star skyrocketing from less than 40,000 to 108,000 degrees Fahrenheit, more than ten times hotter than the surface of our Sun. Now, Reindl and her research team has shown that SAO 245567 is cooling. Reindl speculates the temperature jump was caused by a brief flash of helium fusion that occurred in a shell around the core of the central star. Recently, the star appears to be backstepping into its early stage of stellar evolution.
"We're very lucky to observe it just in that moment," said Reindl. "During such a helium shell flash, it evolves very quickly, and that implies short evolutionary timescales, so we can't usually see how these stars evolve. We just happened to be there at the right time to have caught that."
The team studying the rapid fading of the Stingray nebula can only speculate at this time what's in store for the future of this young nebula. At its present rates of fading, it's estimated the nebula will barely be detectable in 20 or 30 years.
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The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.
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