The life cycle of stars is very long, eternal for the human conception of time. They take millions of years to be born and their adult stage extends over billions of years. When those that are like our Sun deplete the hydrogen nucleus that serves as their energy, they expand and expel their outer layers forming a bright planetary nebula that, in turn, will take tens of thousands of years to fade. Well, the Hubble space telescope has captured something unprecedented: the disappearance of the manta ray nebula, the youngest known, in just two decades. As researchers from the Higher Council for Scientific Research (CSIC) explain, nothing similar has ever been seen in real time before.
The Manta Ray Nebula, or Hen3-1357, was introduced as the youngest known planetary nebula in 1998: it was estimated that the core of the central star had only been producing enough energy for twenty years to ionize the envelope of gas that had formed. formed as a result of the expulsion of the outer layers. The data also showed that the central star had warmed faster than expected considering its low mass. And its size is equivalent to one tenth of the usual dimensions of planetary nebulae
But the quirks of this cosmic stingray don’t end there. A new job shows that it has faded over the past two decades, and that the layers of gas surrounding the central star have faded.
“They are dramatic and strange changes,” says Martín A. Guerrero, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who is participating in the study. “We are witnessing the evolution of a nebula in real time, and we see variations in a few years. We have never seen this so clearly, “he adds.
Comparing the images obtained by Hubble in 2016 and those obtained in 1996 shows how the nebula has lost its brightness and changed shape. The fluorescent gas tentacles and filaments in the central regions have all but disappeared, and the curvilinear edges that suggested their association with stingrays have all but faded.
Researchers have documented unprecedented changes in the light emitted by nitrogen, hydrogen and oxygen expelled by the dying star in the center of the nebula. The emission of oxygen, in particular, dimmed in brightness by a factor of nearly a thousand between 1996 and 2016.
“We have seen changes in nebulae before, but what we have here are changes in the fundamental structure of the nebula,” says Bruce Balick, a researcher at the University of Washington Seattle (USA), who is leading the research. In most cases, the nebula grows larger. Here, instead, it is changing its shape and weakening on an unprecedented time scale. Also, to our surprise, it is not growing; in fact, the inner elliptical ring that was shiny seems to shrink as it fades. ‘
Observations from the ground had shown signs of variability in brightness over time in other planetary nebulae, but these indications have not been confirmed until now. ‘Due to the optical stability of the Hubble Space Telescope, we are very, very confident that this nebula is changing in brightness over time. This is something that can only be confirmed with the visual acuity of Hubble, ”says Guerrero.
The researchers note that the nebula’s rapid changes are a response to its central star, SAO 244567, whose surface temperature soared to 60,000 degrees, ten times the temperature of the Sun, in a short time between 1971 and 2002. Since then it has experienced a gradual descent to 22,000 degrees, so the star is unable to produce enough photons to keep the nebula ionized.
“It is difficult to know what his final destination will be. Perhaps the central star will heat up again and ionize the nebula, or perhaps it never will and Hen3-1357 will become a failed planetary nebula, “concludes Guerrero. At current fading rates, the nebula is estimated to be barely detectable in 20-30 years.