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The stars are quite patient. They can live billions of years and typically make slow transitions, sometimes over many millions of years, between different stages of their life.
So when the behavior of a previously typical star changes rapidly in a few decades, astronomers take notice and get to work.
This is the case with a star known as SAO 244567, which is located in the center of Hen 3-1357, commonly known as the Stingray Nebula. The Stingray Nebula is a planetary nebula: an expanse of material detached from a star as it enters a new phase of old age and then heated by the same star in colored screens that can last up to a million years.
The tiny Stingray Nebula appeared unexpectedly in the 1980s and was first photographed 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 reported in an article accepted by the Astrophysical Journal, the Stingray Nebula has faded significantly and changed form over the course of just 20 years.
If the dimming continues at current rates, in 20 to 30 years the Stingray Nebula will be barely noticeable and was probably already fading by the time Hubble got its first sharp images of it in 1996, according to lead author Bruce Balick, professor emeritus of astronomy at UW. .
“This is an unprecedented deviation from the typical behavior of a planetary nebula,” Balick said. “Over time, we would expect it to light up and expand subtly, which could easily go unnoticed in a century or more. But here we are seeing the Stingray Nebula significantly vanish over an incredibly compressed time frame of just 20 years. brighter interior has contracted, not expanded, as the nebula fades. “
Planetary nebulae form after most stars, including stars like our sun, swell into red giants as they run out of hydrogen. At the end of the red giant phase, the star then expels large amounts of its outer material while gradually, over the course of a million years, it transforms into a small compact white dwarf. The moved material expands outward for several thousand years as the star heats the material, which eventually becomes ionized and glows.
Balick and his co-authors, Martín Guerrero of the Institute of Astrophysics of Andalusia in Spain and Gerardo Ramos-Larios of the University of Guadalajara in Mexico, compared Hubble images of the Stingray Nebula taken in 1996 and 2016. Hen 3-1357 it has changed shape dramatically in 20 years, losing the sharp, sloping edges that gave the Stingray Nebula its name. Its colors have faded overall and the blue expanses of gas near the center have largely disappeared.
“In a planetary nebula, the star is really the center of all activity,” Balick said. “The material surrounding it responds directly to the energy of its parent star.”
The team analyzed the light spectra of Hen 3-1357 emitted by chemical elements in the nebula. Emission levels of hydrogen, nitrogen, sulfur and oxygen all declined between 1996 and 2016, especially oxygen, which decreased by a factor of 900. The resulting fade in color and shape change of the nebula are likely linked 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 that it emits less ultraviolet ionizing radiation that heats the ejected gas and makes it glow.
“Like an extinguished forest fire, the smoke decreases slower than the flames that created it,” Balick said. “Even so, we were stunned when the Hubble images revealed how fast the nebula was fading. It took a month of work to believe.”
Astronomers have yet to understand why SAO 244567 caused the Stingray Nebula to illuminate and then vanish almost as quickly. One theory, hypothesized by a team led by Nicole Reindl at the University of Potsdam, is that the star underwent a brief blast of fresh helium fusion around its core, which shook its outer layers and caused it to shrink and shrink. heat of its surface.
In that case, as its outer layers stabilize, the star may revert to a more typical transition from red giant to white dwarf. Only future observations of the star and its nebula can confirm this.
“Unfortunately, even the best tool for tracking future changes in the Stingray Nebula, the Hubble Space Telescope, is near the end of its life,” Balick said. “We can hope, but the odds are not good for Hubble’s survival as its three remaining gyros begin to fail. It’s a good race to the finish.”
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