The glorious, billowing Southern Ring Nebula is the cocoon of a dying star - and it has a secret. Scientists have discovered that this nebula exhibits a double ring structure, revealing not one, but possibly three stars at its heart.
The Southern Ring Nebula, also called NGC 3132, is a planetary nebula located about 2,000 meters from Earth. light years away in the constellation Vela, the Sails. The name "planetary mist' is a misnomer; such mists have nothing to do with it planets. Instead, they are the last exhalations of dying, Sun-like it stars, which transform into the faint chrysalis until they eventually grow into a white dwarf. A nebula is formed from the outer envelope of the dying star, which is then blown inward room following the star red giant phase.
The Southern Ring Nebula was imaged in December 2022 by the James Webb Space Telescope (JWST), which showed that molecular hydrogen gas forms the nebula's 'exoskeleton'. This refers to warm gas that radiates at a temperature equal to approximately 1,000 Kelvin (1,340 degrees Fahrenheit or 726 degrees Celsius) when illuminated and heated by ultraviolet light coming from the white dwarf yourself. However, that exoskeleton represents only a small fraction of the molecular gas in the nebula.
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A team led by Joel Kastner of the Rochester Institute of Technology set out to hunt for more molecular gas in the nebula, specifically looking for carbon monoxide gas using the Submillimeter Array (SMA), a group of eight radio telescopes on an inactive volcano called Mauna. Kea in Hawaii. Carbon monoxide mixes with hydrogen and other molecular gases in the nebula, so observing carbon monoxide levels is actually indicative of observing all those other molecules that aren't as easy to detect. Sure enough, the SMA was able to measure both the distribution and speeds of the carbon monoxide molecules, showing which parts are moving toward us and which are moving away from us.
"JWST showed us the hydrogen molecules and how they stack up in the air, while the Submillimeter Array shows us the carbon monoxide which is colder than you can't see in the JWST image," Kastner said in a press statement.
As the name of the Southern Ring suggests, it is (from our point of view) mainly shaped like a ring. The SMA observations showed that this ring is expanding, which is expected because the nebula grows slowly before finally spreading. However, the data also allowed Kastner's team to create a three-dimensional map of the nebula's molecular exoskeleton. This offered a surprise. Not only were the researchers able to show that what we see as a ring is just a lobe in a bipolar nebula when viewed from the end, but they also found a second ring perpendicular to the first.
"When we started rotating the entire nebula in 3D, we immediately saw that it was really a ring, and then we were amazed to see that there was another ring," Kastner said.
The whole bizarre arrangement paints a fascinating tail of not one, not even two, but possibly three stars at the heart of the nebula. Only one of these stars, the most massive of the three, will have reached the end of its life - but the stellar trio, if all three actually exist, will likely be too close together or too faint to be seen separately be resolved even if by the JWST.
There is growing evidence that some planetary nebulae, at least those with complex structures, are formed by the intervention of a companion star within the central dying star. For the Southern Ring, Kastner's team proposes that there is a three-tier system consisting of a closure binary is circled by a more distant third star within an orbital radius of 60° astronomical units of the binary number (one astronomical unit, AU, is the distance between Soil and the sun, and in ours solar system 60 AU would be at the very edge of the border Kuiper belt).
The two lobes of the Southern Ring have a narrow or 'pinched' waist, like an hourglass, which is a common feature of planetary nebulae that arise from a binary star system in which one of the stars is reaching the end of its life. The binary star is able to corral the material shed by the dying star so that it escapes in a polar, rather than equatorial, direction, forming the two lobes. JWST's mid-infrared observations support this hypothesis, as it finds an excess of infrared light emanating from the central galaxy, which is a classic feature of a dusty disk formed by interactions between the red giant and a nearby binary star.
So that explains the first ring. The origin of the second ring is less certain, according to the team.
Although the Southern Ring appears bilobed, some material must have been ejected as a roughly spherical or elliptical envelope of material shed by the red giant, a rapid mass loss event that perhaps represented the final exhalation of material to leave the white giant behind. dwarf. The binary star system produces a series of fast, narrow jets, but if a third star is present, the extra star will gravity would act on the inner binary, causing the jets' direction to "wobble", like a spinning top. These precessing jets would have carved a circular cavity into the elliptical component of the nebula, creating the second ring.
Kastner emphasizes that this explanation is still speculative, but that the central ionized cavity of the nebula does show traces of such jets in its structure.
Other annular planetary nebulae, such as the Helix Nebula (NGC 7293 in Aquarius), they have also been shown to have bilobed structures that allow us to look at the end of one lobe. The discovery of the second ring in the Southern Ring Nebula - or should that be Southern Rings, plural? - is prompting astronomers to revisit some of those other well-known ring nebulae to see if they've missed second rings in them, too.
Planetary nebulae don't just mean the death of stars. They also hold the promise of new life - literally, in a sense.
"Where does the carbon and the oxygen and the nitrogen end up the universe come from?" Kastner wonders. "We see it originating in the Sun-like stars that are dying, like the star that just died and created the Southern Ring."
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As an expanding planetary nebula spreads into interstellar space it spreads these molecules through the cosmos, where they end up in gigantic molecular clouds that form the next generation of stars and planets.
"A lot of that molecular gas would end up in planetary atmospheres, and atmospheres could support life," Kastner says. All elements on Earth that are heavier than hydrogen and helium were formed in stars and then ejected into space when those stars died.
We are literally star people, as many experts like to say.
So as we marvel at the beauty of star death in nebulae like the Southern Ring, we can also imagine it as a stellar phoenix that will one day rise from the ashes and begin the cycle of star birth and death all over again. To quote Battlestar Galactica, this has all happened before and this will all happen again.
The findings were published on April 2 in The Astrophysical Journal.
