Astronomers have watched a dead star, in the guise of the Helix Nebula, being recycled back into the interstellar gas from which it originally came, bringing the story of that star full circle.
Stars can last for millions, or even billions, of years, but they do not last forever. When a sun-like star reaches the end, it blossoms into a beautiful planetary nebula. The Helix Nebula, located 650 light-years away in the constellation of Aquarius, the Water Bearer, is one such example of a planetary nebula.
The Helix's central star has run out of hydrogen needed for nuclear fusion reactions in its core. This caused the core to begin to contract and raised the temperature in the star's outer layers, which ignited fusion reactions there instead. This had the effect of causing the star to bloat into a red giant. Then, eventually, the outer layers of that red giant detached and expanded into space to form the Helix Nebula. Meanwhile, the star's inert core contracted and became a white dwarf.
As the nebula expands, it gradually disperses into the interstellar medium (ISM), feeding the elements formed within the star back into interstellar gas clouds. These clouds will eventually produce the next generation of stars. However, astronomers had never actually seen the point at which a dead star's remains are absorbed into the ISM — until now.
"We are seeing material shed near the end of a star's life being broken apart and returned to the galaxy," said Yale University's Pieter van Dokkum, who led the study, in a statement. "That hand-off — from recognizable stellar debris to the diffuse gas between the stars — has been very difficult to observe."
Van Dokkum's team were not actually looking for this. Instead, they were conducting routine calibration of a new astronomical instrument called MOTHRA, the Modular Optical Telephoto Hyperspectral Robotic Array at the El Sauce Observatory in Chile. When complete, MOTHRA will be armed with 1,140 telephoto lenses that will scrutinize the night sky while looking for traces of faint gas in the Milky Way galaxy.
Astronomical instruments require calibration before use, and this is accomplished by targeting a familiar object in the sky and tuning the instruments to how that object should appear. The MOTHRA team used the Helix Nebula to calibrate the telephoto lenses that they currently have operational, and that was when they found something unexpected.
"We thought we were taking a calibration image of one of the best-known nebulas in the sky," Roberto Abraham of the University of Toronto said in the statement. "Instead, we found this extraordinary network of bow-shaped structures. It was immediately clear that the faint outer Helix was telling us a story that had largely been missed."
The Helix Nebula appears donut-shaped and has been imaged countless times before by both professional observatories such as the Hubble Space Telescope and James Webb Space Telescope, and by amateur astronomers whose long exposures can reveal an outer halo of gas around the nebula's bright ring. To give an idea of scale, the outer edge of the bright ring is 5.7 light-years across, and the outer halo exists beyond even this.
The team discovered 22 complete or partial arc-shaped clumps of gas in the outer halo. They are called bow shocks and they arise from the motion of something pressing into another medium, in this case clumps of gas that have expanded out from the planetary nebula running into and plowing through the ISM. It is similar to the bow-shaped waves that ride ahead of the prow of a boat in water.
"The shocks change dramatically with distance from the central star," Imad Pasha of Dragonfly Focused Research Organisation, said in the statement.. "Those nearer the center are large, thin and sharply defined. Further out, they become smaller, fuzzier and increasingly fragmented."
This implies a progression, and that the deeper into the ISM the clumps of the star's ejected material press, the more they become eroded and dispersed back into the interstellar gas. The team estimate that material from the planetary nebula can survive for about 10,000 years after encountering the ISM before they are completely shredded and absorbed back into it.
Stars are born from the molecular gas clouds that lie in interstellar space, and these clouds are enriched with the elements produced by fusion reactions within previous generations of stars. Now, thanks to MOTHRA's observations of the Helix Nebula, we have seen these elements being gifted to the Milky Way at large, and in millions or billions of years they will contribute to a new generation of stars, planets and possibly life.
Our own solar system, and Earth along with it, were formed 4.6 billion years ago from elements produced by dead stars. In another five billion years' time, our sun will also exhaust its supply of hydrogen and its fusion reactions will cease, causing it to unfurl into its own planetary nebula.
"Far in the future the sun will go through a similar process and its material will enter the same cycle," says van Dokkum.
All the elements and molecules that make up you and everything around you will be given back to the Milky Way, and one day it will be reborn again and begin the story of a new star system. What will that story bring, we wonder?
The findings from the Helix Nebula were published on Aug. 12 in the journal Nature.
