A dying star’s last hurrah

Enlarge / The Butterfly Nebula, located just under 4,000 light-years from Earth in the constellation Scorpius, is a striking example of a planetary nebula, the end stage in the evolution of a small- to medium-sized star. The butterfly’s diaphanous “wings” consist of gas and dust that have been expelled from the dying star and illuminated from within by the star’s remaining core. The nebula’s symmetrical, double-lobed shape is a telltale sign that a companion star helped shape the outflowing gases. Both the primary star and its companion are hidden by the shroud of dust in the nebula’s center.

Billions of years from now, as our Sun approaches the end of its life and helium nuclei begin to fuse in its core, it will bloat dramatically and turn into what’s known as a red giant star. After swallowing Mercury, Venus, and Earth with hardly a burp, it will grow so large that it can no longer hold onto its outermost layers of gas and dust.

In a glorious denouement, it will eject these layers into space to form a beautiful veil of light, which will glow like a neon sign for thousands of years before fading.

The galaxy is studded with thousands of these jewel-like memorials, known as planetary nebulae. They are the normal end stage for stars that range from half the Sun’s mass up to eight times its mass. (More massive stars have a much more violent end, an explosion called a supernova.) Planetary nebulae come in a stunning variety of shapes, as suggested by names like the Southern Crab, the Cat’s Eye, and the Butterfly. But as beautiful as they are, they have also been a riddle to astronomers. How does a cosmic butterfly emerge from the seemingly featureless, round cocoon of a red giant star?

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Observations and computer models are now pointing to an explanation that would have seemed outlandish 30 years ago: Most red giants have a much smaller companion star hiding in their gravitational embrace. This second star shapes the transformation into a planetary nebula, much as a potter shapes a vessel on a potter’s wheel.

Enlarge / NASA’s new James Webb Space Telescope has revealed extraordinary details in the Southern Ring Nebula, a planetary nebula that lies around 2,500-light-years away in the constellation Vela. On the left, a near-infrared image shows spectacular concentric shells of gas, which chronicle the history of the dying star’s outbursts. On the right, a mid-infrared image easily distinguishes the dying star at the nebula’s center (red) from its companion star (blue). All of the gas and dust in the nebula was expelled by the red star.

The dominant theory of planetary nebula formation previously involved only a single star—the red giant itself. With only a weak gravitational hold on its outer layers, it sheds mass very rapidly near the end of its life, losing as much as 1 percent per century. It also churns like a boiling pot of water underneath the surface, causing the outer layers to pulse in and out. Astronomers theorized that these pulsations produce shock waves that blast gas and dust into space, creating what’s called a stellar wind. Yet it takes a great deal of energy to expel this material completely without having it fall back into the star. It cannot be any gentle zephyr, this wind; it needs to have the strength of a rocket blast.

After the star’s outer layer has escaped, the much smaller inner layer collapses into a white dwarf. This star, which is hotter and brighter than the red giant it came from, illuminates and warms the escaped gas, until the gas starts glowing by itself—and we see a planetary nebula. The whole process is very fast by astronomical standards but slow by human standards, typically taking centuries to millennia.

Until the Hubble Space Telescope launched in 1990, “we were pretty sure we were on the right track” toward understanding the process, says Bruce Balick, an astronomer at the University of Washington. Then he and his colleague Adam Frank, of the University of Rochester in New York, were at a conference in Austria and saw Hubble’s first photos of planetary nebulae. “We went out to get coffee, saw the pictures, and we knew that the game had changed,” Balick says.

Astronomers had assumed that red giants were spherically symmetrical, and a round star should produce a round planetary nebula. But that’s not what Hubble saw—not even close. “It became obvious that many planetary nebulae have exotic axisymmetric structures,” says Joel Kastner, an astronomer at the Rochester Institute of Technology. Hubble revealed fantastic lobes, wings, and other structures that weren’t round but were symmetric around the nebula’s main axis, as if turned on that potter’s wheel.


In early photos from ground-based observatories, the Southern Crab Nebula appeared to have four curved “legs” like a crab. But detailed images from the Hubble Space Telescope show that these legs are the sides of two bubbles that roughly form an hourglass shape. In the center of the bubbles are two jets of gas, with “knots” that may light up when they encounter the gas between the stars. The Southern Crab, located several thousand light-years from Earth in the constellation Centaurus, appears to have had two gas-releasing events. One around 5,500 years ago created the outer “hourglass,” and a similar event 2,300 years ago created the inner, much smaller one.

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