The last hurray of a dying star

Nearly 4,000 light-years from Earth in the constellation of Scorpio, the Butterfly Nebula is a striking example of a planetary nebula, the final stage in the evolution of a small to medium-sized star.  The butterfly's translucent
Enlarge / Nearly 4,000 light-years from Earth in the constellation of Scorpio, the Butterfly Nebula is a striking example of a planetary nebula, the final stage in the evolution of a small to medium-sized star. The butterfly’s translucent “wings” are made of gas and dust ejected from the dying star and illuminated from within by the star’s remaining core. The nebula’s symmetrical, bilobed shape is a telltale sign that a companion star helped shape the outflowing gases. Both the primary star and its companion are obscured by the layer of dust at the center of the nebula.

Billions of years from now, as our Sun nears the end of its life and helium nuclei begin to fuse at its core, it will puff up dramatically and transform into what is known as a red giant star. After swallowing Mercury, Venus, and Earth with barely a belch, it grows so large that it can no longer hold its outermost layers of gas and dust.

In a glorious dissolution, these layers are hurled into space to form a beautiful veil of light that will glow like a neon sign for thousands of years before fading.

The galaxy is littered with thousands of these jewel-like monuments known as planetary nebulae. They are the normal final stage for stars ranging from half to eight times the mass of the Sun. (More massive stars have a much more violent end, an explosion known as a supernova.) Planetary nebulae come in an amazing variety of shapes, as suggested by names like southern crab, cat’s eye, and butterfly. But as beautiful as they are, they have also been a mystery to astronomers. How does a cosmic butterfly emerge from the seemingly shapeless, round cocoon of a red giant star?

Observations and computer models now point to an explanation that seemed outlandish 30 years ago: most red giants have a much smaller companion star tucked away in their gravitational embrace. This second star is shaping the transformation into a planetary nebula much like a potter shaping a vessel on a potter’s wheel.

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

The prevailing theory of the formation of planetary nebulae previously involved only a single star – the red giant itself. With only a weak gravity on its outer layers, it loses mass very rapidly towards the end of its life, losing up to 1 percent per century. It’s also bubbling like a boiling pot of water beneath the surface, causing the outer layers to flow in and out. Astronomers theorized that these pulsations create shock waves that explode gas and dust in space, creating a so-called stellar wind. But it takes a lot of energy to fully eject this material without falling back into the star. It cannot be a gentle zephyr, this wind; it must have the power of a rocket blast.

After the star’s outer layer escapes, the much smaller inner layer collapses into a white dwarf. Hotter and brighter than the red giant it came from, this star illuminates and heats the escaping gas until the gas begins to glow of its own accord – and we see a planetary nebula. The entire process is very fast by astronomical standards, but slow by human standards, typically lasting centuries to millennia.

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

Astronomers had assumed that red giants are spherically symmetric and that a round star should produce a round planetary nebula. But Hubble didn’t see that — not even close. “It became apparent 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 were not round but symmetrical about the nebula’s main axis, as if being twirled on a potter’s wheel.

In early photos from ground-based observatories, the Southern Crab Nebula appeared to have four curved

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 those legs are the sides of two bubbles that form roughly an hourglass shape. At the center of the bubbles are two jets of gas with “nodes” that can light up when they hit the gas between the stars. Located several thousand light-years from Earth in the constellation Centaurus, the southern crab appears to have had two gas release events. One about 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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