The last hurrah of a dying star

The last hurrah of a dying star

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 final stage in the evolution of a small to medium-sized star. .  The
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 final stage in the evolution of a small to medium-sized star. . The diaphanous “wings” of the butterfly are made of gas and dust that have been expelled from the dying star and illuminated from within by the remaining core of the star. The nebula’s symmetrical double-lobe shape is a telltale sign that a companion star helped shape the outgoing gases. The primary star and its companion are hidden by the veil of dust in 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 in its core, it will swell dramatically and transform into what is known as a red giant star. . After swallowing Mercury, Venus, and Earth with barely a burp, it will grow so large that it can no longer hold its outermost layers of gas and dust.

In a glorious ending, 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 away.

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

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

NASA's new James Webb Space Telescope has revealed extraordinary detail in the South Ring Nebula, a planetary nebula located about 2,500 light-years away in the constellation Vela.  On the left, a near-infrared image shows spectacular concentric shells of gas, which tell the story of the dying star's explosions.  On the right, a mid-infrared image easily distinguishes the dying star in the center of the nebula (red) from its companion star (blue).  All the gas and dust in the nebula was expelled by the red star.
Enlarge / NASA’s new James Webb Space Telescope has revealed extraordinary detail in the South Ring Nebula, a planetary nebula located about 2,500 light-years away in the constellation Vela. On the left, a near-infrared image shows spectacular concentric shells of gas, which tell the story of the dying star’s explosions. On the right, a mid-infrared image easily distinguishes the dying star in the center of the nebula (red) from its companion star (blue). All the gas and dust in the nebula was expelled by the red star.

The prevailing theory of the formation of planetary nebulae previously involved only one star, the red giant itself. With only a weak gravitational grip on its outer layers, it loses mass very rapidly towards the end of its life, losing up to 1% per century. It also bubbles like a pot of boiling water below the surface, causing the outer layers to move in and out. Astronomers have theorized that these pulsations produce shock waves that shoot gas and dust out into space, creating what is called a stellar wind. However, it takes a lot of energy to completely expel this material without causing it to fall back into the star. It can be no gentle zephyr, this wind; it must have the force of a rocket blast.

Once the outer layer of the star has escaped, the much smaller inner layer collapses into a white dwarf. This star, which is hotter and brighter than the red giant from which it came, lights up and heats the escaping gas, until the gas begins to glow on its own – and we see a planetary nebula . The whole process is very fast by astronomical standards but slow by human standards, usually taking centuries or even millennia.

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

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

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

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 those legs are the sides of two bubbles that roughly form an hourglass shape. At the center of the bubbles are two jets of gas, with “nodes” that can ignite when they encounter the gas between stars. The Southern Crab, located several thousand light-years from Earth in the constellation Centaurus, appears to have had two gas release events. About 5,500 years ago, an event created the outer “hourglass”, and a similar event 2,300 years ago created the much smaller interior.

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