Last hurray dying star

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 small to medium-sized stars. The silent butterfly’s “wings” consist of gas and dust that have been ejected from the dying star and illuminated from within by the remaining stellar core. The nebula’s symmetrical, double-lobed shape is a sign that a companion star is helping to form the escaping gas. Both the main star and its companion are hidden by a shroud of dust at the center of the nebula.

Billions of years from now, as our Sun nears the end of its life and the helium core begins to coalesce at its core, it will swell dramatically and turn 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 layer of gas and dust.

In the glorious end, it will eject these layers into space to form a beautiful veil of light, which will shine like a neon light for thousands of years before fading away.

The galaxy is filled with thousands of these gem-like memorials, known as planetary nebulae. They are normal late stages for stars that range from half the mass of the Sun to eight times its mass. (More massive stars have far more violent endings, explosions called supernovae.) Planetary nebulae come in an astonishing variety of forms, as suggested by names such as Southern Crab, Cat’s Eye, and Butterfly. But as beautiful as they are, they are also an enigma to astronomers. How did the cosmic butterfly emerge from the seemingly shapeless, spherical cocoon of a red giant star?

Observations and computer models now suggest an explanation that would have seemed strange 30 years ago: Most red giants have much smaller companion stars lurking in their gravitational embrace. This second star transforms into a planetary nebula, just as a potter forms a vessel on a potter’s wheel.

Enlarge / NASA’s new James Webb Space Telescope has revealed incredible detail in the Southern Ring Nebula, a planetary nebula located about 2,500 light-years away in the constellation Vela. On the left, the near-infrared image shows a spectacular concentric shell of gas, which records the exploding history of a dying star. On the right, the mid-infrared image easily distinguishes the dying star at the center of the nebula (red) from its companion star (blue). All the gas and dust in the nebula is ejected by the red star.

The dominant theory of planetary nebula formation previously involved only one star—the red giant itself. With only a weak gravitational attraction in its outer layers, it sheds mass very rapidly towards the end of its life, losing as much as 1 percent per century. It also churned like a pot of boiling water beneath the surface, causing the outer layer to pulse in and out. Astronomers theorize that this pulsation produces shock waves that blast gas and dust into space, creating what is called a stellar wind. But it takes a lot of energy to eject this material completely without having to fall back into the star. There couldn’t be a gentle breeze, this wind; it needs to have rocket blast power.

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After the outer layer of the star detaches, the much smaller inner layer collapses into a white dwarf. This star, hotter and brighter than the original red giant, illuminates and warms 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 to thousands of years.

Until the Hubble Space Telescope was launched in 1990, “we were pretty sure we were on the right track” towards understanding the process, said 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 the first photographs of a planetary nebula by Hubble. “We went out for coffee, saw the pictures, and we knew the game had changed,” Balick said.

Astronomers assume that red giants have a symmetrically spherical shape, and that a spherical star should produce a spherical planetary nebula. But that’s not what Hubble saw—not even close. “It became clear that many planetary nebulae have an exotically symmetrical axis structure,” said Joel Kastner, an astronomer at the Rochester Institute of Technology. Hubble revealed fantastic lobes, wings, and other structures that are not round but symmetrical around the nebula’s main axis, as if it were spinning on the potter’s wheel.

In early photographs from ground-based observatories, the Southern Crab Nebula appears to have four curved “legs” like crabs. 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 bubble are two jets of gas, with “knots” that may light up when they encounter gas between the stars. The Southern Crab, located a few thousand light-years from Earth in the constellation Centaurus, 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, which is much smaller.

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