Billions of years from now, as our Sun nears the end of its life and the helium nucleus begins to fuse at its core, it will swell dramatically and become 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 spectacular descent, it would blast these layers out into space to form a beautiful veil of light, which would glow like a neon sign for thousands of years before fading away.
The galaxy is studded with thousands of these gem-like monuments, known as planetary nebulae. They are common end stages for stars that range from half the mass of the Sun to eight times its mass. (More massive stars have a more violent end, an explosion called a supernova.) Planetary nebulae come in surprisingly diverse shapes, as suggested by names such as southern crab, cat’s eye, and butterfly. But as beautiful as they are, they remain an enigma for astronomers as well. How does a cosmic butterfly emerge from the seemingly featureless, round cocoon of a red giant star?
Observations and computer models are now pointing to an explanation that would have seemed strange 30 years ago: A very small companion star is hidden in the gravitational embrace of most red giants. This second star shapes the transformation into a planetary nebula, just as a potter shapes a pot on a potter’s wheel.
The leading theory of planetary nebula formation previously involved only one star – the red giant itself. With only a weak gravitational hold on its outer layers, it loses mass very rapidly at the end of its life, losing as much as 1 percent per century. It also churns under the surface like a pot of boiling water, causing the outer layers to pulsate in and out. Astronomers theorize that these pulsations generate shock waves that blast gas and dust into space, called the stellar wind. Yet it requires a great deal of energy to completely eject this material without falling back into the star. It cannot be a gentle zephyr, this wind; It needs the power of a rocket explosion.
After the outer layer of the star is shed, 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, illuminates and heats the remaining 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, typically over centuries to millennia.
Until the launch of the Hubble Space Telescope in 1990, “we were pretty sure we were on the right track” toward understanding the process, says Bruce Bialik, an astronomer at the University of Washington. Then he and his colleague Adam Frank were at a conference at the University of Rochester in New York, Austria, and saw Hubble’s first photographs of planetary nebulae. “We went out to get coffee, looked at pictures, and we knew the game had changed,” Bialik says.
Astronomers assumed that the red giants were spherically symmetric, and that a round star should form a round planetary nebula. But that’s not what Hubble saw – not even close. “It has become clear that many planetary nebulae have exotic axial structures,” says Joel Kastner, an astronomer at the Rochester Institute of Technology. Hubble revealed spectacular lobes, wings and other structures that were not round but were symmetrical around the main axis of the nebula, as if a potter’s wheel had been turned.