Time, even astronomical time, is made of moments. The universe began in a moment around 13.7 billion years ago. The sun was born in a moment some 4.6 billion years ago, when hydrogen atoms began fusing into helium. At another moment, some 5 billion years from now, the sun will exhaust its fuel and begin a slow death. Somewhere in the Milky Way galaxy, a star like our sun dies roughly every month. When that happens, it briefly exhales an extraordinarily beautiful, complex puff of gas known as a planetary nebula.

At the center of the Red Spider nebula lies a white dwarf that is nearly 1 million degrees Fahrenheit. A wind of fragmented atoms blows off it at millions of miles per hour. This star is a recent corpse: The surrounding gas, fluorescing and whipping into waves, was once the star's atmosphere but was cast off when fusion reactions stopped in its core. The nebula's spidery shape is a perspective illusion. “The 3-D structure is somewhat like an hourglass,” says astronomer Garrett Mellema of Leiden University in the Netherlands.


Image Courtesy of ESA

The term is a misnomer: Observing a number of vaguely round, cloudlike objects in the sky during the late 18th century, Sir William Herschel thought they resembled faint planets. Planetary nebulas are the inevitable fate of the run-of-the-mill stars that account for more than 90 percent of the population of the Milky Way. Stars more than eight times as massive as our sun explode as brilliant supernovas, but such events happen just a few times a century in our galaxy. Planetary nebulas are the 15 minutes of fame granted to lesser stars. 

 

Courtesy of NRAO/AUI/NSF

Courtesy of NASA/Hubble/STScI/Aura

Courtesy of NASA/ESA/Hubble

Courtesy of NASA/Hubble/STScI/Aura

Courtesy of NASA/Hubble/STScI/Aura

The Garden Sprinkler (center) is an aging star, not yet a white dwarf, that is shedding its atmosphere in pulsing jets, like the arcs of water that hang in the air as a lawn sprinkler twirls. K3-35, seen as a radio image (top left), also has precessing gas jets. The dark “patty” in Gomez’s Hamburger (top right) is a disk of dust that conceals a dying star; the “bun” is dust that reflects the hidden star’s light. In the Egg nebula (bottom right), high-speed jets from the central star have burst through another disklike cocoon of dust. The Calabash nebula (bottom left), illustrates what happens when high-speed winds of stellar gas (yellow) create shock waves as they ram into clouds of interstellar hydrogen and nitrogen (blue). 

The death of a sunlike star was long thought to be a gentle, orderly process, every star more or less the same. But over the past decade, the Hubble Space Telescope-along with a group of powerful new observatories on the ground-has allowed astronomers to watch this dying stage with new understanding. “With the Hubble, we’ve been able to see things we simply couldn’t see before,” says C. Robert O’Dell of Vanderbilt University, who as project scientist oversaw the construction of the space telescope and has observed many nebulas with it since.

When a star runs out of hydrogen fuel in its center, the core contracts and heats up, inflating the star into a bloated red giant. The sun, in this phase, will reach past the orbit of Venus and may well engulf Earth. Meanwhile, the core becomes hot and dense enough to fuse helium, a flukier fuel. The star starts to pulse ominously. And over the next hundred thousand years, it blows most of its atmosphere off into space like so many smoke rings. What remains behind is a nearly naked core of carbon and oxygen, which collapses to form a white dwarf star, roughly the size of Earth but 100,000 times as dense.