Space / Telescopes

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.

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. 

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.