Every star that now shines will one day die, but some stars live far longer than others. Our 4.6-billion-year-old sun will shrivel into a white dwarf in 7.8 billion years. Now astronomers say a dim red star south of the constellation Orion will outlive any other yet examined. "It actually will live for much longer than the current age of the universe—for literally trillions of years," says Sergio Dieterich, an astronomer at Georgia State University.

Paradoxically, the less mass a star is born with, the longer it lives. Most stars, including the sun, create their energy by using nuclear reactions to convert hydrogen into helium at their centers. Because these stars constitute the vast majority, astronomers call them main-sequence stars. To such a star, mass is both a blessing and a burden. On the one hand, mass provides the fuel that powers the star. On the other hand, the more massive the star, the hotter its center gets, which speeds up the nuclear reactions, making the star shine more brightly and exhaust its fuel more quickly.

The least massive and thus longest-lived main-sequence stars, born with only 8 to 60 percent of the sun's mass, are red dwarfs. Cool, faint and small, red dwarfs outnumber all other stars put together but are so dim that you can't see a single one with the naked eye. The smallest and least massive red dwarfs will live for trillions of years.

There's a limit, though, to how small and long-lived a successful star can be. If an aspiring star arises with too little mass, it becomes not a red dwarf but a failed star known as a brown dwarf. Despite the name, a brown dwarf glows red when young—from both the heat of its birth and nuclear reactions that later dwindle—then fades to black. Because a young brown dwarf looks like a red dwarf, distinguishing the two is often difficult.

To find the dividing line between the two types, Dieterich's team calculated the sizes of 63 red and brown dwarfs near the sun. Both main-sequence stars and brown dwarfs start life large, then contract under the force of their own gravity. A main-sequence star shrinks until it ignites its main supply of nuclear fuel; the less mass the star is born with, the smaller it gets and the longer it takes to contract. Having even less mass than red dwarfs, brown dwarfs take even longer to contract, and those that are young enough to shine appreciably are larger than the smallest red dwarfs. Therefore, if one looks at an assortment of main-sequence stars and young brown dwarfs, diameters should decrease as one proceeds from bright yellow suns to dim red dwarfs, then rise when one reaches the brown dwarfs.

Dieterich computed diameters from the luminosities and temperatures using a rule of physics—the Stefan–Boltzmann law—that links all three quantities. The luminosities required measuring accurate distances to the red and brown dwarfs; the temperatures meant observing their output of visible and infrared light.

Dieterich then plotted diameters against temperatures. "There are those moments when you look at a graph for the first time and your heart skips a beat," he says. As his team is set to report in a future issue of The Astronomical Journal, the plot revealed a minimum diameter of 8.6 percent of the sun's, the same size as Saturn. The small star having this diameter bears the prosaic name 2MASS J0523-1403, and it lies 40 light-years away in the constellation Lepus the Hare. "It appears to be a run-of-the-mill star that happens to be the smallest one that we found and therefore probably has the smallest mass," Dieterich says. This star, he says, is just above the dividing line between successful stars and failed ones, because he found that cooler objects are slightly larger.

Because of its small mass, the star glows feebly. If it replaced the sun, it would look fainter than the full moon. The star takes nearly a year to generate the same amount of energy our sun emits every hour. If its mass is 8 percent of the sun's, it should live for 12 trillion years, slowly growing hotter and brighter as it converts its hydrogen into helium. It will eventually become orange but never attain even 1 percent of the sun's current luminosity. Then it will cool and fade.

The long-lived star has a spectral type of L2.5 and a surface temperature of 2,075 kelvins, much cooler than the sun's 5,780 kelvins. But theoretical models had predicted that the dividing line between red and brown dwarfs should occur at still cooler temperatures.

In fact, J. Davy Kirkpatrick, an astronomer at the California Institute of Technology, is skeptical of Dieterich's result, in part because the number of stars his team studied is small. "You're kind of at the mercy of statistical fluctuations and small-number statistics," Kirkpatrick says. "What he really needs to do is get a bigger sample size and go to [cooler temperatures] and see if this minimum radius he's found actually holds up."

Dieterich plans to observe additional stars in the sun's vicinity, but doing so will take several years. Still, that's a tiny amount of time compared with the life expectancy of the little red star his team has highlighted.