NEWBORN NEIGHBORS: Located just 20 light-years from Earth, the two stars of EQ Pegasi are so young that they shine primarily via gravity rather than nuclear reactions Image: Adric Riedel/CUNY Hunter College/American Museum of Natural History.
Unlike many galaxies, our Milky Way spawns lots of new stars. To study these stellar infants observers often peer across vast distances to places such as the Orion Nebula, a star-making cloud of gas and dust 1,350 light-years away. Now astronomers have discovered that a nearby pair of red stars named EQ Pegasi is so young they shine primarily from the heat of their formation rather than from nuclear reactions. The discovery may give observers the chance to glimpse the glow of newborn planets, because EQ Pegasi is just 20 light-years from Earth—less than five times the distance to Alpha Centauri, the nearest star system.
To be sure, EQ Pegasi isn't so young that it is still swaddled in the gas and dust that gave it life. "My gut feeling is that it's somewhere in the 50- to 100-million-year range," says Benjamin Zuckerman of the University of California, Los Angeles, the astronomer who discovered its youth. If our 4.6-billion-year-old sun were a 46-year-old adult, EQ Pegasi would be no more than a year old.
The discovery was an accident. Zuckerman was studying more distant young stars and noticed that EQ Pegasi might be moving with them. Then he saw that both stars were abnormally luminous—a sign of their youth.
A star forms when an interstellar cloud of gas and dust collapses under its own weight. As gravity squeezes the gas, it heats up, as compressed gas will do, until it shines—and a new star is born, but one that owes most of its light to gravity rather than to nuclear reactions. During this so-called pre–main-sequence phase, the star is larger and therefore brighter than it will be when it is more mature. The star slowly shrinks and fades until it reaches the main sequence, the stage when nuclear reactions at the stellar core convert protons into helium and supply the star with all of its energy.
Earth’s sun shone for 50 million years as a pre–main-sequence star. EQ Pegasi consists of two red dwarfs, stars that are much cooler, fainter and smaller than the sun. Such stars outnumber all other stellar types put together but are so dim that not a single one is visible to the naked eye. A red dwarf evolves slowly and lingers in the gravity-powered pre–main-sequence phase for more than 100 million years, outshining main-sequence stars of the same color. "The two stars in the EQ Pegasi system seem to be sitting above the luminosity that they would have if they were just ordinary main-sequence stars," Zuckerman says. As he and his colleagues report in the November 20 issue of The Astrophysical Journal, EQ Pegasi sports the nearest pre–main-sequence stars to Earth.
Adric Riedel, an astronomer at Hunter College, thinks Zuckerman is right. Two years ago Riedel uncovered the previous record holder, a pre–main-sequence star south of the constellation Orion named AP Columbae, which is 27 light-years from Earth.
Riedel examined unpublished spectra of EQ Pegasi to check the surface gravity of its two stars. The gravity at the surface of a pre–main-sequence star is small, because the star is more extended than a main-sequence star. Riedel says, "Both components of EQ Pegasi are low gravity—convincingly. So yeah, I would say that they are indeed young."
Newborn stars near Earth excite planet hunters: "It makes them very interesting targets for direct imaging searches for extrasolar planets," says Sascha Quanz, an astronomer at the Swiss Federal Institute of Technology in Zurich. Like young stars, young giant planets also glow from the heat of their birth; moreover, because of their proximity to Earth, the planets should appear farther from their star's glare, making them easier to see. Seeing a planet directly gives astronomers the chance to study its atmosphere. Quanz searched for planets around AP Columbae but failed to see any. "We would have been able to find a gas-giant planet with a mass of Jupiter at the separation of five AU, which is the separation of Jupiter in our solar system," Quanz says. One AU, or astronomical unit, is the mean distance from the sun to Earth.
EQ Pegasi is seven light-years closer than AP Columbae, so it's another excellent target for planet hunters. Because EQ Pegasi is a binary, however, planets can't exist just anywhere in the system. The two red dwarfs are about as far apart as Neptune is from the sun. Planets could orbit close to either star so that the other star's gravity doesn't yank it away. And planets could also orbit on a very wide path around both dim suns.
"This discovery is just more proof that we still don't know our nearest neighbors very well," Riedel says. "There's lots of surprises right next door."