Astronomers surveying our solar neighborhood for brown dwarfs have been thrown a surprise: these "failed stars" are a lot less common than previously thought.
During its 14-month mission, NASA’s Wide-field Infrared Survey Explorer (WISE) took snapshots of more than 500 million celestial objects from its vantage point 326 miles (525 kilometers) above Earth’s surface. Scientists originally thought that catalog would include an equal number of normal, hydrogen-burning stars and “failed stars” called brown dwarfs. But new investigations into the WISE data have shown that the number of brown dwarfs in our solar neighborhood is far lower than expected.
Astronomers arrived at the new estimate, which puts the ratio at roughly six normal stars for every one brown dwarf, by looking at WISE objects within 26 light-years of the Sun. Although they found seven new members of the coldest brown dwarf class, called Y dwarfs, that only brought the total number of brown dwarfs in our region of space to 33. That number falls far short of the 211 hydrogen-burning stars in the same volume. Study coauthor Davy Kirkpatrick (Caltech) says it’s likely this ratio is typical of most of the rest of the galaxy: the solar neighborhood contains roughly the same number of stars as other parts of the galaxy — except for those furiously forming stars.
Brown dwarfs form with too little mass to sustain nuclear fusion in their cores. After burning through their initial store of fuel the objects gradually cool off, glowing faintly at infrared wavelengths with the leftover heat of the gravitational collapse that formed them. Y-class brown dwarfs are the oldest and coolest of the group and are notoriously difficult to detect. One such object, WISEP 1828+2650, is only about 80° Fahrenheit (27° Celsius), making it the coldest known star-like body.
Normal stars burn through their constituent materials, fusing heavier and heavier elements inside themselves until they eventually throw much of that material back out into interstellar space. But brown dwarfs never progress beyond the early stages of star formation, meaning they still contain the original (unadulterated) materials they were born with, says Kirkpatrick. “You can think of them as little time capsules,” he says, windows into the chemical history of the region of the galaxy where they formed.
While the low number of dwarfs won't necessarily wreak havoc with star formation theories, it does mean that astronomers have fewer examples to work with.
There may be more Y dwarfs hiding in the WISE data, but almost certainly not enough to match the previous one-to-one estimate, the study’s authors conclude in their paper, which will appear in the July 10 Astrophysical Journal. NASA put the WISE telescope into hibernation mode after it completed its primary science missions last year, so future space-based efforts to locate more brown dwarfs will have to wait for instruments such as the James Webb Space Telescope, currently scheduled for a 2018 launch.