Astronomers have found a gap between "real" and "failed" stars.
What does the smallest star look like? This question is deceptively difficult to answer. Stars spend most of their lives fusing hydrogen in their cores, a prime time of life called the “main sequence.” As you go down the scale of stellar sizes on this sequence, stars become dimmer, cooler, and less massive. But determining the absolute properties of the smallest stars — their mass, radius, temperature, and overall light output — is challenging for at least three big reasons.
First, these stars are extraordinarily dim: even the brightest low-mass stars produce only a few percent of the light seen from the Sun. Second, their atmospheres are very cool, meaning many types of molecules can survive there. Those molecular species make the stars’ spectra much more difficult to understand than those of their high-mass counterparts. And last but not least, the smallest stars have colors and brightnesses similar to the largest “failed stars,” which astronomers call brown dwarfs.
Brown dwarfs are lightweight objects, generally not more than eight percent of the Sun’s mass. They do not have enough mass to create the high internal temperatures and pressures needed to sustain nuclear fusion in their cores.
For objects with surface temperatures near 2,000 K (or about one-third that of the Sun), discriminating between stars and brown dwarfs has been extraordinarily difficult. Now, new results accepted for publication in the Astronomical Journal point to a clear demarcation between “successful” and failed stars.
Sergio Dieterich (Georgia State University) and colleagues assembled a wealth of data on 63 nearby low-mass stars and brown dwarfs. The team meticulously measured distances to each of the objects, along with their colors in multiple optical and infrared wavelengths. Combining the distances, colors, and brightnesses in each wavelength filter, they were able to estimate each object’s temperature, radius, and luminosity by comparing it with expectations from cutting-edge models of what stars look like on the main sequence. The astronomers verified their method by comparing their computed sizes with a handful of radii measured directly using very long-baseline interferometry, a technique that allows astronomers to link telescopic observations together to achieve high-resolution measurements.
A star’s (or a brown dwarf’s) radius is related to its brightness and its surface temperature. Dieterich’s team examined the radius-luminosity and radius-temperature distributions, searching for a gap in sizes that would mark a break between the smallest stars and the largest brown dwarfs. Astronomers expect this gap to exist because although stars and brown dwarfs’ radii are related to their luminosities and temperatures, they’re related in opposite ways: if you increased the mass of a star, it would respond by growing in size; a brown dwarf would shrink.
And the search turned up a gap. “We see that radius decreases with decreasing temperature, as expected for stars, until we reach a temperature of about 2,100 K,” says Dieterich. “There we see a gap with no objects, and then the radius starts to increase with decreasing temperature, as we expect for brown dwarfs."
“We can now point to a temperature (2,100 K), radius (8.7% that of our Sun), and luminosity (1/8000th of the Sun) and say, ‘The main sequence ends there,’” coauthor Todd Henry (Georgia State University) adds. “And we can identify a particular star (with the designation 2MASS J0513–1403) as a representative of the smallest stars.”
Determining the boundary between brown dwarfs and stars is not only interesting to those who study these objects, but it is also important in searches for new exoplanets. Low-mass stars have become increasingly attractive targets for planet searches for many reasons. They are common, long-lived, and have habitable zones nestled closer around them than more massive stars do, often making any planets within those zones easier to detect. But brown dwarfs cool with age, so they would make poor hosts to planets — just imagine what would happen on Earth if the Sun became 20% cooler every 100 million years! Planet hunters can use these new results to ignore any brown dwarfs masquerading as small host stars, limiting their searches to systems with planets that could actually be habitable for several billion years.
Reference: S. B. Dieterich et al. "The Solar Neighborhood XXXII. The Hydrogen Burning Limit." Astronomical Journal, in press.