FOR IMMEDIATE RELEASE: December 9, 2013
RELEASE NO: NOAO 13-11
Dr. Katy Garmany
Deputy Press Officer
National Optical Astronomy Observatory
950 N Cherry Ave
Tucson AZ 857
FOR IMMEDIATE RELEASE: December 9, 2013
RELEASE NO: NOAO 13-11
Dr. Katy Garmany
Deputy Press Officer
National Optical Astronomy Observatory
950 N Cherry Ave
Tucson AZ 85719 USA
The relation between size and temperature at the point where stars end and brown dwarfs begin (based on a figure from the publication) Image credit: P. Marenfeld & NOAO/AURA/NSF.
The SOAR 4.1-m telescope on Cerro Pachón, CTIO
Dr. Sergio Dieterich
Georgia State University
phone: +1 678-350-4741
NOAO/SOAR: Where do stars end and brown dwarfs begin?
Stars come in a tremendous size range, from many tens of times bigger than the Sun to a tiny fraction of its size. But the answer to just how small an astronomical body can be, and still be a star, has never been known. What is known is that objects below this limit are unable to ignite and sustain hydrogen fusion in their cores: these objects are referred to as brown dwarfs.
In research accepted for publication in the Astronomical Journal, the RECONS (Research Consortium On Nearby Stars) group from Georgia State University has found clear observational evidence for the theoretically predicted break between very low mass stars and brown dwarfs. The data came from the SOAR (SOuthern Astrophysical Research) 4.1-m telescope and the SMARTS (Small and Moderate Aperture Research Telescope System) 0.9-m telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile.
For most of their lives, stars obey a relationship referred to as the main sequence, a relation between luminosity and temperature – which is also a relationship between luminosity and radius. Stars behave like balloons in the sense that adding material to the star causes its radius to increase: in a star the material is the element hydrogen, rather than air which is added to a balloon. Brown dwarfs, on the other hand, are described by different physical laws (referred to as electron degeneracy pressure) than stars and have the opposite behavior. The inner layers of a brown dwarf work much like a spring mattress: adding additional weight on them causes them to shrink. Therefore brown dwarfs actually decrease in size with increasing mass.
As Dr. Sergio Dieterich, the lead author, explained, “In order to distinguish stars from brown dwarfs we measured the light from each object thought to lie close to the stellar/brown dwarf boundary. We also carefully measured the distances to each object. We could then calculate their temperatures and radii using basic physical laws, and found the location of the smallest objects we observed (see the attached illustration, based on a figure in the publication). We see that radius decreases with decreasing temperature, as expected for stars, until we reach a temperature of about 2100K. There we see a gap with no objects, and then the radius starts to increase with decreasing temperature, as we expect for brown dwarfs. “
Dr. Todd Henry, another author, said: “We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/80