Astronomers may have systematically erred in estimating the mass of small cosmic objects
The smallest star ever to have its mass reliably measured has been found to actually be nearly twice as massive as expected, astronomers say. This raises doubts regarding a significant part of the existing knowledge about small-mass cosmic debris fragments - objects that are almost stellar. The study, published last week, indicates that astronomers may have systematically erred in estimating the mass of these objects, and therefore erred in defining the smallest objects of their research.
"This discovery will force astronomers to rethink the question of what the masses of the smallest objects produced by nature really are," said Dr. Red Kloss of the University of Arizona, who headed an international team of astronomers who reported the study in the journal "Nature."
Dr. Alan Boss, a theorist at the Carnegie Institution in Washington, defined the study as "a very interesting advance in the field of astronomy." According to him, the research emphasizes the need to anchor astronomical theories in measurements.
The study - in which the mass of the star was calculated by examining its orbit, and not by measuring the intensity of its light, as astronomers usually calculate the mass of stars - is the result of a project aimed at locating, and being able to see, even smaller cosmic particles, and more precisely - Planets orbiting other stars. More than a hundred such objects have been discovered indirectly in the last decade, through an examination of the fluctuations created by the gravitational fields of these planets in the movements of the stars they orbited. Following these findings, an international race developed between astronomers aiming to be the first to observe these planets directly.
In an attempt to win this title, Kloss and Dr. Rainer Lanzen from the Max Planck Institute for Astronomy in Heidelberg, Germany built a special camera designed to distinguish between the light reflected by the planets and the blinding glow emitted by their stars, which shine 10 million to 10 billion times more powerfully. As part of the testing process, the two and their colleagues brought the camera to Chile, to the European Astronomical Union's VLT ("Very Large Telescope") telescope in the Southern Hemisphere, and used it to search for the companion of a star known as AB Doradus A.
This young star is about 48 light years from Earth, and its wobble on its axis is clearly observed. But even the Hubble Space Telescope was unable to locate its satellite planet, because its light is weak and it is close to the main star. However, in the larger telescope, which is equipped with an adaptive optical system that compensates for the smearing of the image caused by the Earth's atmosphere, Kloss's camera managed to photograph a cool red object, whose brightness is about 120 times smaller than the brightness of the main star.
Astronomers usually calculate the mass of a star by its brightness. According to this method, the satellite star, AB Doradus C, should have apparently had a mass approximately 50 times greater than the mass of Jupiter - and hence it is a brown dwarf, a collapsed star.
However, by measuring its orbit, the astronomers were able to determine that AB Doradus C actually has a mass 93 times greater than that of Jupiter - and hence it can be defined as a small star. "Twice as much, it's a huge mistake. "Imagine that you are wrong about your wife's weight by a range more or less than twice her real weight," Kloss said in an interview. The problem, according to him, is that the models used to estimate the masses of stars were calibrated based on measurements of binary systems (systems that have two stars), with stars that are the size of the Sun or even larger. But objects such as AB Doradus C are cool enough for dust clouds to form around them that dim their glow, Kloss said.
"What is exciting is that this is the first time we have discovered the mass of an object whose mass is small, which is cool and young," he said. "This is our first glimpse of low-mass stars."
For a star, mass is everything. The mass determines the degree of density and heat of its core, and also what kind of thermonuclear reactions can occur in it. According to the theory, the fact that its mass is less than 75 times that of Jupiter, there will not be enough energy to ignite and turn hydrogen into helium through combustion, and therefore it will remain a brown dwarf. The Sun, by comparison, has a mass about a thousand times that of Jupiter.
Small stars greatly outnumber the big, bright stars. Recently, some astronomers suspected that a large part of the "dark matter", as it is called, that exists in the universe, is in the form of brown dwarfs - a hypothesis that has aroused great interest in these hard-to-find objects. But the popular explanation today, according to Kloss, is that the brown dwarfs are smaller in their mass, as a group, than faint stars whose size is only a fraction of the mass of the Sun - about 200 Jupiter stars. "Brown dwarfs are now minor players in the dark matter arena," he said.
If the new findings are correct, Kloss said, brown dwarfs may be even more minor players than previously thought. Because it is possible that many objects that have been classified so far as brown dwarfs, are actually stars with a small mass.
Not a dwarf - a small star
The orbit of the star-satellite AB Doradus C (the small red dot) around the star AB Doradus A. Astronomers usually calculate the masses of the stars by the intensity of their luminosity. According to this method, AB Doradus C should have apparently had a mass approximately 50 times greater than the mass of Jupiter - and hence it is a brown dwarf, a collapsed star. However, by measuring its orbit, the astronomers were able to determine that AB Doradus C actually has a mass 93 times greater than that of Jupiter - and hence it can be defined as a small star.
