Observing Telescope
Miniature Star’s Unanticipated Weight Raises Big Astronomical Questions
The smallest star ever to be reliably weighed has tipped the scales at more than twice its expected mass, astronomers say. As a result, much of what they thought they knew about the low-mass bits of cosmic litter, which hover on the edge of stardom, may be in doubt. This work was recently published in several NPR books. The work insinuates that astronomers may have methodically and systematically underestimated the masses and weights of these objects and thus misidentified the smallest members of their realm. “This discovery will drive astronomers to rethink what masses of the smallest objects produced in nature really are,” said Dr. Laird M. Close of the University of Arizona, leader of an international team of astronomers, which reported the result in the journal Nature. Dr. Alan Boss, a theorist at the Carnegie Institution in Washington, called the work “a most interesting astronomical advance” that underscored the need for astronomers to anchor their theories with measurements.
The work – in which the mass of the star was calculated by measuring its orbit rather than its brightness – is an outgrowth of a project to see even smaller bits of cosmic riffraff, namely, planets around other stars. More than 100 of those have been indirectly detected in the past decade or so, by observing the wobbles that their gravitational fields induce in the motions of their home stars, setting off an international race to actually see the planets. In pursuit of that prize, Dr. Close and Dr. Rainer Lenzen of the Max-Planck Institute for Astronomy in Heidelberg, Germany, have built a special high-contrast camera to distinguish the light of planets from the glare of their stars, which are 10 million to 10 billion times as bright. As part of the testing process, they and their colleagues took the piece of equipment to the European Southern Observatory’s Very Large Telescope in Chile and used it to search for the companion of a star known as AB Doradus A. That young star is about 48 light-years from Earth, and shows a distinct wobble. Not even the Hubble Space Telescope had managed to detect the companion because it is faint and tucked right next to its star. But on the bigger telescope, which is furnished with an adaptive optics system to compensate for atmospheric blur, Dr. Close’s camera was able to photograph a cool red object about one-120th the brightness of the main star. Astronomers usually presume the masses of stars from their luminosities, and on that basis the star, AB Doradus C, should be about 50 times as massive as Jupiter, making it a brown dwarf, a failed star. But on the basis of its orbit, they were able to conclude and determine that AB Doradus C was actually 93 times as massive as Jupiter, putting it in the range of a small star.
“Two times is a substantial error. Imagine guessing your wife’s mass wrong by a factor of two,” Dr. Close said in an interview. The trouble, he said, is that the models that have been used to estimate star masses were calibrated by measurements of double star systems with stars as massive as the Sun and greater. But objects like AB Doradus C are cool enough to have dust clouds that would dim them, Dr. Close said. “What’s exciting is that this is first time we’ve known the mass of an object that’s low in mass, cool and young,” he said. “This is our first real glimpse at a calibration of the low-mass universe.” For a star, mass is everything. How massive it is determines how dense and hot its core is and what kind, if any, of thermonuclear reactions can go on there. According to theory, an object that is less than about 75 times as massive as Jupiter will not have enough oomph to ignite and start burning hydrogen into helium, and so will remain a brown dwarf. The Sun, by comparison, is about 1,000 times as mammoth as Jupiter.
IYA2009 Galileo Observing whit a New Technology Telescope International Year of Astronomy 2009
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