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Milky Way population grows

Astronomers identify 20 new stellar systems.
Provided by NOAO, Tucson, Arizona
An artist's impression of our home galaxy - the Milky Way. Our solar system is one of billions in the galaxy. And the galaxy is one of billions in the universe.
NASA
November 15, 2006
Astronomers have identified 20 new stellar systems in our local solar neighborhood, including the twenty-third and twenty-fourth closest stars to the Sun. When added to eight other systems announced by this team and six by other groups since 2000, the known population of the Milky Way galaxy within 33 light-years (10 parsecs) of Earth has grown by 16 percent in just the past six years.

The discoveries were made by a group called the Research Consortium on Nearby Stars (RECONS), which has been using small telescopes at the National Science Foundation's Cerro Tololo Inter-American Observatory (CTIO) in the Chilean Andes since 1999. These new results will appear in the December 2006 issue of the Astronomical Journal.

"Our goal is to help complete the census of our local neighborhood and provide some statistical insights about the demographics of stars in our galaxy - their masses, their evolutionary states, and the frequency of multiple star systems," says RECONS Project Director Todd Henry of Georgia State University in Atlanta. "Due to their proximity, these systems are also excellent targets for exoplanet searches, and ultimately, for astrobiological studies of whether any planets that are found could support life."

The 20 newly reported objects are all red dwarf stars, which now comprise 239 of the 348 known objects beyond our Solar System within the 10-parsec boundary of the RECONS survey. Thus, red dwarfs likely account for at least 69 percent of the Milky Way's residents.

"Red dwarfs are among the faintest but most populous objects in the Milky Way," Henry explains. "Although you can't see a single one with the naked eye, there are swarms of them throughout the galaxy."

The distances to these stars were measured via a classic trigonometric parallax technique using the 0.9-meter telescope at CTIO. The parallax technique for measuring the distance to a star takes advantage of the simple geometry of Earth's changing position in the cosmos as it orbits the Sun each year. The apparent back and forth motion of a nearby star during the year reflects the motion of the Earth around the Sun, much like how your finger appears to jump back and forth in front of your eyes if you blink one eye, then the other.

From Earth, nearby stars appear to make tiny ellipses in the sky because the Earth does not jump from one side of its orbit to another, but slides smoothly around the Sun. The extreme points of the Earth in its orbit are much like the positions of your eyes in your head, and the size of the apparent motion of your finger depends on how close you hold it to your eyes - when nearer, it seems to jump more, relative to distant background objects.

With observations over several years, it is possible to make parallax measurements with an accuracy of 1 milliarcsecond (0.0000003 degrees), or about one two-millionth the width of the full Moon. This allows astronomers to measure distances accurate to better than 10 percent out to more than 300 light-years.

The team of astronomers includes Henry, Wei-Chun Jao, John Subasavage and Thom Beaulieu of Georgia State University, Phil Ianna of the University of Virginia in Charlottesville, and Edgardo Costa and Rene Mendez of the Universidad de Chile. The RECONS long-term parallax program began under the auspices of the National Optical Astronomy Observatory (NOAO) Survey Program in 1999, and continues via the Small and Moderate Aperture Research Telescope System (SMARTS) Consortium.

"We expect to announce more systems within 10 parsecs in the future," notes Henry. "The pool of nearby stars without accurate parallaxes is nowhere near drained."

The purpose of this survey is to discover and characterize overlooked stars and brown dwarfs in the vicinity of the Sun. Objects are scrutinized by measuring their positions (and wobbles), their brightnesses and colors, and by taking spectroscopic fingerprints to examine their atmospheric composition. The estimated "missing" population of solar neighborhood members is expected to be composed primarily of very low mass stars with spectral type M (known as red dwarfs), and objects of spectral types L and T many of which are actually brown dwarfs with too little mass to start long-term thermonuclear reactions.

These L- and T-dwarfs shine feebly, glowing only because of energy leaking out since their gravitational formation many billions of years ago. RECONS has also found several nearby white dwarfs, which are the burned-out cores of intermediate- mass stars, lurking in the solar neighborhood.

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