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[Pete Pete]

This will surely change the Drake Equation!

Cheers,
Pete



http://www.cfa.harvard.edu/press/pr0611.html

Most Milky Way Stars Are Single


Release No.: 06-11
For Release: Monday, January 30, 2006
Note to editors: An image to accompany this release is online at 
http://www.cfa.harvard.edu/press/pr0611image.html.



Cambridge, MA - Common wisdom among astronomers holds that most star systems 
in the Milky Way are multiple, consisting of two or more stars in orbit 
around each other. Common wisdom is wrong. A new study by Charles Lada of 
the Harvard-Smithsonian Center for Astrophysics (CfA) demonstrates that most 
star systems are made up of single stars. Since planets probably are easier 
to form around single stars, planets also may be more common than previously 
suspected.

Astronomers have long known that massive, bright stars, including stars like 
the sun, are most often found to be in multiple star systems. This fact led 
to the notion that most stars in the universe are multiples. However, more 
recent studies targeted at low-mass stars have found that these fainter 
objects rarely occur in multiple systems. Astronomers have known for some 
time that such low-mass stars, also known as red dwarfs or M stars, are 
considerably more abundant in space than high-mass stars.

By combining these two facts, Lada came to the realization that most star 
systems in the Galaxy are composed of solitary red dwarfs.

"By assembling these pieces of the puzzle, the picture that emerged was the 
complete opposite of what most astronomers have believed," said Lada.

Among very massive stars, known as O- and B-type stars, 80 percent of the 
systems are thought to be multiple, but these very bright stars are 
exceedingly rare. Slightly more than half of all the fainter, sun-like stars 
are multiples. However, only about 25 percent of red dwarf stars have 
companions. Combined with the fact that about 85 percent of all stars that 
exist in the Milky Way are red dwarfs, the inescapable conclusion is that 
upwards of two-thirds of all star systems in the Galaxy consist of single, 
red dwarf stars.

The high frequency of lone stars suggests that most stars are single from 
the moment of their birth. If supported by further investigation, this 
finding may increase the overall applicability of theories that explain the 
formation of single, sun-like stars. Correspondingly, other star-formation 
theories that call for most or all stars to begin their lives in 
multiple-star systems may be less relevant than previously thought.

"It's certainly possible for binary star systems to 'dissolve' into two 
single stars through stellar encounters," said astronomer Frank Shu of 
National Tsing Hua University in Taiwan, who was not involved with this 
discovery. "However, suggesting that mechanism as the dominant method of 
single-star formation is unlikely to explain Lada's results."

Lada's finding implies that planets also may be more abundant than 
astronomers realized. Planet formation is difficult in binary star systems 
where gravitational forces disrupt protoplanetary disks. Although a few 
planets have been found in binaries, they must orbit far from a close binary 
pair, or hug one member of a wide binary system, in order to survive. Disks 
around single stars avoid gravitational disruption and therefore are more 
likely to form planets.

Interestingly, astronomers recently announced the discovery of a rocky 
planet only five times more massive than Earth. This is the closest to an 
Earth-size world yet found, and it is in orbit around a single red dwarf 
star.

"This new planet may just be the tip of the iceberg," said Lada. "Red dwarfs 
may be a fertile new hunting ground for finding planets, including ones 
similar in mass to the earth."

"There could be many planets around red dwarf stars," stated astronomer 
Dimitar Sasselov of CfA. "It's all in the numbers, and single red dwarfs 
clearly exist in great numbers."

"This discovery is particularly exciting because the habitable zone for 
these stars - the region where a planet would be the right temperature for 
liquid water - is close to the star. Planets that are close to their stars 
are easier to find. The first truly Earth-like planet we discover might be a 
world orbiting a red dwarf," added Sasselov.

This research has been submitted to The Astrophysical Journal Letters for 
publication and is available online at http://arxiv.org/abs/astro-ph/0601375

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for 
Astrophysics (CfA) is a joint collaboration between the Smithsonian 
Astrophysical Observatory and the Harvard College Observatory. CfA 
scientists, organized into six research divisions, study the origin, 
evolution and ultimate fate of the universe.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar at cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016
cpulliam at cfa.harvard.edu