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Current Science at NOAO

Survey Seeks New Neighbors,
Locals Only Need Apply

Imagine leaving home early one morning and as you pull your car out of your familiar street and around the corner, you are surprised and a bit disconcerted to notice that a handful of new houses have popped up overnight. Some of them sit remarkably close to neighbors, while others are more isolated... yet all of them seem to be as weathered and lived-in as the well-known homes of old friends.

This feeling of neighborly surprise infuses an NOAO sky survey program called "In Search of Nearby Stars: the Cerro Tololo Inter-American Observatory Parallax Investigation." This intriguing investigation aims to discover "missing" members of the small group of stars within a few dozen light-years of the Sun - stars that will one day be destination points for robotic space probes and, sooner or later, humans.

"It seems amazing that there are stars in our own cosmic neighborhood that remain 'undiscovered,' but that truly is the case," says Todd Henry of Georgia State University, Atlanta, one of the leaders of the NOAO survey program, along with Phil Ianna of the University of Virginia, Charlottesville, and Rene Mendez of the European Southern Observatory in Santiago, Chile. "Our goal is to find 250 stars within 80 light-years of Earth. This is 100 more stars than our original goal, which we enlarged given the excellent progress we've made since the survey began in 1999."

[A light-year is the distance that light travels in a year at the amazing speed of 186,000 miles per second, equal to about 6 trillion miles... or 236 million times around Earth's equator!]

The full name of the survey reveals its central method: the parallax effect.

All of us are familiar with the parallax effect, even if we may not know it - simply by winking one eye at a time, objects closer than arm's length appear to jump back and forth relative to more distant objects.

The survey's "eyes" are the 0.9-meter (36-inch) and 1.5-meter (60-inch) telescopes at Cerro Tololo Inter-American Observatory (CTIO) near La Serena, Chile, part of the National Optical Astronomy Observatory. These telescopes take images of candidate nearby stars at opposite points in Earth's long elliptical path around the Sun each year, mimicking the "jump" we would see if our eyes were separated by more than 180 million miles!

When combined with a star's natural linear movement in the sky due to its relative motion as compared to the Sun, a nearby star will trace out a wiggly, snake-like path over the course of observations. Over the three-year duration of the CTIO Parallax Investigation, this path can be measured precisely enough to yield a sharp estimate of the star's distance from Earth.

The length of the observed path is determined by the star's motion relative to background stars. (For example, the fastest moving star in the sky, Barnard's Star, would take 1,800 years to change its location by the diameter of the full Moon.)

At the January 2001 meeting of the American Astronomical Society in San Diego, team members announced the discovery of seven "new" nearby stars, including one triple star system, located between 25 and 50 light-years from Earth.

All of these new discoveries are small stars of the type astronomers refer to as "red dwarfs" (spectral type M), and they are located in the southern sky, where research on nearby stars has been historically less extensive than in the Northern Hemisphere.

Although they are among the nearest few hundred star systems, they have been overlooked because they are thousands of times fainter than stars seen with the naked eye. The nearest of the new neighbors is known as LHS 3746, a small ember of a star only 1/3 the mass and diameter of the Sun, emitting less than 1 percent of the Sun's light.

Since discovering the twentieth nearest star in 1997, Henry and Ianna have expanded their search. It now includes not only the small red dwarf stars that make up 70 percent of stars in our galaxy, but also white dwarfs, which are leftover cinders of massive stars that have blown off their outer layers, and brown dwarfs, bizarre objects up to a dozen times more massive than planet Jupiter that are not quite able to ignite the process of fusion and become a star.

"Right now, the sample of nearby stars seems like a few fireflies hovering near our campfire," Ianna explains. "But we are sure that the real solar neighborhood is more like an entire field full of fireflies - we just haven't seen them all... yet."

Many people have heard the star Alpha Centauri cited as the closest star to Earth. In fact, Alpha Centauri is part of a triple system: Alpha Centauri A and Alpha Centauri B orbit each other every 80 years, separated by roughly the distance from the Sun to the planet Saturn, at a total distance of 4.33 light-years from Earth. Proxima Centauri, the third member of the system, is actually slightly closer to us, at a distance of 4.28 light-years.

At present, there are fewer than 3,000 stars known to be closer than 80 light-years, the distance limit of a sample of stars being studied in great detail by the Nearby Stars (NStars) Project, a somewhat unusual initiative in which the National Science Foundation (NSF), NASA, and its Astrobiology Institute at Ames Research Center in Moffett Field, CA, are teaming to support fundamental research on nearby stars.

"Astronomers estimate that there should be about 10,000 stars in this very local chunk of space," Henry says, "We want to help locate the missing two-thirds of our neighbors."

Some of them could be very close: the team has candidates that could displace current members of the numerically elite "Top 10" closest stars to Earth

This effort is meant to lead eventually to choosing stars as targets for the Terrestrial Planet Finder, a future NASA spacecraft mission that should be able to identify Earth-like planets within the 80 light-year distance horizon of NStars. "These local stars offer the best promise for answers to questions about the census of the Sun's neighbors, how many stars have planets circling them, and whether or not any of these planets harbor life," explains Dana Backman of Franklin & Marshall College, head of the NStars effort.

The CTIO Parallax Investigation is part of a larger effort by Henry and collaborators called the Research Consortium on Nearby Stars (RECONS), which aims to fully characterize our stellar neighbors within 33 light-years by measuring each one's mass, brightness and spectral colors, and how many of them are part of double or triple star systems.

What's the ultimate practical application of results from this survey, and those to follow that will build upon it? Nothing less than a roadmap for future explorers.

"These stars... these are places that we will go to someday," Henry says with certitude. "We're filling in the explorer's map of nearby space. That's what excites me most about the survey, and helps recharge my patience for the search. What could be more cool than picking out the remote targets for the first voyages to the stars?!?"

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