Starz in the Hood
There are more stars in our celestial backyard than we once thought.
- By Michael Milstein
- Air & Space magazine, May 2000
(Page 3 of 5)
The most common means of finding the distance to a star is to determine its parallax, which is an angular measure of its apparent movement against other, more distant objects in the background. The closer a star is to Earth, the greater its parallax. The same principle holds when driving on a multi-lane expressway: Cars in the next lane appear to be moving faster against the surrounding landscape than do cars in the far lanes. Astronomers repeatedly photograph a target star, and by measuring its gradual movement against the background (it usually takes at least two years to get a good parallax), they can use trigonometry to calculate its distance from Earth. It isn’t the most glamorous work in science, which probably explains why parallax data is simply lacking for most stars, particularly in the southern hemisphere, where historically there haven’t been as many astronomers.
The lack of good distance estimates for most stars has long nagged Todd Henry, an astronomer at Johns Hopkins University in Baltimore and the deputy project scientist for Backman’s small NStars research effort at Ames, which, confusingly, has the same name as the larger NASA-NSF program just getting under way. Henry’s first job, funded by the Search for Extraterrestrial Intelligence back in 1991 when NASA was still running that program, was to identify stars that might warrant further scrutiny in a search for radio signals from alien civilizations. Henry worked off a list of the 100 stars known to be closest to Earth, but soon found himself puzzling over four objects that weren’t on the list, even though they were quite bright.
That curiosity led Henry and six other researchers to create an informal group they called the Research Consortium on Nearby Stars, or RECONS, to investigate unidentified shining objects. In 1995 the group got time at a complex of mountaintop observatories in Chile, where they measured the brightness of Henry’s four objects at different wavelengths. The photometry data revealed that three of the objects were in fact stellar giants, very bright but also very distant. The fourth star, which had seemed the most promising from the beginning, turned out to be a more common star known as a red dwarf, but its distance from Earth was not recorded in the catalogs.
As it happened, Henry lucked out. Astronomer Philip Ianna of the University of Virginia had already collected images of this particular star from 1976 through 1990, but hadn’t yet found time to extract parallax data and crunch the numbers to come up with its distance from Earth.
“I said, ‘Phil, do the numbers now!’ ” Henry recalls. It took only a few weeks to determine that the star, which goes by the unassuming tag GJ1061, was just 12 light-years away, making it the 20th nearest star—scarcely three times farther than our closest neighbor, the Alpha Centauri system, which includes the stars Alpha Centauri A, Alpha Centauri B, and, closest of all, Proxima Centauri.
“I’ll let you in on a secret,” Henry says. “We have better numbers now, and they show that it’s actually even closer. A few years ago, no one knew it was there.” The team gave the star another, newer name: RECONS 1.
The discovery led Henry to some simple calculations. Within five parsecs, or about 100 trillion miles, we know of 60 stars and one planet beside our own solar system. Let’s say this represents most, if not all, of the objects actually out there. Simply assuming that the density of stars remains the same out to 10 parsecs, there should be roughly 500 stars within that volume of space. Yet astronomers have counted only about 315.
Within 20 parsecs, there should be about 4,000 stars. “We know of about half that,” Henry says. “I like to compare it to a baseball field, where you have quite a few infielders but far fewer in the outfield. In fact, the density of players in the outfield of space should be about the same as the infield. We just haven’t located them yet. People ask me, ‘Why don’t we know where the rest of them are?’ The simple answer is ‘Because there are a lot of dots in the sky.’ It takes time to map everything out, and we just haven’t been at it that long.”