My first Magazine Sky & Telescope - 01.2019 | Page 37

Astronomers are compiling a census
of the nearest stars to discover
what we know — and what we don’t —
about our stellar neighbors.
Meet the
Neighbors
I
f history had turned out a little differently, Todd Henry
might have become a leading light in the search for extra-
terrestrial intelligence. Instead, for the last quarter of a
century he’s been leading the charge to learn all that we can
about the nearest stars.
Henry graduated from Cornell in the 1980s, where one
of his advisors had been none other than Carl Sagan. With
such inspiration, it’s little surprise that after completing his
PhD he opted to join NASA’s SETI project, which was to be
a huge 10-year quest to search for signals from extraterres-
trial civilizations. Yet just a year after observations began,
the rug was pulled out from under the project as Congress
cancelled its funding.
Still, the questions that SETI posed remained with Henry.
If life exists on other worlds, where are those worlds? The
closest stars would seem to be a reasonable place to start.
However, our stellar neighbors just didn’t seem to interest
most astronomers. “The nearby stars just haven’t been sexy
for all that long,” says Henry (Georgia State University). To
remedy this, in 1994 he formed RECONS, the Research Con-
sortium On Nearby Stars, with the primary goal of mapping
and characterizing all the stars within 10 parsecs (i.e., 32.6
light-years), and later extending that to 25 and 100 pc (81.5
and 326 light-years, respectively).
Obtaining funding was difficult at first. Henry had to con-
stantly emphasize that the nearest stars are worth studying,
particularly since no one else was really looking at them. By
2003, though, his team was able to take over the 0.9-meter at
tu THE CENSUS
As of mid-2017,
astronomers had
tallied 428 stars,
white dwarfs, and
brown dwarfs within
10 parsecs of our
solar system. Each
dot represents a
star and is sized
and color-coded
by type. The tally
includes the Sun.
21
White
dwarfs
19
G stars
50
Brown
dwarfs
7
F stars
284
M stars
43
K stars
4
A stars
the Cerro Tololo Inter-American Observatory in Chile. Most of
RECONS’s work has since been performed on that telescope.
In 2018, the RECONS team released their latest census of
everything currently known within 10 pc — every star, every
brown dwarf, and every planet. Split among 317 different star
systems (including our own), they and other astronomers
have found 378 stars and white dwarfs, 50 brown dwarfs,
and more than 50 planets. There have been surprises, though,
from missing brown dwarfs to the sheer wealth of small stars,
and vital information about the secrets of star formation.
Stars of All Kinds
Our nearest stars are a motley bunch. Spread randomly and
uniformly across space, our neighbors show no discernible
clustering, with an average distance between star systems
of 3 to 4 light-years. The fact that our closest star, Proxima
Centauri, is 4.2 light-years away, might mean that we’re a tad
more remote than average.
While star systems generally keep a polite distance from
one another, about a quarter of systems are multiples —
u WHAT IS A PARSEC? Astrono-
mers’ distance unit of choice is
the parsec, which is based on
parallax. Parallax is the shift in
an object’s position against the
background scene when viewed
from two different locations. Nearby
stars have measurable parallaxes
due to Earth’s motion around the
Sun, which astronomers can use
to calculate the stars’ distances.
A parsec is the distance at which
the difference between a star’s ap-
parent location as seen from Earth
would be 1 arcsecond different than
its apparent location as seen from
the Sun (an arcsecond is 1 / 3,600 °).
Another way to think about it is that
the parsec is how far away you’d
have to be for the distance between
the Sun and Earth to span 1 arcsec-
ond. One parsec is 3.26 light-years.
Apparent
shift = 2 p
Parallax
angle ( p)
Distance
to star
1 a.u.
sk yandtele scope.com • JA N UA RY 2 019
35