http://www.skyandtelescope.com.au 19WXTHE CENSUS
As of mid-2017,
astronomers had
tallied 428 stars,
white dwarfs and
brown dwarfs within
10parsecsofour
Solar System. Each
dotrepresentsa
star and is sized
and colour-coded
by type. The tally
includes the Sun.I
fhistoryhadturnedoutalittledifferently,ToddHenry
mighthavebecomealeadinglightinthesearchfor
extraterrestrialintelligence.Instead,forthelastquarterof
acenturyhe’sbeenleadingthechargetolearnallthatwecan
about the nearest stars.
Henry graduated from Cornell University 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
fromextraterrestrialcivilisations.Yetjustayearafter
observations began, the rug was pulled out from under the
projectasCongresscancelleditsfunding.
Still, the questions that SETI posed remained with Henry.
If life exists on other worlds, where are those worlds? The
closeststarswouldseemtobeareasonableplacetostart.
However,ourstellarneighboursjustdidn’tseemto
interestmostastronomers.“Thenearbystarsjusthaven’t
been sexy for all that long,” says Henry (Georgia State
University). To remedy this, in 1994 he formed RECONS, the
ResearchConsortiumOnNearbyStars,withtheprimarygoal
of mapping and characterising all the stars within 10 parsecs
(ie.32.6light-years),andlaterextendingthatto25and
pc(81.5and326light-years,respectively).
Obtaining funding was difficult at first. Henry had to
constantly emphasise that the nearest starsareworth studying,
particularly since no one else was really looking at them. By
2003, though, his team was able to take over the 0.9-metre atthe Cerro Tololo Inter-American Observatory in Chile. Most of
RECONS’s work has since been performed with that telescope.
In 2018, the RECONS team released their latest census of
everythingcurrentlyknownwithin10pc—everystar,every
browndwarfandeveryplanet.Splitamong317differentstar
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.Starsofallkinds
Ourneareststarsareamotleybunch.Spreadrandomlyacross
space, our neighbours show no discernible clustering, with an
averagedistancebetweenstarsystemsof3to4light-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.
Whilestarsystemsgenerallykeepapolitedistancefrom
one another, about a quarter of systems are multiples —
doubles and triples (or more) of closely orbiting stars orXWHAT IS A PARSEC?Astronomers’ distance
unit of choice is the parsec, which is based
onparallax.Parallaxistheshiftinanobject’s
position against the background scene when
viewedfromtwodifferentlocations.Nearbystars
have measurable parallaxes due to Earth’s motion
aroundtheSun,whichastronomerscanuseto
calculatethestars’distances.Aparsecisthe
distance at which the difference between a
star’s apparent location as seen from Earth
wouldbe1arcseconddifferentthanits
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 arcsecond. One parsec is 3.
light-years.Astronomers are compiling a census
of the nearest stars to discover
what we know — and what we don’t —
about our stellar neighbours.Neighbours
Meet the
CENSUS:
TERRI DUBÉ /
S&T
; PARSEC:
LEAH TISCIONE /
S&T
4
A stars7
F stars19
G stars43
K stars284
M stars50
Brown
dwarfs21
White
dwarfs1 a.u.Apparent
shift = 2pDistance
to starParallax
angle (p)