Sky & Telescope - January 2019 - 36

The Nearest Stars

VEGA The A-type
star Alpha Lyrae lies
25 light-years away.

doubles and triples (or more) of closely orbiting stars or
brown dwarfs. Thus, while at least half of the census stars are
in multiple systems, most systems are not multiples.
The list of our stellar neighbors illustrates perfectly the
initial mass function (IMF), which describes the frequency
with which stars of different masses form. The IMF predicts
that the most massive stars are much rarer than lower-mass
stars. It therefore comes as no surprise to ﬁnd that most of
our neighbors are fairly modest stars, with our Sun actually
standing out as one of the more impressive.
Within 10 pc there are no O- or B-type stars, which are
the hottest, brightest, and most massive stars on the main
sequence. The next class down is the A-type stars, with surface temperatures between 7600 and 11,500 kelvin. There are
four of these nearby, all of which amateur astronomers will
know well: Altair, Sirius, Vega, and Fomalhaut.
Next are the F-type stars, a little cooler than the A-types
but a little warmer than the Sun, and there are seven of
these, including Procyon in Canis Minor. The Sun belongs
to the G-type stars and, again, there are more of these than
the F-types, including our star, Alpha Centauri A, Tau Ceti,
and 16 others. Cooler than the Sun are the K-type stars; these
outnumber all the A-, F-, and G-type stars put together. They
include Alpha Centauri B, both members of 61 Cygni, Epsilon
Eridani, and 39 others ﬁlling their ranks.
However, the most intriguing ﬁnding over the last 24 years
of RECONS is the ubiquity of M-type stars, often referred to

V EG A: A K IR A FUJII; M ULTIPLICIT Y A ND
R ATIO INFOG R A PHICS: TERRI DUBÉ / S&T

Ratio of Brown Dwarfs to Stars

1:2 to 1:5

1:8

SONYC star-forming
regions

RECONS solar
neighborhood

JA N UA RY 2 019 * SK Y & TELESCOPE

as M dwarfs or red dwarfs. These are
the smallest, coolest, and faintest stars,
with a surface temperature less than
4000K and masses from half a solar
mass down to just 7.5% of the Sun's
mass. When RECONS began, it was
thought that M dwarfs might account
for half of all the stars in the galaxy.
However, thanks in part to the work
RECONS has done, we now know that
they are even more common, making
up three-quarters of all stars.
"Three out of four is an awful lot
of stars," says Henry. "That's a bit of a
surprise."
There are also 21 "dead" stars, the
cooling stellar cores named white
dwarfs, with the closest being Sirius B,
8.6 light-years away.

A Shortage of Failed Stars
The other key result relates to brown
dwarfs. Smaller than red dwarfs but
larger than gas giant planets, brown
dwarfs are the awkward in-betweeners,
not quite massive enough to generate
the required temperatures and pressures to ignite the nuclear fusion of
hydrogen within their cores.
Since discovering the ﬁrst of these
failed stars in the 1990s, astronomers
have suspected that brown dwarfs form
the same way that stars do, condensing
from fragmenting clouds of molecular
gas. In that case, one might expect them
to follow the IMF trend and be found
even more frequently than red dwarfs.
But that belief was not data-driven,
Henry explains. "In the early days
people would get very excited about
brown dwarfs and say there are more
brown dwarfs than there are stars," he
says. "And I thought, 'Based on what?'"
Henry's skepticism has since been
borne out. The RECONS data show that
there are 8 times more stars within
10 pc than there are brown dwarfs.
Yet there appears to be a disparity
between the number of brown dwarfs
in the local neighborhood and those
farther aﬁeld. Astronomers are now
routinely detecting them in young star
clusters, with the Substellar Objects
in Nearby Young Clusters (SONYC)
team, led by Alexander Scholz (Uni-

q SEEING DOUBLE (OR
TRIPLE, OR . . .) Of the
317 star systems in the solar neighborhood, 85 have
more than one component - with "component"
meaning either star or
brown dwarf. That's a multiplicity fraction of 27%.

2 systems with

3 systems with

14 systems with

21%

66 systems with

73%

232 systems with

Sky & Telescope - January 2019

Table of Contents for the Digital Edition of Sky & Telescope - January 2019