48 AUSTRALIAN SKY & TELESCOPE November | December 2017
DOUBLE STARS by Ross GouldBinaries, true and false
Aries, the Ram, holds its fair share of captivating doubles.
O
ver the years, I’ve observed more
than 60 doubles in Aries, half
of them brighter than 10th
magnitude and within reach of 20
cm telescopes. First up this month is,
1 Arietis, STF 174, about 1.5 degrees
northwest from Beta Ari (mag 2.6). The
stars are slightly unequal in brightness,
and with a 140-mm refractor were just
separated at 114×. Hartung commented
on the colour contrast, deep yellow
and white. A genuine binary, STF 174
has shown very little change since the
measure by F.G.W. Struve in 1830.
Gamma Arietis is the 3rd-brightest
in the line of naked-eye stars that mark
Aries. A lovely object even with 80 mm,
with 140 mm it showed as equal, pale
yellow, bright stars, with 62× showing
them neatly separated. It has changed
very little over time. Gamma Ari is
about 164 light-years away, at which
distance the current separation (7.4 ̋)
is 370 a.u. in projection (Neptune is 30
a.u. from our Sun), indicating a very
large and therefore slow orbit.
Lambda Ari is about two degrees to
the west of Alpha Arietis. Not a true
binary, but simply a chance line-up of
stars as seen from Earth, it nevertheless
is of interest to the observer — a 37 ̋pair of unequal, yellowish, bright stars,
with two 10th-magnitude field stars
nearby, one of them deep yellow.
Our next object 10 Ari (STF 208)
is a binary with a computed orbit
of 325 years. Nearly 200 years since
discovery by F.G.W. Struve, at present
it is widening, after being at its closest,
slightly less than 0.5 ̋, around 1920. The
most recent good measure is 1.40 ̋. With
my 140mm refractor on a fairly steady
night the secondary star showed at 230×:
a tiny, fairly bright point just separated
from the yellow primary. As usual with
unequal close pairs, the companion
looks smaller/less bright than it is.
Nearly seven degrees east-northeast
from Alpha Arietis is our next object,
30 Ari. This is a very wide pair of bright
stars visible as a double through an
8×50 finderscope. The stars are differing
shades of yellow. Are they connected?
The proper motions are similar, but the
parallaxes differ a little. It’s possible
the stars are co-moving in space from a
birth group that broke up over time.
We’ll now move six degrees southeast
from 30 Ari, to Epsilon Arietis. This
brighter pair is a binary that’s no longer
wide and easy. A preliminary orbit
calculation gives a period of 1,200years. Measures over the 190 years since
discovery suggests an orbit viewed not far
from edge-on. With the 14-cm refractor
it showed as double at 114×, and 160×
showed the similarly bright pale yellow
stars neatly split. A 10-cm scope should
give bare separation of the stars.
Next is Pi Ari (STF 311), 4.5 degrees
south and somewhat west from Epsilon.
A triple star with the three stars in
a line, the 14-cm refractor showed
the wide and dimmer third star with
only 62×, and increasing the power to
114× showed the closer companion,
the unequal A and B stars creating a
delicate pair with lovely aesthetics. The
AB pairing is a very long period binary
that’s changed little in nearly 200 years.
This is not a nearby object, the parallax
suggesting over 500 light-years from us,
so the orbit size must be huge and the
period thousands of years. The wide C
star is merely a field star.
Some 2.5 degrees northwest from Pi
Arietis, STF 291 is a moderately close
pair, near equal brightness, within
reach of 8-cm scopes. It’s reasonably
bright and a neat split at 100×. Despite
the spectral type it looked pale yellow.
Our last double, STF 305, is two
degrees north-northeast of Pi Ari; it
showed well with 14 cm at 80×, a neat
yellow pair just apart and of slightly
unequal brightness. Again a binary,
with a calculated orbit of 530 years,
the separation has increased from
1.6 ̋ at discovery in 1829. The parallax
suggests 110 light-years from us; at
that distance, the current separation in
projection is 120 a.u. The stars are now
near maximum separation, but orbital
closing will be slow so it will remain an
easy pair for the rest of this century.■ ROSS GOULD observes the sky from
the nation’s capital. He can be reached
at [email protected]DoublestarsofAriesStar Name R. A. Dec. Magnitudes Separation Position Angle MeasureDate of Spectrum
1 Ari (STF 174) 01 h 50.1m +22° 17 ́ 6.3, 7.2 2.9 ̋ 165 2016 G3III
Gamma (STF 180) 01 h 53.5m +19 ° 18 ́ 4.5, 4.6 7.4 ̋ 001 2016 A1pSi+B9V
Lambda (H 5 12) 01 h 57.9m +23° 36 ́ AB 4.8, 6.7 37.1 ̋ 048 2016 F0IV+F7V
10 Ari (STF 208) 02 h 03.7m +25° 56 ́ 5.8, 7.9 1.3 ̋ 343 2015 F8IV
30 Ari (STF A 5) 02 h 37.0m +24° 39 ́ 6.5, 7.0 37.8 ̋ 275 2016 F5V+F7V
STF 291 02 h 41.1m +18° 4 8 ́ 7. 5 , 7.7 3. 3 ̋ 117 2 015 B 9.5 V
STF 305 02 h 47.5m +19 ° 2 2 ́ 7.5, 8.3 3.6 ̋ 308 2016 G0V
Pi (STF 311) 02 h 49.3m +17 ° 2 8 ́ AB 5.3, 8.0 3.2 ̋ 119 2 012 B 6 V
" ""5.3, 10.724.1 ̋ 112 2 012 "
Epsilon (STF 333) 02 h 59.2m +21° 20 ́ 5.2, 5.6 1.4 ̋ 211 2016 A2Vs+A2Vs
Data from the Washington Double Star Catalog