Sky & Telescope - March 2021 - 34

Observing Variable Stars

What caused this particularly dramatic episode? Clayton
notes that the obscuring dust must have formed continuously over many years for the star to remain in such a deep,
prolonged decline.

16h 00m

15h 50m

Star magnitudes

87
4
5
6
7
8
9
10

R 100

+28°

J
+26°

Our third RCB variable is V854 Centauri. With a declination of -39.5°, it's accessible to observers at locations south
of roughly latitude 40° north. The star is found a little more
than 2½° north of 2nd-magnitude Eta (η) Centauri. In
March, the star's ﬁeld is visible in the hours before dawn.
V854's variability was ﬁrst detected in 1964, but its status
as an RCB wasn't determined until much later. In July 1986,
Australian amateur Glenn Dawes found a magnitude-7.5 star
on a photographic sky patrol image. A previous photo he took
in 1985 showed no star brighter than magnitude 11 at the
position. Suspecting a possible nova, Dawes quickly alerted

08/28/09

LIGHT CURVE: S&T ILLUSTR ATION /
DATA COURTESY A AVSO

SX COMPARE AND ESTIMATE Use this chart to locate and estimate
the brightness of variable star R Coronae Borealis. On this chart and the
ones that follow, the magnitudes of comparison stars are given with their
decimal points omitted. So, for example, a magnitude-7.8 star is labeled
" 78 " instead.

MARCH 2 021 * SK Y & TELESCOPE

102

E
CORONA
BOREALIS

89
81

16
11/28/05

15h 30m

+30°

Magnitude

X INCONSISTENT LIGHT
In the 200-plus years since
its variability was first
discovered, R Coronae
Borealis has had its share
of ups and downs. As this
graphic illustrates, the
star's brightness changes
dramatically over irregular
time spans.

R Coronae
R
Coronae
Borealis
Borealis

Three to Watch
R Coronae Borealis, along with RY Sagittarii and V854 Centauri, are the easiest RCBs for observers to follow. All three
reach binocular visibility (R Coronae Borealis and RY Sagittarii can even climb to naked-eye brightness), and each has
exhibited unusual behavior recently. The AAVSO recommends
checking these stars once a week when they're at their brightest, and then nightly once fading has begun.
R Coronae Borealis lies within the horseshoe-shaped
curve of Corona Borealis, about 2° north of 4.6-magnitude
Delta Coronae Borealis. A 7.4-magnitude star lies less than
½° northwest of R. During March evenings, Coronae Borealis
rises late in the evening. R's last major decline was in 2016,
though there was a minor fade to 9th magnitude in late 2019.
It's been back at 6th magnitude since the beginning of 2020.
The second-brightest RCB variable is RY Sagittarii, discovered in 1893 by Colonel Ernest Marwick, an ofﬁcer in the
British army then serving in Gibraltar. RY lies 4½° degrees
southeast of 3rd-magnitude Zeta (ζ) Sagittarii, part of the
Teapot asterism. According to the AAVSO, RY Sagittarii has a
brightness range of magnitude 5.8 to 14.0. During March, RY
Sagittarii is observable just before dawn, so you may wish to
wait until later in the year when it's more favorably placed.
During the last 50 years, RY Sagittarii underwent declines
exceeding four magnitudes in 1971, 1978, 1982, 1990, 1993,
1999, 2004, and 2008, before an 11-year gap to 2019. RY Sagittarii has not fully recovered from its most recent dimming
and shines between 9th and 10th magnitude at the time of
this writing.

15h 40m

Sky & Telescope - March 2021

Table of Contents for the Digital Edition of Sky & Telescope - March 2021