14121086M
ag
ni
tu
de1996 2000 2004 2008 2012 2016 2020
YearCORONA BOREALIS5. 4R2. 2αβ 3. 7γ
3. 8δ
4. 64. 2εθ
4. 1π 5. 6ιρ6. 42 °5. 0
7. 48. 166 ASTRONOMY • JUNE 2020OBSERVING BASICS
One of my favorite
backyard astron-
omy activities is
observing variable stars. In
the past, I’ve featured a hand-
ful of my favorites, including
R Leonis (April 2004), Delta
(δ) Cephei and Beta (β) Lyrae
(September 2004), R Scuti
(August 2005), Beta (β) Persei
(Algol; November 2006), SS
Cygni (September 2008), Mu
(μ) and S Cephei (October
2008), and Epsilon (ε) Aurigae
(Aug ust 2009).
But as interesting as these variable stars are, none
fascinates me more than R Coronae Borealis, usually
referred to as “R Cor Bor.” I prefer to call it
“R CurveBall,” though, because this unpre-
dictable star has thrown me plenty of curves
over the years. R Cor Bor is usually visible
as a 6th-magnitude star, and it’s located
about one-third of the way from Epsilon to
Theta (θ) Coronae Borealis. It might shine
this way for several years and then, without
warning, fade rapidly from view. I’ve seen R
Cor Bor drop a few magnitudes and then
return to normal within the span of just
months. I’ve also watched it plunge all the
way down to 14th magnitude and remain
there with occasional f luctuations for years
at a time. The light curve on this page shows
its erratic behavior over the past quarter of
a cent u r y.
R Cor Bor is the prototype of a relatively
rare group of variable stars that includes
some 100 known members, with a few dozen
more awaiting official confirmation. Its
variability was first reported by the English astronomer
Edward Pigott near the end of the 18th century. Among
the facts we’ve learned about R Cor Bor in the two-plus
centuries since its discovery is that it’s a yellow supergi-
ant, having just 80 to 90 percent the mass of the Sun but
100 times the diameter. Because it’s an aging star that
has used up most of its hydrogen fuel, R Cor Bor is
comprised of 90 percent helium and just 1 percent
hydrogen. The rest is mostly carbon and nitrogen. Bycontrast, our middle-aged Sun has a composition of
approximately 75 percent hydrogen and 25 percent
helium by mass. Past distance calculations for R Cor
Bor have been iffy at best, but recent data from the Gaia
spacecraft suggest that the star is about 4,300 light-years
away. From that distance, our Sun would shine feebly
at around 15th magnitude!
A pair of unknowns that have astronomers scratch-
ing their heads are how stars like R Cor Bor form and
what causes their abrupt fadeaways. Two current and
wildly different theories of the star’s creation suggest
it’s experiencing the final helium f lash of an aging star
or the merger of a binary white dwarf system. It’s gener-
ally believed that the dimming is caused by the release
of a cloud of carbon-rich dust that envelops the star’s
photosphere. When outward radiation pressure pushes
the cloud away, or it simply dissipates on its own, R Cor
Bor returns to normal. The mechanism that produces
these sooty clouds is still up for study and debate.
Because its behavior is virtually opposite that of a clas-
sical nova, which undergoes a rapid increase in bright-
ness and then fades back to normalcy, R Cor Bor is
sometimes referred to as a “reverse nova.”
How bright (or dim) will R Cor Bor be tonight? I have
no idea. As I write this in early February, it’s been hover-
ing around magnitude 6.7 since a brief dip to 9th mag-
nitude last autumn. Has it returned to normal or begun
another plunge? The only way to find out is to go outsideand see for yourself. If R Cor Bor is near or at
maximum brightness, you should be able to
pick it up with standard binoculars — even
your unaided eye, if you’re viewing from a
dark-sky location. When near maximum, its
brightness is somewhere between that of the
stars marked as magnitudes 5.6 and 6.4 on
the accompanying chart. When it’s in the act
of fading or returning to max, it might
appear more like its magnitudes 7.4 and 8.1 neighbors.
If R Cor Bor isn’t visible with binoculars, it’s gone into
its disappearing act. Time to get out the telescopes!
Questions, comments, or suggestions? Email me at
[email protected]. Next month: Observing in the
good old summertime. Clear skies!Variable star R Coronae Borealis is a veritable treasure.
Target a changing
crown jewel
ABOVE: R Cor Bor
is an unpredictable
variable star that
resides near the
center-left of Corona
Borealis. In this chart,
the yellow numbers
indicate each star’s
magnitude. ASTRONOMY:
ROEN KELLY
RIGHT: It’s difficult to
predict exactly how
bright “R CurveBall”
will be on any given
night because it can
fluctuate by as much
as about 8 magnitudes
over a relatively short
period of time. ASTRONOMY:
ROEN KELLY, AFTER AAVSOHow bright
(or dim) will
R Cor Bor be
tonight? I
have no idea.BY GLENN CHAPLE
Glenn has been an
avid observer since
a friend showed
him Saturn through
a small backyard
scope in 1963.BROWSE THE “OBSERVING BASICS” ARCHIVE
AT http://www.Astronomy.com/Chaple