Sky & Telescope - March 2021 - 32

Observing Variable Stars

Just how these stars form remains a mystery. Over the
years, astronomers have proposed two very different models to account for them. The ﬁrst is the merger of two white
dwarfs - and not just any two white dwarfs, but a carbonoxygen white dwarf and a helium white dwarf. Most white
dwarfs are composed of carbon and oxygen. However, very
low-mass progenitors can't fuse helium, resulting in a helium
white dwarf. As white dwarfs are also known as degenerate
dwarfs, this model for RCB formation is sometimes called
a double degenerate merger. On the other hand, if the white
dwarfs are massive enough, then the merger leads to a Type Ia
supernova instead.
The other competing, albeit less favored, model of RCB
formation is the ﬁnal-helium-shell ﬂash, in which a star that's
already in the planetary nebula phase rapidly expands and
becomes a supergiant as its helium shell ignites. Thus far,
three stars (Sakurai's Object, V605 Aquilae, and FG Sagittae) have undergone these ﬁnal ﬂash outbursts, and they all
exhibit spectral characteristics similar to those of RCB stars.

More than two million conﬁrmed and suspected variable
stars have been cataloged, according to the American Association of Variable Star Observers (AAVSO). And yet the RCB
count hasn't kept pace, despite the prototype being one of the
earliest variables found. It turns out that RCBs are very rare.
In a seminal 2012 paper, Louisiana State University
astronomer Geoffrey Clayton noted that the list of known or
suspected RCB stars stood at the modest total of around 90,
with 65 lying within the Milky Way and 25 in the neighboring Large and Small Magellanic Clouds. Assuming that RCBs
form from the merger of two white dwarfs, Clayton looked at
the population of known RCBs in the LMC and extrapolated
a total population of around 5,700 RCBs for the Milky Way.
Taking a different approach, he also modeled the likely number of white dwarf mergers per year and the expected lifetime
of RCB stars to arrive at an estimated population of 5,400
Milky Way RCBs - in line with his LMC extrapolation.

X A (VARIABLE) STAR IS BORN This
sequence of illustrations depicts the
formation of an RCB star through the
merger of two different types of white
dwarf stars. The energy generated by
their collision causes the merged stars
to expand and form a supergiant with a
diameter of around 100 million kilometers
(62 million miles). This is the most widely
accepted model explaining the formation
of RCB-type stars.

MARCH 2 021 * SK Y & TELESCOPE

S INCONSPICUOUS JEWEL Corona Borealis, the Northern Crown, isn't
a constellation most telescope enthusiasts spend a lot of time exploring.
There are no bright clusters or galaxies within its borders, but it does
boast R Coronae Borealis - one of the most interesting variable stars in
the sky. The star (indicated with the arrow above) had dimmed to roughly
magnitude 8 at the time this photo was captured.

Since then, astronomers have revised the estimated population downward to around 1,000. At the same time, groundbased and orbital surveys have increased the number of RCB
ﬁnds. New results published in 2020 by Patrick Tisserand
(Australian National University), Clayton, and colleagues
have brought the count up to 147.

CORON A BORE A LIS: A K IR A FUJII;
RCB FOR M ATION SEQ UENCE: G EMINI
OBSERVATORY / JON LO MBERG (5)

An Unusual and Rare Variable

Sky & Telescope - March 2021

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