BOB FRANKE
qR LEPORIS Hind’s Crimson Star (left) blazes with a blood-red glow
that puts G77-61’s shine to shame. R Leporis and its other giant breth-
ren are more deeply tinted than the dwarf because dust clouds surround
their atmospheres and redden their starlight.skyandtelescope.com• FEBRUARY 2020 59their carbon-rich atmospheres, enshrouding themselves in
dust that further reddens their light.
Most of the galaxy’s carbon stars, though, are not the
crimson giants glistening through your telescope. Instead,
they’re mere red dwarfs, much smaller than the Sun.
“There are far more dwarf carbon stars than giant carbon
stars,” says Bruce Margon (University of California, Santa
Cruz), who nevertheless admits that few astronomers have
heard of the stars. “If you go to any assemblage of astrono-Dahn later gave a talk about G77-61. “This just can’t be
— it cannot be a carbon star,” he remembers one astronomer
saying. “There has to be a misidentifi cation.” Yet today G77-
61 is the prototype for the entire class of stars.
“The phrase ‘dwarf carbon star’ should be an oxymoron,”
Margon says. That’s because a red dwarf generates energy by
converting hydrogen into helium and produces no carbon.
A carbon-smothered red dwarf is therefore akin to a kinder-
gartner carrying handwritten calculus equations to school.
Just as you might suspect that the kindergartner didn’t
write those equations but instead had help from an older
sibling, so Dahn’s team attributed the carbon atop G77-61 to
a companion star, whose light no one had ever seen. In this
scenario, the companion was born more massive than the
red dwarf and eventually expanded into a red giant, when it
fused helium nuclei together to forge carbon. Some of this
carbon then reached the star’s surface. The giant showered
carbon onto its red dwarf neighbor and soon shrank into a
hot white dwarf. The white dwarf then cooled and faded so
that the red dwarf now outshines it, which is why no one can
see the white dwarf’s light.
A decade later, a wobble in the red dwarf signaled an orbit-
ing star with just the right mass to be a white dwarf, con-
fi rming this theory for the red dwarf’s carbon. The red and
white dwarfs orbit each other every 245 days.
But G77-61 was still unique, a stellar freak, the only
dwarf carbon star ever seen. Further confi rmation for themers and say ‘What’s a dwarf carbon star?’, you just get
blank stares.”
Yet these little red stars tell two intriguing tales. The
fi rst is one of stellar evolution: How can a red dwarf, which
doesn’t create carbon, get so much of this element as to
become a carbon star? The second is a promise for the future:
possible new insight into the universe’s fi rst years, because
many and maybe most carbon dwarfs belong to the galaxy’s
ancient stellar population, its halo.They Might Not Be Giants
Astronomers found the fi rst carbon dwarf in the 1970s, a
red star in Taurus named G77-61. “G77-61 had extremely
peculiar colors,” says Conard Dahn (U.S. Naval Observa-
tory, Flagstaff), who measured its brightness at different
wavelengths. The star was fairly nearby — about 255 light-
years from Earth, according to modern measures — but was
as dim as Pluto, which meant the star was a red dwarf, not
a red giant. At a 1975 Christmas party, Dahn told another
astronomer how the star was redder than other red dwarfs of
the same luminosity.
That conversation prompted astronomers at Lick Observa-
tory to obtain the star’s spectrum in January 1976. One of
the observers, James Liebert (University of Arizona), phoned
Dahn. “He says, ‘You’re not going to believe this, but it looks
like a carbon star,’” Dahn recalls. “Knowing that carbon
stars were giants, I said: ‘Come on, what are you talking
about here?’” The little red star in Taurus had the same car-
bon compounds as “normal” carbon stars: diatomic carbon
molecules (C 2 ), methylidyne (CH), and cyanogen (CN).THE PROTOTYPE
Name: G77-61
Constellation: Taurus
Distance from Earth: 255 light-years
Apparent Visual Magnitude: 13.9
Absolute Visual Magnitude: 9.4