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Astronomers spot
a new type of
supernovaASTRONOMERS HAVE LONG KNOWN that stars
more massive than about eight Suns die in powerful blasts,
known as type II or core-collapse supernovae. But in 1980, a
paper in Publications of the Astronomical Society of Japan sug-
gested that stars in a specific mass range — eight to 10 times
the mass of the Sun — would die in a unique explosion called
an electron-capture supernova. Prior to such a star’s death, its
core is supported by electron degeneracy pressure, generated
in dense environments where physics prevents free electrons
from getting too close. But as magnesium and neon atoms in
the core absorb (i.e., capture) these free-f loating electrons, the
pressure drops and the star’s inner regions collapse into a neu-
tron star as the outer regions rebound as a supernova.
While some supernovae have exhibited hints they could
be an electron-capture supernova, none exactly matched the
theory. Until March 2018, that is, when Japanese amateur
astronomer Koichi Itagaki spotted a supernova in the galaxy
NGC 2146, which lies about 31 million light-years away.
Researchers raced to image the explosion, called SN 2018zd,
with the Hubble Space Telescope. Comparing their data with
previous Hubble images of the galaxy, they identified the pro-
genitor star. And after fully analyzing the star and its explosion,
the team published a paper June 28 in Nature Astronomy
announcing it neatly fits all six expected criteria for an electron-
capture supernova.
“We started by asking, ‘What’s this weirdo?’ Then we
examined every aspect of SN 2018zd and realized that all of
them can be explained in the electron-capture scenario,” said
the study’s lead author, Daichi Hiramatsu at the University of
California, Santa Barbara, and Las Cumbres Observatory, in a
press release. “It was such a ‘eureka moment’ for all of us that
we can contribute to closing the 40-year-old theoretical loop.”
The progenitor was the specific mass and type of red giant
theorized to produce an electron-capture supernova, and the
density and composition of its stellar winds prior to its demise
also matched expectations. The explosion itself — its light
lingering as the shock wave hit material the star had sloughed
off before its death — behaved exactly as astronomers modeled
for this type of stellar blast. Finally, the particular chemical
fingerprints of the debris matched calculations of what an
electron-capture supernova leaves behind.
Now, researchers can slot this information into the bigger
picture of how stars live and die. The masses of stars that cre-
ate electron-capture supernovae fit neatly between stars that
leave behind a white dwarf, like the Sun, and those that
explode in a core-collapse supernova. “One of the main ques-
tions in astronomy is to compare how stars evolve and how
they die,” said study co-author Stefano Valenti at the
University of California, Davis. “There are many links still
missing, so this is very exciting.”The recent supernova SN 2018zd, which occurred in the galaxy NGC 2146 (seen
at upper left), fits all six criteria for a long-theorized but never-confirmed type of
stellar explosion called an electron-capture supernova. JOSEPH DEPASQUALE, STSCICONNECTIONS TO THE PAST
The discovery of a new type of supernova may also solve a
centuries-old mystery. Researchers have long believed the
supernova of A.D. 1054, which produced the famous Crab
Nebula (M1), was an electron-capture supernova. Despite
the fact that it popped off long before astronomers had
instruments such as telescopes and spectrometers to chart
the skies, this blast was previously the best-fitting example
of such an event. From the neutron star it left behind to the
way ancient astronomers recorded its light changing after
the initial blast to modern observations of how the nebula
has slowly expanded over time, the Crab meets several crite-
ria for this particular flavor of supernova. Now, SN 2018zd
provides firm evidence linking M1 to an electron-capture
supernova — and that gives researchers a current picture
of how these
events evolve
in the centuries
after they
explode. — A.K.
Based on
ancient records
and current
observations,
astronomers
think the famous
Crab Nebula
(M1) in Taurus
was likely an
electron-capture
supernova. ESO