28 Scientific American, December 2020
Tying together the evidence from different tele-
scopes, we concluded that after shining for millions of
years, the star did something surprising and mysteri-
ous: it abruptly cast off layers of gas from its surface,
forming a cocoon around itself. A few days or a week
later the star exploded. The debris from the blast col-
lided with the cocoon, producing an unusually bright
and short-lived flash of light. Because the explosion took
place in a galaxy far away—the light took almost a bil-
lion years to reach Earth—it was too dim to be seen with
the naked eye but bright enough for our observatories.
Through a retrospective search of telescope data, we
were even able to detect the star in the act of shedding
two weeks before it exploded, when it was one one-hun-
dredth as bright as the explosion itself.
This was just one of several recent discoveries that
have shown us that stars die in surprisingly diverse
ways. Sometimes, for example, the remnant of a star’s
core that is left over after a supernova remains active
after the star has collapsed—it can launch a jet of mate-
rial moving at hyperrelativistic speeds, and the jet itself
can destroy the star with more energy than a normal
supernova. Sometimes, in the final days to years of its
life, a star blows away a significant fraction of its gas
in a series of violent eruptions. These extreme deaths
appear to be rare, but the fact that they happen at all
tells us there is much we still do not understand about
the basics of how stars live and die.
Now my colleagues and I are amassing a collection
of unusual stellar endings that challenge our tradition-
al assumptions. We are beginning to be able to ask
and answer fundamental questions: Which factors
determine how a star dies? Why do some stars end
their lives with eruptions or violent jets, while others
simply explode?
A NEW STAR
The sTory of sTellar birTh, life and death is a tale of
competing forces. Stars are formed in interstellar
clouds of hydrogen gas when the force of gravity pulls
part of the cloud inward strongly enough to overcome
the outward push of magnetic fields and gas particles
O
n sepTember 9 , 2018 , a roboTic Telescope on iTs rouTine paTrol of The
night sky detected what looked like a new star. Over the next few
hours, the “star” grew 10 times brighter, triggering a flag by soft-
ware I had written to identify unusual celestial events. It was night-
time in California, and I was asleep, but my colleagues on the oth-
er side of the world reacted quickly to the alert. Twelve hours later
we had obtained enough additional data from telescopes on Earth
and in space to confirm that this was the explosion of a star—a supernova—in a distant galaxy.
But this was no ordinary supernova.
Anna Y. Q. Ho is a Miller Fellow in the astronomy
department at the University of California, Berkeley.
She studies the catastrophic deaths of massive stars.
