Sky & Telescope - May 2024 - 19

of heavy elements incorporated into stars, says Prasiddha
Arunachalam (Rutgers University). One example of a Type Ia
remnant is SNR 0509-67.5, which looks like a delicate ring.
The bubble is currently some 23 light-years wide and expanding
at 5,000 km/s (11 million mph). Arunachalam recently
used an algorithm to mix and match various explosion possibilities
and find the most likely explanation for the shock
motions detected inside the remnant. She calculated the
explosion's energy was about 1.3×1051 ergs, in keeping with
what's expected from a white dwarf's death.
Alas, it isn't always obvious what kind of star created a
given remnant. Astronomers know of more than 350 supernova
remnants in the Milky Way alone, and more in other
galaxies. But they've only classified about 100 as either Type Ia
or core-collapse. Often, observers rely on finding a neutron
star or particular elemental abundances to finger the latter.
Having multiple wavelengths helps. X-rays from the
superheated gas in the remnant's interior can reveal ejecta's
composition and velocities, for example. But for the remnant
Kes 75, X-rays don't tell us much. Kes 75 contains the
youngest known pulsar, whirligigging away at the tender age
of 500 years. The wind the pulsar creates in the remnant's
heart heats ejecta and a smattering of mixed-in dust, which
together glow in infrared, enabling Temim and her colleagues
to measure the material's velocity and simulate the
remnant's evolution.
Based on the oxygen and carbon emission detected, the
researchers can estimate how much mass the pulsar's wind
has swept up, from which they extrapolate that the star that
exploded had at most 12 Suns' worth of mass. Calculations
suggest the star was stripped down, perhaps by a companion
star. Astronomers have seen signs of binary stripping in a
handful of remnants, Temim says.
But, she adds, the only remnant of a massive star for which
we know exactly what kind of supernova made it is Cas A.
When Cas A's progenitor blew up, the supernova's flash scattered
off nearby dust grains. The scattered light preserves the
explosion's spectrum, including which elements were present.
Studying these light echoes - which, just like a sound echo,
reach us after the original event - reveals that the explosion
was a Type IIb supernova, the death of a big star stripped
of most of its hydrogen shell. The star likely began life with
between 15 and 25 solar masses, then was stripped down to
about 5 Suns (perhaps by a companion) before it exploded.
Mysterious Circumstances
One of the most iconic supernova remnants is the Crab
Nebula, beckoning to us from the constellation Taurus, the
Bull. With an apparent magnitude of 8.4, it's a nice target
for small telescopes on evenings early in the year. A haze fills
the remnant, emitted by high-energy electrons and positrons
that have been corkscrewing around the magnetic field of the
pulsar at the Crab's heart.
The Crab supernova lit up Earth's skies in AD 1054, which
makes it roughly 1,500 years older than Cas A. Oddly, there's
no sign of a blast wave or rebound shock. " Everything that we
observe in the Crab is the pulsar wind that just swept up all
of the stuff that was ejected, " Temim says. Photons emitted
by this wind of particles are ionizing the star's ejecta, some 7
solar masses' worth, and making it glow. The ejecta travel out
at the comparatively low speed of 1,200 km/s.
These results suggest that the explosion packed less than
10% of the typical pow of a core-collapse supernova. The star
that died probably had only 8 to 10 solar masses and perished
in a low-energy explosion.
Astronomers debate the details of how such a thing might
have unfolded. It might have been a hypothetical event called
an electron-capture supernova, in which a star just over the
edge of being big enough to die by core collapse meets its end.
The implosion would happen when the core is made up of
oxygen, neon, and magnesium, instead of iron.
Temim and others are currently analyzing JWST spectra
of the Crab to determine the remnant's composition. If they
find an abnormally high level of nickel compared with iron,
that could mean the explosion was this never-before-confirmed
type of supernova. The work is immensely challenging:
The researchers must tease apart a forest of roughly 100
spectral lines, all blended together by the mash-up of different
ejecta clumps' positions and velocities when flattened
into our 2D field of view.
Lasting Memories
Based on the supernovae we see in other galaxies and how
long it takes for remnants to fade away (some 100,000 years),
there may be anywhere from 1,000 to 4,000 supernova
remnants in the Milky Way. Taken at face value, that suggests
that we've only found one-tenth of what's out there.
New remnants are turning up thanks to endeavors like
the MDW Hydrogen-Alpha Sky Survey, conducted by the
late David Mittelman and S&T's Dennis di Cicco and Sean
Walker. These include the ancient G107.0+9.0 in Cepheus,
which astronomers estimate may be 100,000 years old. In
fact, most known remnants in our galaxy lie somewhere in
the age range of 10,000 to 100,000 years.
But by the time a remnant is several thousand years old,
the supernova's blast wave has usually finished its passage
through the gas shed in the star's death throes and moved on
to the surrounding interstellar medium. Although the remnant
may still be visible, it won't provide as much information
about the star that died. That's why astronomers devote
so much attention to young remnants like Cas A, the Crab,
and Supernova 1987A - all subjects of JWST's first observing
cycle. They're also turning JWST's gaze on supernovae in
other galaxies soon after the initial explosion, to detect the
infrared glow of newly created dust. It may well be that one
of JWST's lasting contributions to science will be revealing
where all this darn dust comes from.
¢ Science Editor CAMILLE M. CARLISLE has a soft spot in her
heart for Supernova 1987A.
sk yand tele scope .o r g * MAY 2024 19
http://skyandtelescoper.org

Sky & Telescope - May 2024

Table of Contents for the Digital Edition of Sky & Telescope - May 2024

Contents
Sky & Telescope - May 2024 - Cover1
Sky & Telescope - May 2024 - Cover2
Sky & Telescope - May 2024 - 1
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