This graphic features data from NASA’s Chandra X-ray
Observatory of the Cassiopeia A (Cas A) supernova remnant that reveals that the
star’s interior violently rearranged itself mere hours before it exploded. The
main panel of this graphic is Chandra data that shows the location of different
elements in the remains of the explosion: silicon (represented in red), sulfur
(yellow), calcium (green) and iron (purple). The blue color reveals the
highest-energy X-ray emission detected by Chandra in Cas A and an expanding
blast wave. The inset reveals regions with wide ranges of relative abundances
of silicon and neon. This data, plus computer modeling, reveal new insight into
how massive stars like Cas A end their lives.
X-ray: NASA/CXC/Meiji Univ./T. Sato et al.; Image
Processing: NASA/CXC/SAO/N. Wolk
The inside of a star turned on itself before it spectacularly exploded,
according to a new study from NASA’s Chandra X-ray Observatory. Today, this
shattered star, known as the Cassiopeia A supernova remnant, is one of the
best-known, well-studied objects in the sky.
Over three hundred years ago,
however, it was a giant star on the brink of self-destruction. The new Chandra
study reveals that just hours before it exploded, the star’s interior violently
rearranged itself. This last-minute shuffling of its stellar belly has profound
implications for understanding how massive stars explode and how their remains
behave afterwards.
Cassiopeia A (Cas A for short) was
one of the first objects the telescope looked at after its launch in 1999, and
astronomers have repeatedly returned to observe it.
“It seems like each time we closely
look at Chandra data of Cas A, we learn something new and exciting,” said
Toshiki Sato of Meiji University in Japan who led the study. “Now we’ve taken
that invaluable X-ray data, combined it with powerful computer models, and
found something extraordinary.”
As massive stars age, increasingly
heavy elements form in their interiors by nuclear reactions, creating
onion-like layers of different elements. Their outer layer is mostly made of
hydrogen, followed by layers of helium, carbon and progressively heavier
elements – extending all the way down to the center of the star.
Once iron starts forming in the
core of the star, the game changes. As soon as the iron core grows beyond a
certain mass (about 1.4 times the mass of the Sun), it can no longer support
its own weight and collapses. The outer part of the star falls onto the
collapsing core, and rebounds as a core-collapse supernova.
The new research with Chandra data
reveals a change that happened deep within the star at the very last moments of
its life. After more than a million years, Cas A underwent major changes in its
final hours before exploding.
“Our research shows that just
before the star in Cas A collapsed, part of an inner layer with large amounts
of silicon traveled outwards and broke into a neighboring layer with lots of
neon,” said co-author Kai Matsunaga of Kyoto University in Japan. “This is a
violent event where the barrier between these two layers disappears.”
This upheaval not only caused
material rich in silicon to travel outwards; it also forced material rich in
neon to travel inwards. The team found clear traces of these outward silicon
flows and inward neon flows in the remains of Cas A’s supernova remnant. Small
regions rich in silicon but poor in neon are located near regions rich in neon
and poor in silicon.
The survival of these regions not
only provides critical evidence for the star’s upheaval, but also shows that
complete mixing of the silicon and neon with other elements did not occur
immediately before or after the explosion. This lack of mixing is predicted by
detailed computer models of massive stars near the ends of their lives.
There are several significant
implications for this inner turmoil inside of the doomed star. First, it may
directly explain the lopsided rather than symmetrical shape of the Cas A
remnant in three dimensions. Second, a lopsided explosion and debris field may
have given a powerful kick to the remaining core of the star, now a neutron
star, explaining the high observed speed of this object.
Finally, the strong turbulent flows
created by the star’s internal changes may have promoted the development of the
supernova blast wave, facilitating the star’s explosion.
“Perhaps the most important effect
of this change in the star’s structure is that it may have helped trigger the
explosion itself,” said co-author Hiroyuki Uchida, also of Kyoto University. “Such final internal activity
of a star may change its fate—whether it will shine as a supernova or not.”
These results have been published
in the latest issue of The Astrophysical Journal and are available online.
To learn more about Chandra, visit: https://science.nasa.gov/chandra
Source: NASA's Chandra Reveals Star's Inner Conflict Before Explosion - NASA
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