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METALS - how did they get here? -
Before exploding, this star puffed out a sun's worth of mass. This supernova could prove to be a lynchpin
in our understanding of massive star deaths.
The massive star, in the final year or so of its life, ejected large
amounts of matter into space before going supernova.
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METALS - how did they get here?
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- This supernova exploded in the Pinwheel Galaxy in May, 2024,
and it appears to have unexpectedly lost approximately one sun's worth of
ejected mass during the final years of its life before going supernova. This
new discovery reveals more about the mysterious end days of massive stars.
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- This star had exploded in the nearby
Pinwheel Galaxy (Messier 101), which is just 20 million light-years away in the
constellation of Ursa Major, the Great Bear.
That's pretty close.
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- Amateur astronomers around the world started
gazing at “SN 2023ixf” because the Pinwheel in general is a popular galaxy to
observe. Haste is key when it comes to supernova observations: Astronomers are
keen to understand exactly what is happening in the moments immediately after a
star goes supernova. Yet all too often, a supernova is spotted several days
after the explosion took place, so they don’t get to see its earliest stages.
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- The race to decode a supernova began
measuring the supernova's light spectrum, and how that light changed over the
coming days and weeks. When plotted on a graph, this kind of data forms a
"light curve." The spectrum
from “SN 2023ixf” showed that it was a type II supernova.
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- This is a category of supernova explosion
involving a star with more than eight times the mass of the sun. Archival images of the Pinwheel suggested
the exploded star may have had a mass
between 8 and 10 times that of our sun. The spectrum was also very red,
indicating the presence of lots of dust near the supernova that absorbed bluer
wavelengths but let redder wavelengths pass.
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- Normally, a type II supernova experiences
what astronomers call a 'shock breakout' very early in the supernova's
evolution, as the blast wave expands outwards from the interior of the star and
breaks through the star's surface. Yet a bump in the light curve from the usual
flash of light stemming from this shock breakout was missing. It didn’t turn up for several days. Was this a
supernova in slow motion?
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- The delayed shock breakout is direct
evidence for the presence of dense material from recent mass loss. Observations revealed a significant and
unexpected amount of mass loss, close to the mass of the sun, in the final year
prior to explosion.
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- An unstable star puffing off huge amounts
of material from its surface creates a dusty cloud of ejected stellar material
all around the doomed star. The supernova shock wave therefore not only has to
break out through the star, blowing it apart, but also has to pass through all
this ejected material before it becomes visible. This took several days for this supernova.
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- Other massive stars often shed mass. Betelgeuse has shed mass over late 2019 and
early 2020, when it belched out a cloud of matter with ten times the mass of
Earth’s moon that blocked some of Betelgeuse’s light, causing it to appear dim.
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- Betelgeuse isn’t ready to go supernova
just yet, and by the time it does, the ejected cloud will have moved far enough
away from the star for the shock breakout to be immediately visible. In the
case of SN 2023ixf, the ejected material was still very close to the star,
meaning that it had only recently been ejected, and astronomers were not
expecting that.
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- The only way to understand how massive stars
behave in the final years of their lives up to the point of explosion is to
discover supernovae when they are very young, and preferably nearby, and then
to study them across multiple wavelengths.
Using both optical and millimeter telescopes they effectively turned SN
2023ixf into a time machine to reconstruct what its progenitor star was doing
up to the moment of its death.
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- Stars, they're just like onions. An evolved massive star as being like an
onion, with different layers. Each layer is made from a different element,
produced by sequential nuclear burning in the star's respective layers as the
stellar object ages and its core contracts and grows hotter.
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- The outermost layer is hydrogen, then you
get to helium. Then, you go through carbon, oxygen, neon and magnesium in
succession until you reach all the way to silicon in the core. That silicon is
able to undergo nuclear fusion reactions to form iron, and this is where
nuclear fusion in a massive star’s core stops.
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- Iron requires more energy to be put into
the reaction than comes out of it, which is not efficient for the star. Thus the core switches off, the star
collapses onto it and then rebounds and explodes outward.
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- One possibility is that the final stages of
burning high-mass elements inside the star, such as silicon (which is used up
in the space of about a day), is disruptive, causing pulses of energy that
shudder through the star and lift material off its surface.
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- What the story of SN 2023ixf does tell us
is, at the very least, that despite all the professional surveys hunting for
transient objects like supernova.
Without early detection we would have missed the opportunity to gain
critical understanding of the evolution of massive stars and their supernova
explosions.
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May 30, 2024 RARE
EARTH METALS - how
did they get here? 4488
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