- 4177 -
SUPERNOVA - spotted the minute it started? The tumultuous massive star, in the final
year or so of its life, ejected large amounts of matter into space before going
supernova. This massive star that
exploded in the Pinwheel Galaxy in May, 2022,
appears to have unexpectedly lost approximately one sun's worth of
ejected mass during the final years of its life before going supernova.
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4177 - SUPERNOVA
- spotted the minute it started?
-
- Astronomers want
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.
Considering how close, relatively speaking, SN 2023ixf was to
us and how early it was identified, it was a prime candidate for close study.
-
- Astronomers
measured 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. This is a category of supernova explosion
involving a star with more than eight times the mass of the sun. In the case of
SN 2023ixf, searches in 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. This was all fairly
typical, but what was especially extraordinary was the shape of the light
curve.
-
- 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.
-
- The delayed shock
breakout is direct evidence for the presence of dense material from recent mass
loss. Our new observations revealed a
significant and unexpected amount of mass loss, close to the mass of the sun,
in the final year prior to explosion.
-
- The unstable star
was puffing off huge amounts of material from its surface. This 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.
-
- Massive stars
often shed mass. Betelgeuse’s has shed
mass from late 2019 and to 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.
-
- However,
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.
-
- The Submillimeter
Array on Mauna Kea in Hawaii is able to see the universe at long wavelengths.
Astronomers could see the collision
between the supernova shockwave and the circumstellar cloud.
-
- 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
astronomers effectively turned SN 2023ixf into a time machine to reconstruct
what its progenitor star was doing up to the moment of its death.
-
- The question then
becomes, what caused the instability?
Stars, they're just like onions 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.
-
- 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 because 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
outwards. 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.
-
-
October 4, 2023 SUPERNOVA
- spotted the minute it started? 4170
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Thursday, October 5, 2023 ---------------------------------
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