- 4216 -
SUPERNOVA - witness to an early explosion? 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, 2023,
appears to have unexpectedly lost approximately one sun's worth of
ejected mass during the final years of its life before going supernova.
--------------------- 4216 - SUPERNOVA - witness to an early explosion?
- Exploring light
before exploding, this star puffed out a sun's worth of mass. It was a supernova, pinpointed by amateur
astronomers, and it could prove to be a lynchpin in our understanding of
massive star deaths.
-
- This star 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.
-
- 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.
-
- 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. What was especially extraordinary was the
shape of this light curve.
-
- Normally, a type
II supernova experiences 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 this 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?
-
- The delayed shock
breakout is direct evidence for the presence of dense material from recent mass
loss. 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.
-
- This 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.
-
- Massive stars
often shed mass. Betelgeuse over late
2019 and early 2020, 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.
-
- 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.
-
- 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 turned SN 2023ixf into a time machine to reconstruct what its
progenitor star was doing up to the moment of its death.
-
- What caused the
instability? Stars are 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.
-
- The outermost
layer is hydrogen, then helium. Then 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.
-
- 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.
-
-
November 11, 2023
SUPERNOVA - witness to an early explosion? 4216
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Saturday, November 11, 2023 ---------------------------------
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