- 4187 - SUPERNOVA - new discoveries with close encounter? The massive star, in the final year of its life, ejected large amounts of matter into space before going supernova. This occurred n the Pinwheel Galaxy in May, 2023, and appears to have unexpectedly lost approximately one sun's worth of ejected mass during the final years of its life.
------------ 4187 - SUPERNOVA - new discoveries with close encounter?
- The night before
exploding, this star puffed out a sun's worth of mass. This supernova, pinpointed by amateur
astronomers, could prove to be a lynchpin in our understanding of massive star
deaths.
-
- On the night of
May 19, 2023, Japanese amateur astronomers were conducting regular supernova
sweep using telescopes based in three
remote observatories dotted around the country. They were located in Yamagata, Okayama and on the island of
Shikoku.
-
- Amateur astronomers
have a long history of discovering exploding stars before the professionals
spot them. 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. Cosmically speaking, that's close.
-
- Soon enough,
amateur astronomers around the world started gazing at “SN 2023ixf” because the
Pinwheel in general is a popular galaxy to observe. However, 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. Alerted to the supernova, several
professional telescopes were used, including the 6.5-meter Multi Mirror
Telescope (MMT) at the Fred Lawrence Whipple Observatory on Mount Hopkins in
Arizona. They 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. Was
this a supernova in slow motion?
-
- The delayed shock
breakout is direct evidence for the presence of dense material from recent mass
loss. Imagine an unstable star 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.
Seemingly, this took several days.
-
- Massive stars often
shed mass. Just look at Betelgeuse’s
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. 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, and astronomers were not expecting that.
-
- 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.
-
- We can think of 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 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. 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.
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-
October 13, 2023 SUPERNOVA - new discoveries with close encounter? 4187
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