- 4485 - SUPERNOVAE - close to home? - Before exploding, this star puffed out a sun's worth of mass. The tumultuous massive star, in the final year or so of its life, ejected large amounts of matter into space before going supernova. The star exploded in the Pinwheel Galaxy in May, 2023. It unexpectedly lost approximately one sun's worth of ejected mass during the final years of its life before going supernova.
------------------------------------- 4485 - SUPERNOVAE - close to home?
- On the night of May 19, 2023, Japanese
amateur astronomer Kōichi Itagaki was conducting his 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.
-
- When Itagaki spotted the light of SN
2023ixf he immediately knew he'd found something special. That’s because 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.
-
- 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, astronomers immediately followed-up with several professional
telescopes at their disposal 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, or was something else afoot?
-
- The delayed shock breakout is direct
evidence for the presence of dense material from recent mass loss. A significant and unexpected amount of mass
loss, close to the mass of the sun, occured in the final year prior to
explosion.
-
- 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.
-
- 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.
-
- 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.
-
- Astronomers were able to observe SN 2023ixf
with the Submillimeter Array on Mauna Kea in Hawaii, which sees the universe at
long wavelengths. They were able to 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 we effectively turned SN
2023ixf into a time machine to reconstruct what its progenitor star was doing
up to the moment of its death.
-
- Astronomers 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.
-
-
May 28, 2024 SUPERNOVAE
- close to home? 4485
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--------------------- --- Wednesday, May 29, 2024
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