- 4468 - STAR EXPLOSIONS - how astronomers learn? 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 appears to have unexpectedly lost one sun's worth of ejected mass during the final years of its life before going supernova.
------------------------------ 4468 - STAR EXPLOSIONS - how astronomers learn?
- On the night of May 19, 2024, Japanese
amateur astronomer 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 amateur astronomers spotted the light
of “SN 2023ixf” they immediately knew they had 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. Cosmically speaking, that's
pretty 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. The race to decode a
supernova 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 which 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?
-
- The delayed shock breakout is direct
evidence for the presence of dense material from recent mass loss. These 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 like the star
Betelgeuse’s shenanigans 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.
-
- Submillimeter Array on Mauna Kea in Hawaii
sees the universe at long wavelengths. It was 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.
-
- 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 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.
-
- 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 supernovas, amateur astronomers can still make a
difference.
-
-
May 13, 2024 STAR
EXPLOSIONS - how astronomers learn? 4468
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