Wednesday, May 15, 2024

STAR EXPLOSIONS - how astronomers learn?

 

-  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.

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-    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.

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-    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.

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-    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."

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-    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.

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-    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.

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-   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?

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-    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.

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-    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.

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-    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.

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-    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.

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-   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.

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-    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.

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-    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.

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-  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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-    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.

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May 13, 2024          STAR  EXPLOSIONS  -  how astronomers learn?           4468

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--------------------- ---  Wednesday, May 15, 2024  ---------------------------------

 

 

 

 

 

           

 

 

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