What would we see of a devastating interstellar war between alien civilizations within 1000 Ly?

Question

I have two interstellar civilizations, both of which are extremely advanced and within a few thousand light years of earth. They go to war and make some stars explode in the process.

The Combatants

I have in my space setting two civilizations who are extremely, extremely advanced. To get an idea of their capabilities let me list the ones relevant to this discussion. Both sides were capable of causing a star to go super nova, had micro black hole arrays as their "small arm weaponry" and were able to warp from star system to star system by willing it. There is a tech gap between them that is balanced by the less advanced ones, who I will refer to as the Industrials (of whom you can imagine as extremely advanced machines), being much more tenacious than the Archivists. The Archivists, whose design is very serpentine and fuming in blinding energy are ancient beings who really just wanted to collect data on the universe. They predate the empire, but that part of this overbuilt cosmic history isn't important here.

So why are they at war? Here's a bit of context for anyone who needs it.

Context

There is a somewhat complicated history behind why such a cataclysmic war occurred in the first place. Since it's not the meat of my question and really unrefined in terms of completion i'll just say the following long sentence. While humans were figuring out agriculture a powerful, seemingly all encompassing empire spanning the Orion Arm fell apart violently due to a miasma of catastrophes that stem from the Archivists launching a decapitation raid on the empire's oligarchs in response to the empire destroying their observatory world located around a star in the Aquila rift. Said observatory held quite a few Archivists. Given the distance of the named place, you may see why i'm asking this question.

Fast forward a thousand years and the empire is pretty much dead. A procession of events leads the Industrious to have a civilization that is slowly murdered by a cryptic force known as the Celestials who appear on the scene a few centuries after the empire's decapitation. They just start killing anyone who is self-aware and a pain inducing war against a would-be Empire Successor polity called the Eternal Republic which no one but the Celestials won. The Eternal Republic was exterminated, but the Industrious had enough time to go into hiding as the celestials massacred their society as well. Industrious than spent a few millenniums in hiding as the Celestials continued hunting down life that was sensed by them as self-aware, purifying world after world of such life. The Industrious however, rebounded and rebounded hard.

The Industrious deep within the mantle of multiple worlds uninhabitable managed to advance to technological levels that rivaled the Archivists to a degree and begun a uphill crusade against the Celestials, at first being very outnumbered. However the Industrious at this point were militarily far more advanced than the Celestials, meaning they easily fought the Celestials despite being out numbered and rapidly expanded in numbers as they exterminated the Celestials. Once the Celestials were killed off, the Industrious inexplicably turned their weapons and vast military on the Archivists. Thus began the Annihilation.

In the Annihilation (which was a war that only lasted a century), multiple stars were super novae'd. But the Industrious, despite managing to do far more damage to the Archivists than the Empire ever could would lose the war. The Industrious as a species were let off surprisingly easy, being only reduced to a primitive, stone age civilization. The Archivists however, instead of coming to dominate the Orion Arm simply went into further hiding. They still are out there to this day, observing as they always had.

I am aware that by cosmic history has opened way more questions than answers (like about the culture/nature of all these aliens and the way the empire was ran), but I rapidly came to realize a massive, massive trouble with all this star exploding and interstellar massacring:

Wouldn't we have noticed all this?

I ask this since there is no way a war where stars went into supernova in such rapid procession wouldn't be noticed by astronomers and raise tons of questions. The super novae would after all leave behind planetary nebulae in their wake, such as the crab nebula which came from a super nova seen around the world in 1200 AD. The question of super novae also has been bothering me since wouldn't multiple super novas in short procession would cause some very noticeable planetary nebulae complexes? These would be extremely noticeable cosmic features within a thousand light years and their cultural impact may have history altering conditions if I have this in my mind right.

One solution I had was simply to write these anomalies into the history books, but than I realized that having multiple super novae go off in the sky in rapid procession may completely alter human culture (the Annihilation happening just as our civilizations were taking shape) to the point where i'd have to rewrite all of human history, something I don't really want to do. I like to make alien societies from scratch, but find remaking human history a extremely hard task despite looking up history in my free time, go figure.

So, what would a nearby interstellar where super novas occur as a direct product look like from earth and is there a way to hide it or have it be simply dismissed in our time as just a natural occurrence? I don't really know what to do and I don't want to move all this to the other side of the galaxy. I used to have it on the other side of the galaxy, but decided that I didn't want a galactic setting anymore and so compacted my space opera into our corner of the galaxy. Why? Because I like space being massive and being able to traverse the galaxy has always undermined that feeling for me. So a empire that only spans the Orion Arm is extremely, extremely massive to me.

To the main question.

Ignoring the subjective part (on my placement of the empire), my assessment on the product of a interstellar war between extremely advanced societies where some stars are detonated accurate? If not, what would the impact of a war of this magnitude of destruction in the space? Assume a variable distance of 450-1700 Ly or so from earth towards the general direction of the Aqulia Rift.

(I think a anomaly of many planetary nebulae in one area would be a bit anomalous, not even taking into account the cultural impact of seeing supernovae in the sky as a reoccurring event for two centuries on our ancient ancestors.)

Answer 1

I think a lot of it would depend on how far back you are setting the war. Humans became anatomically modern @ 200,000 years ago, but the cultural explosion happened much closer in time (@ 50,000 years ago).

If you push your time line far enough back that the light from the distant suns passes Earth between 200,000 and 50,000 years ago, essentially there would be no one there to notice. Humans and their cousins from that time period exhibited few or no signs of culture, language, arts or other signs of sapience beyond that of extremely smart pack animals. Modern astronomers would obviously be aware that something is amiss in the night sky, but this would be a result of late 20th century astronomy using the entire sophisticated arsenal of observation tools and in multiple wavelengths (from radio astronomy to visible light, infrared, UV and even x-ray astronomy).

Given the relatively short time any individual astronomer can have on a particular instrument, and spreading the observations over long periods of time and space (a Spanish astronomer observes star "x" in Infrared and notes an anomaly in 1988, but the next time the star is studied it is a Japanese astronomer using a radio telescope in 1996. The Hubble space telescope takes a look as part of an American project in 2002, and the Kepler observes this in passing while looking for a habitable planet in 2014). It takes a long time before these various observations are brought together, and even longer before anyone starts noticing something is amiss.

What they are seeing are large nebula formed from the explosions of stars, huge swaths of ionized gasses and molecular clouds punctuated by a rather unusual number of pulsars (neutron stars rotating at extremely high speeds). What will eventually tweak someone's curiosity are the sheer number of nova and supernova remnants in a relatively small area, and the puzzle that many of the remnants do not seem to match modern theories of supernova formation. The estimated masses of the nebula and former stars will be far lower than the generally accepted lower bound for a type 2 supernova (8 solar masses. The supernova is caused by the collapse of the core when the fusion process has reached iron, which produces no net energy. The radiation pressure of the core ceases and gravity causes the mass of the star to collapse into the core, triggering the supernova).

Considering the number of stars you seem to be postulating, there should also be a fair number of type 1 nova in the region as well, as the neutron star remnants of the former stars move into binary orbit of an unexploded star over time and begin transferring mass from the active star. The resulting hydrogen "atmosphere" will eventually reach a pressure and density that triggers a fusion reaction, which both ancient and modern astronomers will certainly notice.

A bit of tweaking of the backstory would be required to ensure that the number of observed type 1 nova matches the historic record. You could also invoke some sort of handwavium by having the cores of the murdered stars ejected from the galaxy at high velocity due to whatever caused the supernova explosion in the first place (they will not have reached the edge of the galaxy yet, but are moving fast enough that they will not interact with other stars).

So if the events happened far enough away in time and space that there were no "modern" humans to see the actual events in the sky, then modern astronomers will have lots of clues scattered about, but due to the dispersed nature of astronomy (no singe astronomer spends his life at one telescope or observatory any more, unlike Tycho Brahe or John Herschel), it will take a long time to put the subtle clues together.

Answer 2

From your intro we have: _{There is a somewhat complicated history...While humans were figuring out agriculture...in the Aquila rift...(Archivists attacked)...a few centuries after...(we have Celestials)...(over) a thousand years (from the Archivists' attack)......(Everything is quiet)...(Industrious) spent a few millenniums in hiding (and then kicked butt)...(so over a thousand years is actually a few milleniums (3+))}

Which gives us a timeline:

• Humans Discovering Agriculture (11,500 yrs ago)
• Archivists vs Empire (11,500 yrs ago)
• Celestial Cleansing (+X00 yrs)
• Industrious Revolution (+Y000 yrs)
• Annihilation (+100 yrs)

So we need to overcome some issues. Mainly surviving records and detectable remnants.

For records:

• 4,485 BC is about our latest date if you max out the X to 9 and bump the Y to 4. We're talking the time when the Sumerians were inventing the patriarchy. No more clay Mother Earths. The world had more important social issues and stars were probably not high on the list.
• The Earliest recorded supernova is from 185 AD. Well after any of your dates. So we're looking at so long ago that there wouldn't be remaining records or ways of viewing after the initial burst. Although we're close enough that you would have a near-Earth supernova and you'd be likely dead anyway. If you want humans to survive better you need more of your burst to be neutrinos.
• Wikipedia's article on Kepler's Supernova seems to suggest that many would be obsessed with viewing it if they could see it (large amount of stargazing contemporaries and whatnot). But if you we're struggling to survive there's very little reason to record what the sky is doing (oral tradition in those days) which leaves your 185 AD record intact.

Detecting the remnants:

• Tycho's supernova highlights issues for detecting the remnant and also for possible advances or cultural shifts during that time. Of note is the fact that advances can slip away from humanity if that culture gets conquered so no net gain is acceptable, and once again the the advancement is not likely due to survival pressures.
• RX J0852.0-4622, is closer than your distance and seems to have eluded us until 1998/2001.
• Your method of explosion determines how visible and for how long. Material of star does as well. Roughly speaking the heavier the material the brighter and more short-lived it is, whereas lighter material is brighter longer. The majority of 55% of our supernova are bright and fast. If you move more of the products into neutrinos you become less detectable. Maybe they could only blow up stars with lots of heavy metals?
• There's a decent amount of protostars nearby that are attracting the clouds of The Rift but not yet accreting disks. Your supernovas could have been dispersed by them.
• Supernovae should occur about three times every century in our galaxy (source). Yet I don't think we're discovering remnants at that rate.
• At your distances active stars are already being obscured by the water clouds in The Rift. As long as the remnant's thermal signature is low enough they should be all but invisible even if they're standard supernovae.

In conclusion, I'm not an expert but after a fair amount of research it seems like you could just wing it and you'd be good. Disruptions in the clouds are still being explained (magnetism, etc.) so as long as you explosions are roughly uniformly distributed (or so densely packed we missed it), your probably good to go.

Answer 3

Supernovas beyond 26Ly are just pretty, bright lights and without telescopes to figure out what happened, they will remain historical curiosities. Human astronomers will event whatever explanations suit their theology and philosophy; just as they did for thousands of years before telescopes and science.

Given the superstitious nature of pre-science humans, seeing that many bright lights in the sky would be taken as an omen or omens of some kind. What that would be depends on the culture and the situation at the time. Halley's Comet, a giant ball of ice with no impact whatsoever on Earth was nevertheless able to cause commotions when it sailed by.

stars went into supernova in such rapid procession wouldn't be noticed by astronomers and raise tons of questions

The OP doesn't specify what "rapid procession" means so I'm going to assume that it's within a span of 10 years. Either these stars will need to explode in a staggered fashion "timed" so they show up in quick succession or they will be physically close together and detonated about the same time. Having stars simultaneously go supernova in a range between 400 and 1700 light years away means that the light from those explosions will reach us over a range of 1300 years; not exactly rapid succession.

Events like supernovas definitely leave behind stellar remnants and glow for a good long time afterwards. Even at 400 to 1700 Ly, those remnants should still be identifiable with good enough telescopes. Saying that the stars were destroyed in a war will be the ideas of science fiction writers unless there is clear evidence that distinguishes the destroyed stars from naturally occurring supernovas.