# Time scale and mechanism to “realistically” remove Sol from the Milky Way

What is the fastest rate a star could be flung out of our galaxy and by what mechanism(s)? (i.e. from a gravity kick by a hypothetical passing hypervelocity black hole)

Secondary question: how far away would it need and how long would it take for Earth to have no naked-eye visible stars in the night sky?

Alternatively, I am trying to build an alternate reality where some or all of human history was not influenced by the backdrop of the star field in the night sky, if leaving the galaxy is too unfeasible, I’ve considered applying these questions to a scenario where Sol is flung into a dark nebula—would that be better? (Thinking about the timescales, it would need to be shrouded from the rest of the stars long enough to cover a significant portion, if not the entirety of human history)

• It's nice to see that your question made it's way here from the East India Company Discord. Welcome to Worldbuilding Stackexchange! Nov 28, 2021 at 21:23
• The Frederik Pohl story The World at the End of Time has the protagonists in a universe where there are no visible stars in the sky. An alien sun-entity accelerated their star system at the speed of light and time-dilated them to the heat death of the universe. Poor Wan-wan-wan. Nov 29, 2021 at 14:44
• If you don't want folks looking up at the sky and wondering, you'll have to get rid of the moon and the other planets. Not for nothing do many of the planets have the names of ancient gods. Removing just the galactic backdrop might in fact intensify interest in the handful of visible planets and moon.
– Tom
Nov 29, 2021 at 22:03
• I have considered that, but the moon and planets are still important for developing math, science, and global navigation in this alternate civilization. I envision that mythology and religion will still spring up about the sun, moon, and planets, and in some respects may be even more intense since there’s not much else to pull focus from the few things that can be seen in the sky. Nov 29, 2021 at 22:41
• Ultimately I want this population to become cynical in a way that is more extreme than real life. These people don’t look up at the sky and see a goal to reach or a universe they are connected to. Maybe they discover there’s nothing outside of our solar system within reach and so they just stop caring about the cosmos, they look down and focus purely on agrarian efficiency, ‘knowing’ that this is the only home and the only pool of resources they will ever have. Not that all of the population feels this way, just the vast majority, leading to a status quo that will play into later conflicts. Nov 29, 2021 at 22:46

There are a couple of ways to produce hypervelocity stars. A common one involves an interaction between a supermassive black hole and a binary system, leading to one star being ejected at speeds potentially exceeding 1000 km/s. I believe the current record is held by S5-HVS1, which appears to have been ejected from the Galactic center at a speed of $$\sim1800$$ km/s (Koposov et al. 2019). Assuming that the star doesn't slow down significantly, it could reach a distance of about 100,000 light-years (roughly corresponding to the edge of the Milky Way's disk) in about 17 million years. Not bad!

This isn't ideal for your scenario, though, since you want to remove the Sun from the galaxy, and we're about 25,000 light-years from our central supermassive black hole, Sgr A*. It would take an extremely improbably sequence of events to send the Solar System to the Galactic center and have it undergo encounters with multiple bodies in order to be properly ejected. So we might want to look elsewhere.

Another scenario would be to have the Sun, when newly-born, be bound to a companion star which subsequently goes supernova. The explosion would cause the system to become unbound and send the Sun moving away at speeds akin to classic hypervelocity stars. US 708 (Geier et al. 2015) was likely ejected in this manner, and has achieved a speed of $$\sim1200$$ km/s. If the same thing was to happen to the Sun, it could travel the remaining 75,000-ish light-years to the edge of the disk in roughly 19 million years. As supernova progenitors typically live for no more than millions or a couple tens of millions of years, the whole process could happen very quickly compared to the lifespan of the Sun and the time it would take life to subsequently evolve on Earth.

Would planets survive the explosion - and, furthermore, remain bound to the Sun? Well, we've found planets orbiting supernova remnants, so it's quite possible for a system to have planets after a supernova has taken place. That said, it's unlikely that this alternate-history Solar System would look the same as ours; I'm worried about the outer giant planets in particular. It seems quite possible that planets in tighter orbits, like Earth, could be retained, though - particularly if they formed in the wake of the supernova.

Another possibility, as noted by Adrian Colomitchi, is to utilize interactions with another galaxy, such as the future collision between the Milky Way and Andromeda. While most stars won't be ejected, some certainly will, accompanying the formation of features like tidal tails.

• Just for the sake of completeness - if you wait enough (like 4.5 Gy) it may happen to see other possible causes for stellar ejection. But there are no strong warranties for that one, galaxy collisions are somehow like mixing rarefied gases. Nov 29, 2021 at 0:54
• @AdrianColomitchi Yeah, fair point - I had been focusing on (I guess relatively) low-impact changes, but that would definitely have a chance of working. Nov 29, 2021 at 4:23
• Thank you so much for all the detailed information. I’m wondering, if a sufficiently sized black hole (ejected from a different galaxy) where to pass through our galaxy, would that be enough? How big would it have to be, how fast could it be moving to still drag us out of the galactic disk, how long (estimated range) would such a process take? And within that how long would it take before no more individual stars in the Milky Way are naked-eye visible from Earth? Nov 29, 2021 at 6:34
• In theory Sol doesn’t even need to be going at escape velocity, in fact the characters could discover at some point that Sol is indeed being pulled back into the galaxy at some point long into the future. Nov 29, 2021 at 6:37

If you assume that we've been kicked out of the Andromeda galaxy, that galaxy is still visible to the naked eye along with a few others

If you want a night sky devoid of stars, your best bet would be to hid Sol in a dark nebula..

However, you'd still have planets to deal with.

Sol could be lighting up the nebulae gasses from the inside. So, you might need to deal with that too.

If you need the planets' lights blocked out too, you'd need to convince Hactar to surround Earth like he did with the planet Krikkit.

• The galaxies can be visible because to the naked eye they are just blurry smudges, but for instance we can’t resolve individual stars from Andromeda. From some certain distance we should not be able to resolve individual stars from the Milky Way too. It’s okay if the planets are visible, the effect I’m going for is a “lonely” sky that doesn’t hypnotize and feed our wonder and explorer’s spirit the way our vast night sky star field does. I’m shooting for an extreme version of humanity that feels deeply isolated and disinterested in looking up. Nov 29, 2021 at 20:31