It doesn't really matter how long it takes. There will be a day in that period when it becomes visible.

Disclaimer: This is not my field of expertise

Okay I will try going through this piece by piece:

• has there been or could there be a time where someone actively looking at the sky sees a new star appear before their eyes - The probability to experience a new star with naked eyes or even a regular telescope is very low. You will have to know the exact location where you would like to look into the sky and perhaps the timing too. - As @Bjorn says we have observatories working 24/7 around the globe and new stars are always popping up.
• does it have a limited range before it would just always be invisible beyond a certain range based on size and luminosity, like a flashlight? - Photons can travel the vacuum forever. Flashlight has limited range only due to photons getting diffused or barricaded by some other molecules present in their path.
  - However, we know space is not exactly pure vacuum and that means signals we receive from the stars are still going to be inaccurate.

Since our discussion was mostly on eyes, I found this article for you, hopefully this satisfies your curiosity. :) https://www.johncbarentine.com/astronomical-visibility.html

It doesn't take much going wrong to "hallucinate" light, its not that you physically produce the light the eye starts to see. Its just a byproduct of the processes happening

There are many more types of radiometric dating than just Uranium and Carbon mostly mentioned here.

e.g. K-Ar dating would be entirely independent of bombs AFAIK

https://en.wikipedia.org/wiki/Radiometric_dating

A huge amount of what your brain does is keep other parts of your brain from speaking out-of-order. The most energy intensive parts of the brain functions are those ones, so they fail first. When that happens you get a lot of brain noise or different bits talking over each other. That can happen in other instances too, often called a "seizure".

Maybe only the synapses related to the visual cortex?

I know nothing about brains so I’m it’s guessing.

Visual processing is an overlay that happens on top of sensory data. You can actually meditate to the point of getting some of that processing to shut down while you are still alive. (See Buddhist 6th fetter as an example) One of the outcomes of this kind of training is that all things are said to shine with their own internal light and become "heavenly". In relation to your question, it would really just be a modification or a failing of that visual overlay that your brain adds to your visual data all the time.

"Somehow" being plants using it in photosynthesis and then it going up the trophic food chain.

I think because they remembered something halfway so rather highlight both aspects of it to make them understand what it was they remembered. Bomb pulses affect carbon dating after all. Though they are not necessary for carbon dating of course.

I get your point but I genuinely think it's helpful to clarify. Didn't mean to step on your toes, this was more for OP than for you.

I answered a similar question a few weeks back so I'm mostly just gonna copy/paste that

But TL;DR- Yes, we'd still have radiometric dating, arguably it would be less complicated since we wouldn't need to account for nuclear testing and the bombs dropped on Japan messing with the ratio of different isotopes found in nature, but it does also offer some new opportunities for dating certain things. For example, since we started setting off nuclear bombs basically all steel we've made has been a tiny bit radioactive, so you can pretty reliably date steel to being from before or after WWII based on the presence of radioactive contaminants. (Those traces of radioactive material are pretty harmless and don't affect things for most applications, but there are certain cases where it is actually very important so there's a market for recycling WWII battle ships and such to get steel that's free of radioactive contaminants)

There are 3 naturally occuring types of carbon, called isotopes, ¹²C, ¹³C, and ¹⁴C. These are all basically the same except for the number of neutrons in the nucleus of the atom. Living things can use all 3 types to build sugars and proteins and all those other organic molecules needed for life. Animals get it from the food they eat, plants get it from the air. These 3 isotopes all exist in nature in a pretty consistent ratio

¹⁴C, however, is unstable (radioactive) and over time decays into more stable elements.

But it also is constantly getting replenished from cosmic rays interacting with other gases in the atmosphere making more, so as long as something is alive the ratio of carbon 14 in that plant or animal to the other types of carbon stays pretty consistent. It breathes/poops out carbon and takes more in, keeping that ratio pretty consistent.

But once it dies, it's not taking any new carbon in, so what was there when it died is all it's ever going to have until it decomposes away entirely or gets eaten by scavengers or whatever. The ¹²C and ¹³C are basically going to stay put, while the ¹⁴C slowly decays.

So to date something you just need to look at the ratio of the different types of carbon and you can figure out how old it is.

Let's say ¹⁴C has a half life (the time it takes for half of it to decay) of 1 year (its actually something like 5 or 6 thousand years but this makes the math easy with smaller, easier to digest numbers)

And let's say the ratio is 1:1, ¹²C+¹³C to ¹⁴C (again, it's more like 1% carbon 14 IRL)

So after a year, half of the ¹⁴C will be gone, and the ratio will be 2:1

After another year half of that remaining ¹⁴C will be gone and it'll be a 4:1 ratio, another year and it's 8:1, etc.

This of course has limitations. After a long enough time basically all of the carbon 14 will be gone and you can't really date things that way anymore. I think the limit is something like 50,000 years.

It's also only really useful for plants, animals, and stuff made from them (leather, textiles, the straw in mud brick buildings, food, etc.) so not particularly useful for dating inorganic matter.

There's also a few things that can cause more ¹⁴C to form than normal, humans caused one with our nuclear testing, there was another event around the 8th century that we've discovered from measuring the carbon 14 in tree rings from that time that we think may have been caused by a supernova sending more cosmic rays our way than usual. So those events also need to be accounted for.

But there's other similar techniques using elements with longer half lives that we can use to date older or inorganic materials.

For example uranium-lead dating. When the mineral Zircon forms, it tends to include uranium but exclude lead. But over time uranium decays into lead, so if you have a sample of zircon, you can measure how much lead is in it and figure out how long ago it must have formed because that lead must have once been uranium that has decayed at a steady pace over the centuries.

Even for high-mass stars, the time it takes for a star to form from a molecular cloud is on the order of a million years (see Figs 5 and 6 of this paper). A human lifetime is essentially just a snapshot of this process; you're not really going to see a new star form over such timescales (unless you count supernovae as "new stars").

Also, all of the stars visible to the unaided eye are relatively close: only a few thousand light years away at most. All of the other stars in the milky way are too far away to see as anything more than a collective haze.

That's OK, we made a brand new market for pre 1940s steel so we can make scientific tools that aren't contaminated with radiated air when making the steel.

Atomic bombs 80 years ago doesn’t have anything to do with assessing the age of things over 80 years old. I’m not even sure if that is used for anything but steel. Also, way more bombs than just those have been detonated in atmospheric (above ground) nuclear tests.

Carbon dating is based on the idea that cosmic rays interact with nitrogen in the upper atmosphere and produce carbon-14, which is then consumed somehow by organisms. When an organism dies, it stops absorbing new carbon-14, and the isotope decays. Measuring the extent of that decay is how the age is estimated.

Well... I mean.... The name makes the question pretty self explanatory. Knowledge of this system of measurement existing answers the question itself, so why bother.

Radiocarbon dating is the whole field of dating things by the amount of carbon isotopes as opposed to the environment these things are in.

I think you are referring specifically to bomb-pulse carbon dating. This bomb pulse carbon dating is more precise for recent "things". Anything that formed after the bomb basically.

So even if we lose bomb-pulse carbon dating (which we will in approximately the next few years) we still have "classic" radiocarbon dating. It's a loss of one specific process but not the general method.

Disclaimer: not a scientist in that field, please correct me as needed.

They are talking about bomb-pulse radiocarbon dating which is way more precise for more specific applications.

You're talking carbon dating, and the analysis probably takes into account the 2000+ atomic bombs going off these past decades.

I'm replying to this post because you and others need to know that:

2000+ nuclear bombs went off on earth - obviously not simultaneously - but we DO live on a rather radioactive planet now.

Radiometric dating uses the proportions of different radioisotopes in a substance to determine age, it has nothing to do with atomic bombs.

It has certainly happened. We know of stars that are younger than humans.

But has someone directly observed this happening? Well, it depends. As already noted, star formation takes a long time. It's not like dark, cold dust suddenly collapses and ignites into a star. During the formation process, the dust is compressed, and as you might recall, as volume decreases on a mass, pressure and temperature increase. As it heats up, it starts emitting radiation. So it's possible to observe this even before becoming a true main sequence star. And we have definitely observed a lot of those, and if you go outside and point your sufficiently powerful telescope in the right direction, you too can see one with your own eyes!

https://astronomy.stackexchange.com/questions/49792/has-the-birth-of-a-new-star-ever-been-observed-scientifically-speaking

There’s not really one instant where a new star goes from not shining to shining where a human looking up at night would see a star “turn on.” The process of turning on takes between 10,000 - 1,000,000 years, depending on the size of the star and how many stages you include in the process.

We know of old supernovae because ancient astronomers catalogued them as new stars (nova means new). Maybe they also managed to find new stars.

The new Vera C. Rubin Observatory should help us with finding new stars and other objects. It will scan the whole southern night sky in three days, marking all changes when it gets to the same patch of sky again.

If sharks have a culture they might have recorded the appearance of Polaris, the north star. Sharks have been longer on earth than Polaris has existed.

Well yeah, but I mean has there been or could there be a time where someone actively looking at the sky sees a new star appear before their eyes and it wasn't a super nova of a star dying or a newly formed star relatively nearby? IE the star has been there longer than Earth existed, but it is so far away it took until today to finally arrive where we could see it.

I know it has limited speed, but does it have a limited range before it would just always be invisible beyond a certain range based on size and luminosity, like a flashlight? I am ptetty sure atmosphere has something to do with that, but not 100% certain if light could travel in a vaccum forever.

Yes, it has already happened. When you look.up the sky, you are pretty much looking at ghosts. Due to the speed of light being finite.

Yes. New stars are still being born. We can also see stars going supernova that we couldn't see with our bare eyes before.

It sure is.