FWIW, here’s Wikipedia’s page on Algol variables and it lists 9 of up to 6th magnitude but R CMa would be borderline visible at it’s brightest and not visible at it’s dimmest if you take 6th mag. as the delimiter. YMMV.
It would definitely be subtle, but that wasn’t the question. If any stars were far enough away thousands of years ago to be just outside of naked eye visibility, proper motion could have brought them just inside that zone.
I agree that in practical terms, thing like light pollution would swamp those effects. But strictly speaking there should be some stars visible today that weren’t visible thousands of years ago - and vice versa.
The problem is that there are far more factors about whether a given marginally visible star can be seen at any one time. Besides light polution, the brightness and distance of the star, there’s
[ul]
[li]whether the star is variable[/li][li]altitude of the star[/li][li]darkness of the observing site[/li][li]turbulence of the atmosphere[/li][li]temperature[/li][li]wind[/li][li]humidity[/li][li]how long the observer has been in darkness[/li][li]altitude of the observing site[/li][li]age of the observer[/li][li]is the observer using averted vision[/li][/ul]
And probably several more that don’t occur to me at the moment.
Now you might say that you’ll keep all those constant. I’m not even sure you could do that in theory, but in practice you can’t even keep them all constant during a single observation session, or for that matter, from second to second. So there’s no such thing as a simple set of naked-eye stars.
However, I suspect the OP is really more interested in those that aren’t marginally visible. For those, the vast majority have not changed their visibility. But besides the “missing Pleiades sister”, there’s also eta Carinae. We don’t really know what η Car was like 4000 years ago (it’s too far south for the Babylonians, Greeks, and Chinese to have seen), but just in the past two hundred years, it’s been highly and very irregularly variable. At one point is was brighter than all stars except Sirius, some time later it was too dim to see. It’s currently around 4th magnitude. Much of that variability is due to material the star has expelled. In the not too distant astronomical future, it (or at least one of them; it’s a double star) is expected to supernova.
In a few years:
“Before their meeting the two stars were too dim to be seen by the naked eye, but in 2022, the newly formed Red Nova will burn so brightly in the constellation Cygnus that everyone will be able to to see it.”
I don’t know what this answer is even referring to. I was assuming the OP meant stars at the edge of the visible universe whose light was just now arriving at Earth, based on the first sentence.
Quoth Sasquatch:
Any new stars you can see now that you couldn’t 4000 years ago would likely just be visible due to relative motion. A star previously occluded by another so not visible, may now be out from behind.
Hmm, I didn’t actually go and calculate this, but just threw out that specific example. Intervening dust clouds, two stars lined up that look like one, but 4000 years of proper motion make them visibly separated, it seems there are lots of ways stars can hide. Space is really big though, I admit, so curious what the calculations are for the several thousand nearby, naked-eye visible stars. Are there any that look like 1, but a telescope can resolve 2 (non-binary), or stars with intrinsic brightness that should make them visible, but aren’t due to something in the way outside our or its system?
From the phrasing of the thread title – ‘stars in the night sky’ – I took it to mean "stars visible with the naked eye in the night sky of Earth, which under optimal conditions is said to be about 2000 stars at one time, or about 4000 total. Apologies if I misunderstood.
Using David Nash’s Astronexus site (highly recommended for galactic explorers) 61 Cygni, one of the fastest moving visible stars, was at mag. 5.4 four thousand years ago, so still visible even back then. There really aren’t that many stars with high proper motion that are visible with the naked eye, although it may be possible to find one or two very dim ones that fit the OP’s definition.
Sort of a reverse of the question, how far back in time would you have to go before one of the stars visible with the naked eye today stopped being visible to the naked eye?
This is in true in the sense that we can see em radiation from this galaxy, albeit heavily red-shifted and via a near-infrared camera on the Hubble telescope, but approx. 380 million years ago it would be theoretically impossible to observe em radiation from this galaxy from the Earth.