So, it’s a well known “fact” that many of the stars ion the sky died long ago but we are still receiving light from them. The question is how many of the stars we see are really still there?
I assume there is no way of knowing with any certainty, but with what we know of the average age of the visible stars, the average distance and the average lifespan of those types of stars how, many of the stars I see in the night sky are likely to to still exist and how many have probably gone kablooie?
Hmmm…this is an interesting question. Let’s start out with, the overwhelming majority of the stars that exist, you can’t see – they’re in other galaxies, the galaxy itself only visible through a telescope and the individual stars probably not resolvable even with the largest telescopes, except in the closest galaxies.
In our own galaxy, the overwhelming majority of the stars are too dim and/or too distant to see with the naked eye. And your basic question is, more or less, “If I go out in the back yard and look up at the stars, how many of the ones I see are still there?”
So let’s set this up: When you look at the sky, you can perceive the Moon, five planets (six with excellent viewing conditions and exceptional eyesight), one asteroid (again with special conditions), three galaxies (M-31 in Andromeda and the Magellanic Clouds, plus M-33 in Triangulum with the aforementioned special conditions), about 6,000 stars, and a small assortment of nebulae and other odd stuff that can be disregarded. The Milky Way is among this assortment of loose ends.
Stellar lifetimes range from a few million years on the main sequence for bright type O stars to hundreds of billions of years for small dim type M stars. The post-main-sequence helium-burning phase typically runs for about a million years. The fusion of more complex elements is a much shorter time frame.
I wasn’t able to quickly pull down a list of sizes and types for all 6,000 naked-eye stars, but it appears that if the samples I looked at are accurate, they’re about 70% ‘average’ close stars and about 30% big intrinsically-bright stars at a greater distance. While I can’t nail down figures (obviously) the “average” stars are within about 50 lightyears of earth, the big bright ones range from a few hundred to a few thousand lightyears.
Now, insofar as I can nail this down, of those 6,000 stars that show up as individual points of light to the naked eye, something over 4,000 of them are in the same general range as the Sun in terms of lifetime: happily chugging along as main sequence hydrogen-burning stars with a lifetime on the order of the Sun’s ten billion years – say 2 billion to 50 billion to approximate a range. That means none of them has blown up, run out of fuel, or anything else in the 4-50 years it’s taken their light to get to Earth.
Almost all the other 2,000 are giants or supergiants with a shorter lifespan, but are presumably mostly in the helium-burning phase, taking a million years or so to go through that … a substantially longer time frame than the few thousand years maximum distance. It’s of course possible that one or a few of them were at the end of that phase as of when their light started to Earth and have gone on to the next, catastrophic steps, in the interim. But in my opinion that’s unlikely.
There are three stars that are supergiants which are irregular long-period variables with the potential to go supernova. Two of them are Betelgeuse (Alpha Tauri) and Eta Carinae; I recall from past supernova discussions there being a third but don’t remember the name of it. These three, it’s possible that in the 450 years (Betelgeuse) and 7000 years (Eta Carinae) their light has been travelling to Earth, they have gone off the deep end, into a catastrophic explosion. No certitude here, of course, but the chance that they have is substantially greater than anything else. (I’m sure a relativist will raise an issue about our talking about simultaneity with reference to objects at astronomical distances, but let’s for purposes of this discussion say that we’re talking about what might be perceived at some reference frame where our “now” and what we’ll see from Betelgeuse in 450 years are effectively equidistant.)
The Milky Way, on the other hand, is a concentration of stars along the disk of the Galaxy at distances too great to permit them to be naked-eye observed as individual stars, but with the cumulative light concentration from the lot of them adding up to produce a “stellar haze.” These can be up to the tens of thousands of lightyears away, and it’s quite possible that a few of them are in that instability range.
The Magellanic Clouds are 179000 (Large) and 200000 (Small) lightyears away. They are not resolvable into individual stars without a significant telescope, but it’s quite probable that at least some of their brightest stars have since gone to their reward. That probability approaches certainty in the Andromeda Galaxy, M-31, two million lightyears away, where the brightest main sequence stars, the Type O stars, have a lifetime on the order of their distance in lightyears, and the visible giants and supergiants would have had lifetimes in years shorter than the distance in lightyears.
Distant galaxies and galaxy clusters, observable only with large telescopes, are almost certainly “fossil light” – the product of stars no longer in existence as regards that imponderable simultaneity between us and them.
If you’re referring only to the set of stars we can see with the naked eye, in Earth’s sky, then all or nearly all of those stars still exist right now.
Stars have lifespans in the tens of millions to hundreds of billions of years, and the ones in our sky are only a few hundred light years away, give or take. (Some a few thousand, but most much less.) So the “delayed image” you see when you look up isn’t all that delayed, relatively speaking.
Another argument for this is the fact that novae of naked-eye stars are very rare. Only a small handful in all of human history. The visible stars just don’t change that much.
But plausibly one of them went boom 50 years ago, and we won’t see it happen for another 300?
Can astronomers tell if a star is just about to nova? And what would be the timeframe of “just about to happen?” 200 years, or so? 2,000? 200,000?
“About to happen” could be a hundred thousand years, easily.
Several visible stars will go supernova (not the same thing as a nova) sooner rather than later, in stellar terms. Betelgeuse - bg and close enough to be visibly red - is doomed, IIRC.
Sorry for the hijack but this seems like a good place to ask my question:
When I’m out flying into the desert on NVGs when I look up I see this line that looks like it’s going, I don’t know, up and down maybe? And it look like it is clouds. There are stars in it, but the surrounding is cloud like. I’ve seen this in both the deserts in New Mexico and around Las Vegas.
I wish I could describe it better, but what is it?
That’s the Milky Way galaxy, the galaxy we’re in. The cloudiness is the light of billions of stars, too remote to be distinct individually.
Exactly. And the reason that it appears as a band in the sky is that our galaxy’s shape is that of a flattened disc.
Milky Way on Wikipedia (including pictures of it as you’ve likely seen it)
As previous posters have said, it sounds like the Milky Way, but it could also be the zodiacal light.
Wow - I find it pretty depressing that our skies are so light-polluted that people can reach adulthood unaware of the Milky Way! To my mind the Milky Way is about the most prominent thing you see when you look up, unless you’re in a big city.
I’ve heard the following story in several sources.
A minor earthquake in the LA area (maybe in the late 70’s?) knocked out the power to a large area. For several days afterward, radio call-in shows and emergency services recieved calls from people wondering what the silvery cloud that they had seen in the sky that night was, and could it have been related to the earthquake? Of course, it was the Milky Way, but the urbanites had no idea what they were seeing.
Wow - I find it pretty depressing that our skies are so light-polluted that people can reach adulthood unaware of the Milky Way! To my mind the Milky Way is about the most prominent thing you see when you look up, unless you’re in a big city.
Ooops. Note to self – refreshing browser will repost and make you look like a fool…
The furthest object visible to the naked eye are Galaxies, M33 or for the very keen sighted perhapse M81 even these are less than 12 million lightyears away. So since stars tend to last tens of billions of years then only about 1/1000th of the stars within those galaxies are no longer there.
Another tangental question:
Going by this image of a milky-way type galaxy there’s a big bright spot in the center. Is that simply a higher density of stars (which I assume is at least partially the reason) or is there some midpoint star that is particularly huge? And if there is (or was) that one midpoint star, is it still around and visible?
Eh, no biggie. Relative velocities between the Sun and the other stars in our Galaxy are pretty low, relativistically speaking, so one can reasonably define a frame “the rest frame of the stars in our Galaxy”. Which is the logical frame to choose, here. And even if you don’t want to set a frame, you could also say that the deaths of almost all naked-eye stars are probably in our proper future: That is to say, light that we’re sending off now could reach those stars before they die. That’s a statement independant of reference frame.
Nitpick: alpha Orionis. alpha Tauri is Aldeberon, which is also a red giant, but not quite as precarious as Betelgeuse.
There’s a whole bunch of dust and stuff, which is dense enough for stars to be born out of. But there may also be a whacking great black hole too.
So in a very real sense, our neighbourhood does suck.
And to answer the tangential question that just snuck in, the bright bulge in the center of a galaxy is just a denser concentration of stars. At least, what you’d see with a naked eye, in a current galaxy, is. Most galaxies have a supermassive black hole in the center, and matter falling into it will result in energy being radiated away, but nowadays, that energy is mostly in radio waves, and a supermassive black hole can’t really be considered a “star”. If you’re wondering, such black holes last an incredibly long time, over 10[sup]97[/sup] years (yes, nearly a googol) for the largest (about ten billion times the mass of the Sun).
Google Eta Carinae for an interesting debate on that topic. Conventional wisdom says it’s teetering on the edge of kablooeyhood, but nobody really knows what it’s going to do.
So this is yet another fact that isn’t. Basically the chances of any of the stars I see not being there is incredibly remote.
See, this is why I pay my membership.
