How far back in time would the Milky Way be recognizable?

I played around with SkySafari, and investigated the sky 100,000 years ago. Many constellations are completely unrecognizable. The Big Dipper looks very strange, and only Orion bears some resemblance to its modern appearance.

The Milky Way texture is however not changed due to program limitations. My question is, how much would the Milky Way have differed? Would famous dark nebulae, such as the Coalsack, be in a different position? Would the spiral arms look different?

Screenshot of how the sky probably looked about 100,000 years ago:

A similar, heavily related question: How far away from Earth would one have to travel to see a noticeably different Milky Way in the sky?

Moving to Deep Sky Observing.

smp

I am going to guess that 50,000 years ago, early hominids didn't understand the stuff they saw at night - so  50,000 years works for me.

But more seriously, if one were to be in a non-Milky Way viewpoint, then they would see a changing Milky Way as it wends itself in a galactic plane rotation that circles around every couple of million years. 

So, the CoalSack would not be where it was a million years ago.

For the purposes of Human history, the Milky Way would not have changed much at all. After all, the Milky Way is our galaxy, which was in existence long before Earth came into being. And, the time that Humans and our predecessors have been around, is but the blink of an eye in the history of the Earth. So for all intents and purposes, you could say that, as long as the time Humans have existed, the Milky Way has been an unchanging element of the night sky.

Ron Abbott

This is largely a matter of distance. All of the sky's bright stars are quite close to us -- otherwise they wouldn't appear bright. Orion's stars are among the most distant -- visible because they're extremely young and luminous -- but even they are just 1,500 light-years or thereabouts, making them still well within the same spiral arm as our own Sun. And aside from the "runaway" star Betelgeuse, Orion's shape hasn't changed significantly in the course of human history.

All of the major Milky Way features visible to the unaided eye are quite a lot farther, meaning that they appear to move much more slowly from our perspective.

This is largely a matter of distance. All of the sky's bright stars are quite close to us -- otherwise they wouldn't appear bright. Orion's stars are among the most distant -- visible because they're extremely young and luminous -- but even they are just 1,500 light-years or thereabouts, making them still well within the same spiral arm as our own Sun. And aside from the "runaway" star Betelgeuse, Orion's shape hasn't changed significantly in the course of human history.

 

All of the major Milky Way features visible to the unaided eye are quite a lot farther, meaning that they appear to move much more slowly from our perspective.

That was my thought when I read the title as well.  The Milky Way as we see it from Earth will have changed little, but the constellations and individual star positions and magnitudes will have changed by a lot over tens of thousands of years.  

Most of the stars we can see are within 1000 light years from earth, and are very bright, much brighter than the sun. Vega is 25 light years. Acturus 400.

The earth is 25,000 light years away from the center of the milky way and orbits it every 60 million years.

Sources I have read indicate ~220 to 250 million years = 1 orbit of the Sun around the Milky Way or 1 galactic year.

Thank you all for explaining. I keep forgetting that the Milky Way is made up of stars at a very great distance. Ok, so SkySafari is correct to assume the Milky Way has not changed appearance noticeably since 100,000 BC. In fact, many stars are still in roughly the same position in the sky. Some stars are much brighter, such as Aldebaran and Capella, but others, such as Alpha Centauri, are almost too faint to see with the naked eye because of the increased distance compared to today.

Individual features such as the Coal Sack might change, but the overall appearance should be pretty much the same going back to when the Sun was formed.

Individual features such as the Coal Sack might change, but the overall appearance should be pretty much the same going back to when the Sun was formed.

Really? We talk a time period of 4.6 billion years, over which the Milky Way would have rotated more than 20 times around its centre (Sun's orbit). Would that really keep the overall appearance?

Really? We talk a time period of 4.6 billion years, over which the Milky Way would have rotated more than 20 times around its centre (Sun's orbit). Would that really keep the overall appearance?

To some extent the answer is "nobody knows." The dynamics of spiral galaxies are still poorly understood. There's still no consensus even on something as fundamental as why spiral arms form at all.

But the short answer is that even if the grand plan stays the same, each and every detail changes dramatically within one billion years.

This is also interesting: https://en.wikipedia...brightest_stars

Some stars 'pass by' such as e.g. Epsilon Canis Majoris, which is now still +1.8, but was -4 4 million years ago.

To some extent the answer is "nobody knows." The dynamics of spiral galaxies are still poorly understood. There's still no consensus even on something as fundamental as why spiral arms form at all.

 

But the short answer is that even if the grand plan stays the same, each and every detail changes dramatically within one billion years.

Interesting! I had no idea we knew so little about galaxy formation and rotation. I hope to live to see more detailed scientific models on spiral galaxies.

This is also interesting: https://en.wikipedia...brightest_stars

 

Some stars 'pass by' such as e.g. Epsilon Canis Majoris, which is now still +1.8, but was -4 4 million years ago.

Yes, I've seen that link too. Very interesting! I would have liked to see Aldebaran at its peak. Just as bright as Sirius is today, but red! I envy those Neanderthals who got to see that.

This is also interesting: https://en.wikipedia...brightest_stars

 

Some stars 'pass by' such as e.g. Epsilon Canis Majoris, which is now still +1.8, but was -4 4 million years ago.

Adhara back then was almost as bright as Venus.

Interesting to note some of the brightest stars in the past are in Canis Major.

Edited by Ptarmigan, 28 July 2021 - 10:52 PM.

There's another interesting fact you can derive from that chart: Canopus is far more luminous than the other stars in that list. Canopus starts out being 177 ly away in its first appearance as the sky's brightest star and nearly doubles that distance to 346 ly at its fourth and last appearance--all are triple-digit distances. Every other star on that list (except one) is under 50 ly away when it's the brightest, and when it moves far enough away Canopus is there in the distance, ready to take over again.

The exceptional star is Albireo at 80 ly away when it becomes the sky's brightest star 3.8Myr in the future. Between now and then it will have moved 5x closer to us, so its two stars should spread from their current separation of ~0.5 arcmin to ~2.5 arcmins--well within the human eye's visual resolution. You'll see their blue and gold colors naked eye!

Interesting! I had no idea we knew so little about galaxy formation and rotation. I hope to live to see more detailed scientific models on spiral galaxies.

Oh, there's no shortage of models. The problem is that the models don't always agree with one another.

Also, the underlying data is controversial. It's surprisingly hard to tell a galaxy's structure by looking at it from just one angle. Consider the case of edge-on spirals. No doubt they have spiral arms, but we can't see them directly because our perspective doesn't allow us to see the gaps between them. It's not like you can pick up a galaxy and rotate it at will! Our own galaxy is one of the worst, because our perspective from inside one of the spiral arms is truly awful. People are still debating how many spiral arms the Milky Way has, and whether it has a bar.

One thing that has been known about spiral arms more or less forever is that they're not actual physical objects. If they were, they would get wound around the core hundreds of times over a galaxy's lifetime. Instead, they're best viewed as waves of star formation. The stars and the gas from which stars are made rotate the core at their own pace, and the spiral arms sweep through them.

A complicating factor is that over an orbit of the galaxy, the Sun's orbit is tilted 60 degrees, so it will weave back and forth through the disk, seeing it from above/below and in-line.  That will change the appearance of the galaxy considerably even if it remained with the same wedge of disk throughout and even if the disk were unwarped.  And it most likely would not remain with the same wedge due to the tilt and accelerating/deceleration through the local bulk masses..  I think you can see this weaving impact in the changing shape of the star stream rotation patterns in the Wiki galactic year animation.  The disk is made up of many streams.

EDIT: The 60 degree figure is for the ecliptic plane tilt.  I should have checked this more closely because it seemed far higher than I expected.  Unfortunately, when I was looking for this many of the diagrams were unclear or stated things poorly while showing the oscillation I am talking about.  The actual oscillation is on the order of ~63 million years, so roughly 1/4 of a galactic year.  The distance above and blow the plane of the galaxy is not as large as some of the illustrations suggested, only about 17 parsecs above at present according to a 2016 study (link.)  I am unsure how far above and below it goes, but an answer to this claims 100 parsecs on each side every 70 million years (link), but the actual research paper is behind a paywall.  Anyway, perspective relative to the galactic plane will change by +/- 100 parsecs as we orbit, versus a distance of ~25,600 years from the center of the galaxy.  This is not a large angular change in perspective along the disk as a whole...although locally 200 parsecs movement up and down (N/S galactic plane) might appear so relative to nearby bright MW features.

Edited by Redbetter, 30 July 2021 - 03:42 AM.

There's another interesting fact you can derive from that chart: Canopus is far more luminous than the other stars in that list. Canopus starts out being 177 ly away in its first appearance as the sky's brightest star and nearly doubles that distance to 346 ly at its fourth and last appearance--all are triple-digit distances. Every other star on that list (except one) is under 50 ly away when it's the brightest, and when it moves far enough away Canopus is there in the distance, ready to take over again.

 

The exceptional star is Albireo at 80 ly away when it becomes the sky's brightest star 3.8Myr in the future. Between now and then it will have moved 5x closer to us, so its two stars should spread from their current separation of ~0.5 arcmin to ~2.5 arcmins--well within the human eye's visual resolution. You'll see their blue and gold colors naked eye!

That would look very pretty in the night sky. Hopefully some sort of human ancestor with appreciation of the wonders of the night sky is still around by then.

Oh, there's no shortage of models. The problem is that the models don't always agree with one another.

Also, the underlying data is controversial. It's surprisingly hard to tell a galaxy's structure by looking at it from just one angle. Consider the case of edge-on spirals. No doubt they have spiral arms, but we can't see them directly because our perspective doesn't allow us to see the gaps between them. It's not like you can pick up a galaxy and rotate it at will! Our own galaxy is one of the worst, because our perspective from inside one of the spiral arms is truly awful. People are still debating how many spiral arms the Milky Way has, and whether it has a bar.

One thing that has been known about spiral arms more or less forever is that they're not actual physical objects. If they were, they would get wound around the core hundreds of times over a galaxy's lifetime. Instead, they're best viewed as waves of star formation. The stars and the gas from which stars are made rotate the core at their own pace, and the spiral arms sweep through them.

I see why it is a problem. Hopefully astronomers will develop new tools and methods to help see the structure, or simulate different galaxies and help develop a model that can be generally accepted.

A science-fiction scenario would be to send a probe, or even humans, outside of the Milky Way to observe it. We only need to find out how to travel faster than light, that's all!

A complicating factor is that over an orbit of the galaxy, the Sun's orbit is tilted 60 degrees, so it will weave back and forth through the disk, seeing it from above/below and in-line.  That will change the appearance of the galaxy considerably even if it remained with the same wedge of disk throughout and even if the disk were unwarped.  And it most likely would not remain with the same wedge due to the tilt and accelerating/deceleration through the local bulk masses..  I think you can see this weaving impact in the changing shape of the star stream rotation patterns in the Wiki galactic year animation.  The disk is made up of many streams.

Do you mean the Sun's orbit around the Milk Way is inclined? I've never heard that. I know that the plane of the Milky Way is inclined about 60 degrees compared to the orbit of the Earth, are you sure that's not what you mean?
 

 

Do you mean the Sun's orbit around the Milk Way is inclined? I've never heard that. I know that the plane of the Milky Way is inclined about 60 degrees compared to the orbit of the Earth, are you sure that's not what you mean?

 

Yes and no.  I have updated my post after looking at this further.  I thought the number looked huge relative to the diagrams and should have investigated further, turns out it was.  You are correct that the Sun's poll and ecliptic plane tilt is 60 degrees relative to the Milky Way and that was what most things refer to.  There is an additional harmonic that occurs on a shorter time scale, and that is not well explained/documented in the diagrams, although it is shown on some of them...hence the confusion.   This is a periodic wobble relative to the galactic plane on a 60 to 70 million year time scale.  This results in the Sun moving up and down ~ +/- 100 parsecs vs. the plane of the galactic disk.  That is not large compared to the scale of the galaxy and distance to center.  It would be large for more localized bright MW features such as the Trapezium (roughly 400 parsecs away at present.) 

Hopefully astronomers will develop new tools and methods to help see the structure, or simulate different galaxies and help develop a model that can be generally accepted.

Don't get your hopes up too high. Galaxies are ultimately big blobs of fluid, and fluid dynamics is notoriously complex and chaotic. Just to take one example, I doubt that weather forecasts will be a whole lot more reliable 1,000 years from now than they are today. Weather is provably inherently unpredictable.

Add to that self-gravitation, which is nonexistent on the terrestrial scale, plus supernova explosions, which are poorly understood, plus galactic-scale magnetic fields, which have barely been measured, plus the underlying effects of dark matter, which increasingly defies all explanations. And you have a pretty seriously intractable problem.

It’s not my area of specialty, but back when I was a student all the talk was about density wave theory. I’m not sure where it stands now, but aside from galactic dynamics not being well understood we don’t even know for sure why galaxies form spiral shapes, or barred spirals to be more exact. The most likely explanation seems to be that the shape is influenced by dark matter, which surrounds the galactic halo. Since we don’t even know what dark matter is yet, we can’t give a model yet that everyone agrees on.