Technically I’m an engineer, but I’ve always had a soft spot in my heart for astronomy. With that in mind, I have a few questions I was hoping one or more experts could help answer.
I forget the exact numbers I’ve seen before, but let’s assume that we see an upper-bound of ten thousand stars in ideal conditions, ignoring ambiguous galactic hazes. Obviously this is only a tiny percentage of stars in our own galaxy, implying that our eyes kinda suck for astronomical observation. The hypothetical premise, therefore, is that a human observer magically gets teleported to a point along the galactic rotational axis far enough out that the whole galaxy fits into their field of vision. What would they see, if anything at all?
I once worked out just the absolute and apparent magnitudes of the situation and IIRC, got something like the galaxy having an apparent magnitude of -4 (which I admit could be way off), obviously making it visible. However, I don’t put any stock in this because I assume it implies that the light comes from a concentrated area. And beyond this, I have no idea what the light distribution of a galaxy is like, from the core all the way out.
If that’s not enough, I would also like to know how big our eyes would have to be to get a real-time view that was roughly as good as those long telescopic exposures of other galaxies. And finally, completing the circle, roughly how many distinct objects would we see in the sky if we had eyes that big here?
Thanks in advance for any thoughts/calculations. I’ve thought for a while that it would have to be one of the most awe-inspiring sights imaginable to have a whole galaxy fill your field of view and was just wondering how possible that actually is.
I’m skeptical though because you’re still seeing from within. Assuming you want to fit the whole thing into a 90 degree area (my guess for more or less being able to see it all at once), then the closest visible matter is roughly 50 kly away. Obviously as a whole, enough light is emitted to see from up to millions of light years away, a la Andromeda. But with it filling up so much sky space, is the light concentrated enough?
Pretty much I won’t be completely satisfied until someone walks in with a model of the light distribution of a galaxy and the know-how to use it.
Seeing from “within” is actually a problem that makes it LESS bright, because when you are looking from the edge inwards there is ALOT of dust and gas that blocks the view.
It just occurred to me. If you want to know what the Milkyway would look like from outside, all you need is the right sized telescope and a suitable galaxy to point it at.
Your fully dilated eye’s pupil is roughly 6mm.
If I take a 600mm (24 inches) diameter telescope and use a 100x magnification that will give me a 6mm exit pupil (600mm/100X = 6mm). That fills the eye with as much light as it can take in. If I aim the scope at galaxy it looks a hundred times closer/bigger. Minus some minor optical losses (about 10-30 percent), that galaxy pretty much looks just like it would if I were able to view it with my naked eye from a position a 100 times closer.
I’ve looked at various galaxies that filled most of the field of view of a “low powered” eyepiece on such large telescopes from dark viewing sites. The view was rather nice, but not bright and any colors to be seen were subtle.
It would be exactly as visible as it is from here on Earth. The “surface brightness”, or “brightness density”, or whatever you want to call it, is exactly the same no matter what distance you’re at. The total brightness only decreases because the area it takes up in the sky decreases. So you’d see a band of Milky Way 90 degrees across.
So you’re saying that the ratio of brightness decrease due to light that misses you because you move farther away and brightness increase because the source occupies less of a solid angle is 1:1? This seems counter-intuitive to me but obviously I’m no optics expert.
Also, why would it look that same as from a point within the galaxy? Surely the fact that our field of view of the Milky Way is a full 360 must change it.
Also, for Bill: how do the galaxies you’ve seen look when comparing the core region to the outskirts? Also, I assume telescope eyepieces give you much less than a 90 degree field of view. Though if the argument above is correct, that wouldn’t matter.
On the first point, I believe that is exactly what Chronos is saying, and it is correct. You will notice that the colors of objects do not get darker as they get further from you.
I think seeing a full 360 degrees - or, really we are talking solid angles, so a full 2 pi steradians - isn’t any different as far as what the surface looks like (I use “surface” to mean the appearance compressed to a single unknown distance along our line of sight, reducing sight to two dimensions). It is a bit different to see edge on, as you are seeing more light emitting stars and more dust absorbing clouds along any line of sight; in the same way, a colored liquid is more strongly colored if you look along the length of a long skinny container than if you look through its width.
Generally galaxies have a bright core and gradually weaker emission as you get further out. IIRC they say our solar system is 2/3 of the way out from the center of our galaxy, but if you point to a similar location in Andromeda, in typical pretty pictures the location is beyond what looks like the end of the galaxy.
Chronos is right about the distance/surface brightness thing. Let me explain the math a bit so it will be clearer. Imagine looking at a glowing golfball. Move twice as far away from it. The total amount of light you will see will drop by a factor of 2 squared, or 1/4 the original amount. 3x further and its 1/9… And so on. Now, when its twice as far away it only appears half as wide right? Well, .5 times .5 equals 1/4. So you have 1/4 the light but it is distributed over an “area” thats 4 times smaller, so the brightness per area has remained constant.
If thinking about things in the sky having a “surface brightness” bothers you, imagine an image of the object being being created by a camera on a piece of film instead. The math works out the same way.
Which reminds me of a factoid that often bothers people but is a result of the above. When taking pictures of an object with a camera and flash, in order to determine the right f-stop or flash setting, the distance from the object to the camera is irrelevant. Only the flash to object distance matters. This relationship breaks down at extreme distances, but if the object appears “bigger than a dot” to the camera, it works.
As Napier points out, the fact the you want to change the view of our galaxy from edge on more to face on complicates things. I need to run for a bit, but I can address that too and will later.
Thanks for the replies. Certainly my first impression that you might not even see anything is shattered. If someone knows more details on my follow up questions I would appreciate that too. Given the brightness invariance, I guess all you would need to know would be the size of the telescope and the length of the exposure for those nice photos. Do you guys have experience you can pull some numbers out of?
Oh, you’ll definitely see something. The only question is how spectacular or not it will be. And much of that would be based on your expectations.
Here as some numbers. The Milkyway is a band in the sky due to us seeing our own galaxy edge on. Because we are about a 1/3 the way in from the outer edge, its brightest looking towards the center, and faintest looking the opposite direction.
This band is, depending on how you want to define it, is between 10 degrees and 30 degrees wide. 10 degrees wide by 360 degrees long is about 4000 square degrees. On the high end it would be about 12000 square degrees.
You wanted a view where you saw the galaxy from above the plane face on rather than edge on, with it being 90 degress wide. Lets call it a 100 and square that, which makes that 10000 square degrees.
So, you will have the about the same amount of light distributed in about the same “area” of sky, but this time in a big disk in the sky rather than a long band.
So, if the Milkyway as you can currently see it impresses you visually, then the new view would as well. If not, the new view aint gonna be much better.
Yeah, yeah, I know its more complicated than this, but I’ve convinced myself this is close enough for gubment work.
If you wanna get an idea of what it would look like, here you go.
I’ve seen this galaxy, M-51 in medium to large telescopes in dark sights, and this a reasonable image. The reddish H II regions color would be more subtle, and maybe not even noticable as being red in color.
Yeah, structurally it probably looks more like that.
I picked the M-51 photo more because of the fact that its a big enough and bright enough galaxy that with not unreasonably large telescopes you can actually get a real world visual impression of what the OP was asking about.