I know, you might see lots of images, especially on social media, showing all those beautiful Milky Way arcs.
But, what you might not know is one important fact:
The Milky Way, as we see it from our point of view in this galaxy, is no arc at all!
This arc shape is due to a few conditions which arise during capturing our galaxy in photographs.
First, the Milky Way, in reality, is a disk shaped spiral. We are pretty far away from the center, on it’s outskirts, as shown in this image:
This means, we are sitting on the brim of a plate. When we look to the Milky Way center, we do so from this brim. The Milky Way center, or core, is the beautiful big area, with all its colors. It’s usually visible to us on the southern hemisphere from around February to October.
Back to our position… As mentioned, we are sitting on the outskirts of our disk shaped galaxy. This means, if you take my plate-example, looking towards the core of our galaxy will show it in a pretty straight line, naturally, like the plate looks like a line if you look at it from the side.
If you are out there in a clear night, in a really dark place, you will see the Milky Way with the naked eye. You will also notice, it IS actually a straight line.
So how do we get those mysterious arcs then?
Well, this happens due to the way a camera is used, what sort of lens you have on and the cameras' angle of view.
The curvature of our planet plays also a role (sorry flat-earthers), as our horizon isn't a straight line in the first place.
There are two main techniques to shoot such an arc, given the Milky Way is in a somewhat horizontal position, not too far or too close to the horizon. The bigger that distance, the more pronounced the arc(s) get:
The first one is to use a very wide angle (or even fish-eye) lens. Something around 15mm focal length on a full frame camera. With this sort of setup the arc is pretty obvious and can be done in a single shot. The arc effect comes from the lens distortion. It’s very obvious if the center of the lens is pointing somewhere to the middle between horizon and the Milky Way. Without correcting this distortion, you will see the the horizon curving downward and the Milky Way pointing upward.
Now when you post process such an image, most programs will find the actual horizon and straighten it, resulting in curving the galaxy even more – and the arc is born.
In some cases the software might adjust to the galaxy, and so create a downward ‘arc’ of the horizon, which can usually be corrected, depending on the software used. Like in the image below, which shows the Milky Way correct but the horizon curved.
The second technique is to shoot a panorama with a somewhat longer lens (say 24mm on a full frame). Naturally this doesn't work in a single shot. The advantage is, you catch more detail. The disadvantage is, you need to be fast, as Earth rotates within the galaxy, so the Milky Way ‘moves’ during the shooting. This technique is often used by more advanced astro shooters.
You'll see distortion effects similar to single shot images (fish-eye/super-wide-angle lens), when you stitch your individual shots together. This is because the overall focal length of the lens 'mimics' that of a fish-eye. However, you will have a much higher resolution.
Some of those images above are single shots, some are panoramas. Because the effect is the same, I didn't bother splitting this up any further
The top image is a panorama of 19 individual shots, image #4 is a panorama of 24 shots in 2 rows.
You see something similar when you shoot a panorama with your phone, for example parallel to a street. The street will show a pretty weird angle in some of those shots. This is due to the same effect, shooting several images from different angles – because that’s exactly what the phone, too, does.
You might also have seen images of the Milky Way, rising steep over the horizon. In those you won’t get any arc. This is due to said steep angle, very roughly around 90 degrees towards the horizon. With that, the software needs only to straighten the horizon, but this has no visible effect on such a steep Milky Way.
Those images below are vertical panoramas of 5 or more individual shots.