It sounds like you know what you're talking about until one realizes the earth is spinning. Wide field photos can be shot up to thirty seconds depending on the back and lens.

Anything more zoomy than 50mm uncropped you're getting streaks in < dozen seconds. There's a rule of thumb but I don't remember it.

Best course of action is to take a video and let a stacking program deal with it, especially if you use a real telescope.

Also the Sony a7r have like "150,000 ISO " and iirc cost like $3500 with a kit lens. That's a bit above consumer, but I may have mixed up models.

My camera is not an a7r. I paid $1200 for it used.

The exposure was 10 seconds, so by your own explanation, the spinning of the earth is not a problem ( as you can clearly see in the photo I linked )

An astronomy photo can commonly require hours or even tens of hours of exposure time.

The noise from a single image of that duration would be unworkable. Even if there were no planes or satellites, nobody would take that as a single exposure.

Signal scales with exposure time, noise scales with the root of exposure time. So although the noise increases with time, the signal-noise ratio increases faster. When dealing with astronomy photos, you want to maximise SNR.

(This is a simplification, there are many types of noise including shot noise and thermal noise, but in general this is the rule of thumb you use)

your method of shooting the stars with a single long exposure is called a "wide-field" image. They're cool, but if you want to get any details of anything other than the milky way you either have to track or you have to stack. that's all there is to it.

I was going off memory, and with 50mm, it looks like 10 seconds is about when you start to get streaks, assuming no cropping. I have cropped sensors so even with a 10mm lens i can only do 12-15 seconds. I have a barn door tracker, and i know how to use https://www.autostakkert.com/

you need to have black frames for it to work, and ideally a lot of them (iirc at least a half dozen if not more) - because this is how the stacker knows which pixels are noisier than other ones, and also what the thermal noise of the sensor "looks like", at least the smarter stackers use it that way. you take a bunch of pictures with the lens cap on in a dark room at around the temperature you're gunna use it at.

real amateur astrophotography is done with CCD cameras, usually monochrome with filters (or a filter wheel), done in video mode, and the frames are processed after the fact. It takes hours with clear skies to make a decent deep field astrophotograph.

Tens of seconds is about right. It's something like 500/f(mm) in seconds, but you get a feel for what will blur and what won't. Here's an example I shot with a Nikkor 50mm f/1.4 @ 20s: https://www.instagram.com/p/C-mU6iIp0re/?igsh=cm16bWx1cGp3OG...

Rule of 500. Divide your lens focal length into 500. That's the longest exposure to avoid trails. 500 / 24mm = ~21s so your example would be 500 / 50mm = 10s.

Some random search result:

https://astrobackyard.com/the-500-rule/