Last week I went to Tahoe to do some astrophotography.

I watched this video (https://www.youtube.com/watch?v=Rydg7JGTAbw) about image stacking to reduce noise in my photos so I decided to give it a shot.

My setup: Sony A7ii with Batis 2.8/18 lens. I shot all my photos with ISO 1600 with a tripod for 25 seconds of exposure. My aperture is f/2.8 and I set my focus distance to my hyperfocal distance for all my shots

This is what the sky of my first image looks like:

And I followed the youtube video: I aligned just the sky and converted it to a smart object and applied the median filter. This is what it looks like:

Upon applying the median filter, I realized that aside from the center of the sky, most stars have disappeared in the newly stacked smart object.

I suspected that the movement of the stars could have contributed to this. so I changed the stacking mode to maximum and saw the star trails:

So here are my questions regarding my photos:

1. If the stars moved significantly between each frame, how can auto-align function properly?
2. How can I apply photo stacking to my current set of photos?
3. What can I do in the future to prevent this from happening?

Last week I went to Tahoe to do some astrophotography.

I watched this video (https://www.youtube.com/watch?v=Rydg7JGTAbw) about image stacking to reduce noise in my photos so I decided to give it a shot.

My setup: Sony A7ii with Batis 2.8/18 lens. I shot all my photos with ISO 1600 with a tripod for 25 seconds of exposure. My aperture is f/2.8 and I set my focus distance to my hyperfocal distance for all my shots

This is what the sky of my first image looks like:

And I followed the youtube video: I aligned just the sky and converted it to a smart object and applied the median filter. This is what it looks like:

Upon applying the median filter, I realized that aside from the center of the sky, most stars have disappeared in the newly stacked smart object.

I suspected that the movement of the stars could have contributed to this. so I changed the stacking mode to maximum and saw the star trails:

So here are my questions regarding my photos:

1. If the stars moved significantly between each frame, how can auto-align function properly?
2. How can I apply photo stacking to my current set of photos?
3. What can I do in the future to prevent this from happening?

EDIT

Would you guys mind submitting answers so I can reply individually?