Look at this timelapsevideo:
https://vimeo.com/16917950
What are he doing in After Effects to make the northern lights move so natural, and at same time let the stars almost not move at all?

Looks like some type of "dissolve" transition added between each frame, so they blend together more smoothly.

I think you are right, when i read one of the comments to the video, he talks about laying in dissolve in to each image.

Can anybody tell which effect to use in AE to get this kind of dissolve?

Looks like he slowed down the footage with Twixtor or something like that.

So you can´t do this in After Effects?

Twixtor is an "add on" (can't remember what it's called) for AE and it essentially smooths out video that doesn't have enough frames for smooth slow mo.

Possibly he just turned on "Frame Blending".

Twixtor is a pretty expensive plugin DerekMellott!

I have now tried frame blending, and can not get the stars and aurora to move so sloooow and nice. I then have timestretch to 300%. If i increase it to 500%, the clouds become too choppy.

I think I figured it out - at least one potential way to do this. The key is to import all the still images *individually* - NOT as a sequence. Then from the AE project panel, select all the image files and drag them into a new composition. You can probably play with these settings, but here's what I tried:

Still Duration: 6 frames (eg, 5 fps in a 30fps video)
[x] Sequence Layers
[x] Overlap
Duration: 4 frames
Transition: Dissolve Front Layer

This seemed to give a pretty smooth transition between frames. It was still a bit too fast, so I set Time-Stretch to 300%. I tried this with some aurora timelapse pics I took in Iceland last year. Each image was a 4-second exposure, with little or no interval between shots (set to multiple-shot mode, locked down the shutter release).

Here's the quick & dirty results (password = "aurora"):

There you got it Jim!
Exactly what i was looking for!!
Just one question: In "still duration" and "duration" in the "New composition from selection"-box, how to get "frames" instead of time (0:00:00:00)?

Thank you for helping me

The last :00 in the time is in frames. I think all of the "time" fields in AE use this same format by default:
Hours : Minutes : Seconds : Frames

Aha! Learning something new every day! Thanks again