Thanks for the comments.
Here's the settings: 15 sec, f/2, ISO 6400
I first shot at 30 sec, but peeking at the photos I noticed some star trails even the 24mm should be fine up to 30sec. So I decided to use faster shutter and higher ISO.
Not really, using the 600 rule a 24mm is only good to 25 seconds. For those unfamiliar with the 600 rule, take 600 and divide it by your focal length and that is the exposure time in seconds before the stars become trails. In this case 600/24=25.
That is a generalistic rule, and not necessarily accurate in light of the fact that final output size can vary greatly. It does not take into account varying pixel pitch either. For web scale, you can usually bump the 600 "rule" up to a 700 or 800 "rule". For print, you usually want to drop it down to 500 "rule". If you usually publish online, then you could get away with at least 30 seconds, if not a bit more. Larger pixels can also handle longer exposures than smaller pixels.
To get entirely accurate maximum exposure times for any camera setup, you can use the math on this page:
It is in french, but it is extremely useful stuff. It factors in atmospherics, diffraction, and pixel size in determining what the best exposure time is to avoid star trailing.