Completely over-exposed 40 min long star trail.?

Hi guys, I tried to take a 40 min long single exposure, but by the end the photo(RAW) came out completely over exposed. Aperture was set at F4, ISO 250 and shutter speed Bulb for 40 minutes. Did I do somethings wrong with the set up or 40 min long single exposure is supposed to be over exposed? Can I correct the... show more Hi guys,

I tried to take a 40 min long single exposure, but by the end the photo(RAW) came out completely over exposed. Aperture was set at F4, ISO 250 and shutter speed Bulb for 40 minutes. Did I do somethings wrong with the set up or 40 min long single exposure is supposed to be over exposed? Can I correct the error in Lightroom? Any suggestion how to improve such a exposure next time?

Note: original Raw image was converted to jpeg for posting.
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