Nebulas - could reduce sensors to hide ships from enemies

Here's a thought. This could also come under the category of space anomalies or Mega-eventa:

You can have stars that are going to super nova, or pulsar stars where at random intervals, the star goes through pulsing intervals or episodes of expansion and contraction. During these intervals (and there should be some sort of warning given a few turns in advance that this is going to happen), the surrounding hexes or parsecs could not be travelled through. If your ship or fleet happens to be in that area when the event occurs, the ships would be wiped out. If you happen to have a colonized planet in the affected area when this happens, you lose a random number of the planets population AND some of its buildings. In worst cases, you might even lose a land tile or two permanently.

A warning could be given 3 or 4 turns before this happens - something like one of those messages that occurs after every turn where it says something like: "Your scientists determine that the star "your name here" will pulse or expand in 3 turns". That way you have some advance warning not to have ships in a particular area. You could also achieve the same result maybe by having the star start blinking in the game - as a visual indicator.