Long green bar.
Not sure what this artifact is???
Welcome to Galaxy Zoo, Owen Williams!
This is an #artifact, of the #satellitetrail kind. When a bright light moves quickly through the field of view of the SDSS camera, it leaves a streak like this. Because such a light will be 'seen' by only one filter (the SDSS camera has five, but the images we see use only three of those), it will have just a single color, green, red, or blue (though for some reason green satellitetrails are more common than red or blue ones).
Satellites are the most common of these artifacts, in SDSS image (hence the tag!), but airplanes and meteors can produce them too.
Happy hunting! 😃