What is it?
What is the black line?
#artifact. For this type of artifact, however, I do not know the cause. The nice, highly-inclined spiral which appears close to the center of the image is not, it would seem, the object intended to be classified! 😮 Rather, that's a small yellow smudge in its outskirts. That's likely a background elliptical, making this an #overlap (more than one, most likely).
Oh, and somehow
<b>got set; in the OP? This post showed up as bold, in a larger type, in Preview, until I added
</b>at the beginning of this post (of course, that is non-display HTML tag, so you can't see it!). This is the second time today that I've had to do this.