Star in forum icon?

In the round icon to the left of the topic title there sometimes appears a star. It doesn’t have to do with the Thanks Toplist which are the starts under the title, nor does it have to do with new posts. How are we supposed to know these things?

From inspection, it seems that the little star shows for threads to which you have replied.

Hmm, then why did this thread have a star before I replied to it?

I thought it might mean threads which I started but it’s not that, either.

It means it’s a thread in which you participated (i.e., started or replied).

And when the blue/red icon is scrolling, what does that mean?

Red : you have not read it all/Blue : you have already read it all !

The scrolling means it’s a “hot” topic, with more than 20 posts. (Is that the number?)