Similar because the subject of the post is No two (similar) Suns have ever collided. This goes for planets, atoms, electrons, protons and any other similar so-called mass.
Are you sure you want to prompt him for definitions one at a time like this? In reviewing his posts so far it seems to me that there are three or four dozen words or word combinations that he will need to define before we can see that he has painted himself into a corner.
