Apersuasive speech exploring the truth or falsity of an assertion is known as

It is known as a speech on a question of fact.
Facts are pieces of information which are always true - so if you want to determine whether an assertion is true or false, you will first have to determine whether it is a fact. If it is a well-known fact, then it must be true. Facts deal with the truth or falsity of various pieces of information.

White dwarf is very hot, but also very small and that is why it may appear dim.