Yes, a history but an historical event or a unicorn but an undershirt. You breathe out slightly pronouncing the second of each pair and the separating "n" makes less effort of following the article with an aspirated word like these. [Articles are a, an, the.]
You probably don't think about it ever, but the same thing happens with "the." "Thuh"precedes the normal words, "thee" the aspirated ones. See, I bet you even did it in your head while reading that last sentence. [Now, who can tell us why "e" works in front of that aspirated vowel, because I sure don't know.]


