"A Nova," He Repeated

I sat at a table in a hallway at school working on today's crossword puzzle. The hint for 3-down was "exploding star" and I stared at the blank boxes. Nova. I knew that the last four letters had to be nova. The first five, however, I just couldn't get. After moving on to other hints, I noticed my Astro lab professor walk behind me and go for a drink from the vending machine. He selected a drink and knelt to get it out when it came. I pretended to fill in some other squares. He got up and started walking behind me again and I turned around.
"Professor, what do you call an exploding star?"
"A nova."
"No, no, I know that. But what kind of nova?"
"A nova."
"And one that explodes?"
"Is this for your...?" He pointed to my puzzle.
"Yeah."
"Supernova. A supernova is a star that explodes."
I thanked him and filled it in. He walked away and I realized that I should've figured it out on my own.