A monthly lecture on the Johns Hopkins University campus will focus Tuesday on novae, massive nuclear explosions that occur in stars known as white dwarves. They involve the interaction of a white dwarf with another star, with the white dwarf pulling in gases from its neighbor. The lecture is titled “Novae: Stars that Explode – Repeatedly” and will be at 8 p.m. in the auditorium of the Space Telescope Science Institute, 3700 San Martin Drive.