To answer you query:
The white dwarf, which is the dead remnant of a star, is about the size of Earth but has the same mass as the sun. Meanwhile, the aging red giant is a dying star that's shedding material out into space. The white dwarf's massive gravitational pull is hauling in the ejected material from the red giant. Once the white dwarf has accumulated enough material, the heat increases so much that it causes a runaway thermonuclear reaction. That explosion is called a nova.
The prior nova from this star system occurred in 1946. It's a cycle that's been going on since it was first discovered more than 800 years ago.