Nova

The classical novas described above are just one part of a group of stellar systems called cataclysmic variable stars. All cataclysmic variables are systems of two stars, one of which is a small dense star and the other a normal star. Two of the other cataclysmic variables most closely related to classical novas are dwarf novas and recurrent novas. Dwarf novas brighten and return to normal on an irregular cycle of weeks to months. Their maximum brightness is much less than that of classical novas. Recurrent novas brighten on a cycle on the scale of decades. Their maximum brightness is again less than that of classical novas, but their return to normal brightness is more sudden.

A dwarf nova’s brightest point is only about 100 times brighter than the system at minimum. These outbursts reach maximum in a matter of hours and stay bright for only a few days. A dwarf nova’s outbursts are not caused by the same runaway thermonuclear reactions that fuel classical novas. Instead, the brightness of the system changes according to how fast the gas in the accretion disk flows onto the primary star. When the flow of gases from the secondary to the primary star is fast, the system glows brighter. When the flow of gases slows, the system dims.

All novas have repeated outbursts, but it will take hundreds or thousands of years for the hydrogen fuel to flow onto the white dwarf and build up the explosive layer to fusion temperatures. Astronomers call novas that go through the explosion on the scale of decades recurrent novas. Recurrent novas brighten more than dwarf novas but less than classical novas. In a recurrent nova system, the secondary star is a red giant star. Red giants lose material faster than the normal stars of the classical novas, so the layers in the accretion disk build up faster and, thus, they explode sooner than in classical novas. Recurrent novas build up another explosive layer every few decades. They eject only a small fraction of the accreted layers during the explosion and the white dwarf grows in mass until it explodes as a supernova.