There are many close binary stars in our galaxy which consist of a white dwarf which accretes material from its ordinary main-sequence companion. Under the right circumstances, a layer of hydrogen accumulates on the surface of the white dwarf. At some point, the temperature at the base of the hydrogen layer reaches a few million degrees and hydrogen atoms begin to fuse into helium. The reaction spreads throughout the hydrogen, blowing the layer off the surface of the white dwarf and into space at speeds of 1000 km/s. We call this explosion a "classical nova." The star rises in brightness by 10 to 12 magnitudes in a few days, then drops slowly back to its normal level over several months. If a carbon-oxygen white dwarf in a close binary manages to accrete and hold onto enough material, its mass may rise to a critical point of about 1.4 solar masses. At this point, the temperature and density at its center reach the ignition point for carbon fusion, starting a runaway thermonuclear reaction which consumes the entire star. The resulting "Type Ia supernova" explosion destroys the star, blowing the remnants outwards at over 10,000 km/s. With an absolute magnitude of -19, these supernovae can outshine an entire galaxy for several months. There are other supernovae, Types Ib, Ic, and II, which arise when young stars, at least 8 times the mass of our Sun, run out of fuel in their centers, creating a core of iron atoms. Because iron does not release energy when it fuses, the core cannot resist the pull of gravity and collapses. A shock wave generated during the violent collapse races outwards through the main body of the star, heating it to millions of degrees and flinging it outwards at speeds up to 10,000 km/s. These core-collapse supernova may leave behind a neutron star or black hole.