An average star shines because nuclear fusion in its core generates energy in the form of x-rays and gamma rays, which become visible light by the time they reach the star's surface. However, scientists have recently found that some stars have nuclear fusion occurring just below their surfaces. These stars are white dwarves, or dense stars that have used up their nuclear fuel, in orbit around other ordinary stars. (See The Life and Structure of a Star.) In this type of arrangement, the dwarf siphons off gas and other materials from the surface of its companion. This material congregates on the dwarf's surface and initiates the unusual type of stellar fusion described above. As a result, the dwarves emit large quantities of x-rays with "soft" wavelengths. When the dwarves accumulate too much gas and thus gain too much mass, they become unstable and either collapse into a very dense neutron star or explode into a Type Ia supernova.
Hard x-rays are much more commonly researched than their soft counterparts. With typical energies of 1-20 kiloelectron volts (keV), they are much easier to detect, much higher in energy, and can be generated by a number of cosmological processes. They usually imply temperatures of 10-100 million kelvins and often represent neutron stars or black holes "digesting" large amounts of matter, often from a companion star. Soft x-rays, on the other hand, basically represent the boundary line between hard x-rays and ultraviolet light (See The Electromagnetic Spectrum.) Their wavelengths are typically 50 to 1000 times smaller than those of visible light, putting their energy in the 0.09-2.5 keV range. They are generally emitted by objects with temperatures in the hundred thousands of kelvins.
Scientists studying supersoft sources first thought they were neutron stars or black holes in orbit around ordinary stars. Both of these systems would involve the dense star remnant in question siphoning off matter from its companion and using it for nuclear fusion. However, in neutron stars, a large amount of energy released in the form of energetic hard x-rays balances out a lower fusion rate. The x-rays produced by black holes are generally softer than neutron stars because 46% of the matter accreted by a black hole disappears into it. For this reason, supersoft x-ray sources were originally thought to be black holes. However, it was quickly shown that the sources were much softer than any known black-hole system. It was concluded that the sources in question were white dwarves accreting matter onto their surfaces and releasing much lower energies due to their lower gravity and greater efficiency in fusion.
Supersoft sources begin as binary systems whose stars have significantly different lifespans. Eventually one will use up all the fuel in its core and cease nuclear fusion, becoming a red giant. The orbit then begins to tighten and the giant releases its outermost layers, becoming a white dwarf with no internal nuclear fusion. At this point, the type of the initial system determines its future: if the companion star is an ordinary star or a large red giant, it essentially surrenders its outer layers of gas to the white dwarf. If it is a smaller red giant or is in a wide orbit, its solar winds can drive the supersoft source. When the white dwarf has accumulated enough mass to become unstable, it either collapses still further into a neutron star (thus becoming a hard x-ray source) or explodes as a Type Ia supernova. Supersoft x-ray sources, depending on the rate of accretion of material from the companion star, may also provide explanations for certain types of novae.
The death of supersoft sources can occur in three ways: first, as described above, the white dwarf can collapse into a neutron star and thus become a classic source of hard x-rays. Second, the companion star can begin to "feel" the effects of the loss of its outer layers and thus cease the donation of its matter. Third, the white dwarf can explode into a Type Ia supernova. This occurs when the white dwarf reaches the Chandrasekhar limit, or the maximum mass it can stably support, and either possesses carbon or was initially smaller than 1.1 solar masses.
Some white dwarves that reach the supernova stage simply amass too much helium and explode completely. In most, however, the helium layer reaches its own critical mass and explodes independently. The explosion then ignites the carbon core of the star, which is converted almost within seconds to nickel and elements between silicon and iron. The nickel then falls into space and decays to cobalt, then iron, in the space of a few hundred days. This is the supernova Type Ia, in which no spectroscopic evidence of hydrogen or helium participation is evident (this distinguishes it from Type Ib, Ic, and II). Type Ia supernovae are thought to be major sources of iron and similar elements in the universe. Type Ia supernovae are also often used by astronomers as "standard candles" to determine distances to other, less well-understood objects. Additionally, these supernovae are now being used to analyze the rate of expansion of the universe.