Theory Novae

A nova erupts when a white dwarf has siphoned off enough hydrogen from a companion star to trigger a thermonuclear runaway. As hydrogen builds up on the surface of the white dwarf, it becomes hotter and denser until, at the point of a 'flah point' -when a fusion of the hydrogen occurs- it detonates like a colossal hydrogen bomb, leading to a more than a thousandfold increase in brightness in a little more than one day, or a period of a few weeks as the outer layers of the white dwarf are blown away. Dwarf nova systems are binary system consisting of a white dwarf and a brown dwarf. Classical novas can be considered to be 'miniature' versions of supernova explosions. Each nova explosion releases a total of 10,000 to 100,000 times the annual energy output of our Sun. Astronomers discover about 10 novae each year in our galaxy. Novas are less violent and more common than supernovae as they also are the source to material injected into the interstellar medium. Novae are specific also in that their explosion is repetitive. A star can pass from the 15th to the 5th magnitude, a 'new star' disrupting the familiar pattern of a constellation. Astronomers call erupting stars novae, Latin for 'new,' because they abruptly appear in the sky. A nova quickly begins to fade in several days or weeks as the hydrogen is exhausted and blown into space. As energies and masses associated with classical novas are more modest than those in a supernova, novas' remnants evolve more quickly. Novas occur more frequently than supernovas. Novae are recurrent phenomenons, which occurs regularly, and more frequently than supernovae. Each nova explosion releases up to 100,000 times the annual energy output of our Sun. Astronomers estimate that between 20 and 50 novae occur each year in our Milky Way Galaxy. Novae almost always also produce gamma rays, the most energetic form of light. Gamma rays are due to collisions among multiple shock waves of different speeds in the rapidly expanding shell of debris the explosion is yielding. Ejecta from earlier outbursts may stay in the vicinity of the star and form a disk of debris around the nova. The material continues expanding outward along the system's orbital plane, in a disk and not a sphere-shape, but it does not escape the system. The companion star likely plays a role in shaping how material is ejected, presumably along the system's orbital plane. Any new blast lights up the disk. As far as binary systems pairing a normal star with a black hole are concerned, they have gas from the normal star streams toward the black hole and forms a disk around it. Friction within the disk heats the gas to millions of degrees, hot enough to produce X-rays. At the disk's inner edge, near the black hole, strong magnetic fields eject some of the gas into dual, oppositely directed jets that blast outward at about half the speed of light. Higher-energy ("hard") X-rays are triggered by a region of very energetic electrons that form a corona around the innermost part of the disk. When these electrons run into photons of visible light, the collision boosts the photons to hard X-ray energies, a process known as inverse Compton scattering. Lower ('softer') energies seem to then come from the dense gas in the accretion disk. At the same time, the hot disk quenched whatever process powers the jets and shut them down. Then a cycle keeps on with the accretion disk cooling enough and switching the jets back with X-rays fainter but higher in energy, again. In the jets, electrons and positrons moving at a substantial fraction of light speed emit the X-ray radiation as they encounter magnetic fields, a process called synchrotron emission with energies of a trillion electron volts -- billions of times the energy of visible light. High-speed blobs of matter that the jets had hurled into space during previous eruptions in turn are becoming a source of X-rays. The study of the mechanism of the matter flowing from the companion star to a black hole, with a evolved star orbiting in 1.7 days, a 10-solar masses black hole, has shown that when matter is falling, the accretion disk is reaching nearly to the black hole as when the flow is reduced, gas close to the black hole is heating up and evaporating the innermost part of it. In the case of the pair, X-rays can be traced to within 20 miles of the black hole, or the inner edge of the accretion disk can retreat as much as 600 miles, showing that the inner disk thins to a tenuous but hotter gas, with a temperature of 20 billion degrees, instead of the regular 20 million degrees Fahrenheit. Polar jets however keep being produced by such occurrences. System containing a compact white dwarf and a red giant star are called symbiotic binary. The system goes nova when the red giant is so swollen that its outermost atmosphere is just leaking away as the white dwarf intercepts and captures some of this gas, which accumulates on its surface. As the gas piles on for decades to centuries, it eventually becomes hot and dense enough to fuse into helium. This energy-producing process triggers a runaway reaction that explodes the accumulated gas. The white dwarf itself, however, remains intact. Such a process has been seen in 2010 like forming a resulting shockwave, composed of high-speed particles, ionized gas and magnetic fields, expanding at 7 million mph (4,3 million km/h). The magnetic fields trapped particles within the shell and whipped them up to tremendous energies, up to velocities near the speed of light producing gamma rays when these accelerated particles smashed into the red giant's wind. That was the first evidence that nova events can produce gamma-rays as such novas would allow astronomers to study along a few days the events which take thousands of year to occur in a supernova remnant. An X-ray nova is a short-lived X-ray source that appears suddenly, reaches its emission peak in a few days and then fades out over a period of months. The outburst arises when a torrent of stored gas suddenly rushes toward one of the most compact objects known, either a neutron star or a black hole. Such a system, like a low-mass X-ray binary (LMXB) system may include a normal, Sun-like star. At certain rates, the accretion disk fails to maintain a steady internal flow and instead flips between two dramatically different conditions, a cooler, less ionized state where gas simply collects in the outer portion of the disk and a hotter, more ionized state that sends a tidal wave of gas surging toward the center. Each nova outburst clears out the inner disk, and with little or no matter falling toward the black hole, the system ceases to be a bright source of X-rays. Decades later, after enough gas has accumulated in the outer disk, it switches again to its hot state and sends a deluge of gas toward the black hole, resulting in a new X-ray outburst. This phenomenon is called the 'thermal-viscous limit cycle'

A peculiar type of a exploding binary system is a one which eventually results into AM Canum Venaticorum (or, AM CVn) with two white dwarfs involved, revolving extremely rapidly around eachother in 18 minutes. Such a system is predicted to generate gravitational waves, those ripples in space-time predicted by Einstein. Such a binary originate like one white dwarf and a other one much heavier and more compact already giving off gravitational waves, as the heavier start eventually begins to pull material from the smaller and explode at its surface after 100 million years. Such a explosion is categorized a Type Ia supernova. Some very rare events in which stars explode due to a merger with another star bring a event more brilliant than a nova but less a supernova. Such a event may be termed a 'luminous red nova,' or LRN, with a distinct red colour and a resurgent brightness in the infrared

->Binary Events in More Details
Recent studies have found that a binary system may erupt as a nova each 10,000 years in full-blown explosions, as, in-between, they are feebly hiccuping every few weeks. The full-blown explosions are leaving shells of expanding matter behind. Such events are 'nova', strictly speaking, due to their repetitive nature
Novae, on the other hand, of the type Ia are thought to be originating, as far as the white dwarf involved is concerned, into white dwarfs of a relatively high mass, not having, hence, to gain much mass before reaching their limit and exploding. Such events seem not that rare, as they do occur on small scales of time
In-between the major explosions, the fact that one of the binaries orbit into the solar wind of the other one, brings to that cold places are generated into which the lower temperature allows atoms to turn into dust molecules. Those dust molecules arrange themselves into a pinwheel pattern around the binary system. That pattern of dust is blown up each time the binary system goes into an explosion. A nova, strictly, is when gas from one of both stars in a binary system accumulates on the other's surface and eventually explodes as that process is of a cyclical type. X-ray binaries (XRBs), on a other hand, consist of pairs of objects where a compact star, either a neutron star or, more rarely, a black hole, is capturing material from an orbiting companion star. The infalling material is accelerated by the intense gravitational field of the compact star and heated to millions of degrees, producing a luminous X-ray source. The black holes with massive stellar companions are consistently bright over ten years as high-mass stars in these X-ray sources also have strong winds that allow for a steady stream of material to flow onto the black hole