Although most stars shine steadily day after day, year after year, some change dramatically in brightness over very short periods of time. One type of star, called a nova (plural: novae), may increase enormously in brightness—by as much as a factor of 10,000 or more—in a matter of days.

The word nova means "new" in Latin, and to early observers, these stars did indeed seem new, as they appeared suddenly in the night sky. Astronomers now recognize that a nova is not a new star at all. It is instead a white dwarf—a normally very faint star—undergoing an explosion on its surface that results in a rapid, temporary increase in luminosity. Figure 21.1 illustrates the brightening of a typical nova. Novae eventually fade back to normal, usually after a few weeks or months. On average, two or three novae are observed each year. Astronomers also know of many recurrent novae—stars that have been observed to "go nova" several times over the course of a few decades.

Figure 21.1 A nova is a star that suddenly increases enormously in brightness, then slowly fades back to its original luminosity. Novae are the result of explosions on the surfaces of faint white-dwarf stars, caused by matter falling onto their surfaces from the atmosphere of a larger binary companion. Shown is Nova Herculis in (a) March 1935 and (b) May 1935, after brightening by a factor of 60,000.

What could cause such an explosion on a faint, dead star? The energy involved is far too great to be explained by flares or other surface activity, and as we saw in the previous chapter, there is no nuclear activity in the dwarf's interior. (Sec. 20.3) To understand what happens, we must reconsider the fate of a low-mass star after it enters the white-dwarf phase.

We noted in Chapter 20 that the white-dwarf stage represents the end point of a star's evolution. Subsequently, the star simply cools, eventually becoming a black dwarf—a burned-out ember in interstellar space. This scenario is quite correct for an isolated star, such as our Sun. However, should the star be part of a binary system, an important new possibility exists. If the distance between the two stars is small enough, then the dwarf's tidal gravitational field can pull matter—primarily hydrogen and helium—away from the surface of its main-sequence or giant companion, as illustrated in Figure 21.2. The system becomes a mass-transferring binary, similar to those discussed in Chapter 20. A stream of gas leaves the companion through the Lagrange point and flows onto the dwarf. (Sec. 20.6)

Figure 21.2 A white dwarf in a semidetached binary system may be close enough to its companion that its gravitational field can tear material from the companion's surface. Compare with Figure 20.21. Notice that unlike in the earlier figure, the matter does not fall directly onto the white dwarf's surface. Instead, as discussed a little later in the text, it forms an "accretion disk" of gas spiraling down onto the dwarf.

As it builds up on the white dwarf's surface, the stolen gas becomes hotter and denser. Eventually its temperature exceeds 10⁷ K, and the hydrogen ignites, fusing into helium at a furious rate (Figures 21.3a—d). This surface burning stage is as brief as it is violent. The star suddenly flares up in luminosity then fades away as some of the fuel is exhausted and the remainder is blown off into space. If the event happens to be visible from Earth, we see a nova. Figure 21.3(e) is a photograph of a nova apparently caught in the act of expelling mass from its surface.

Figure 21.3 In this artist's conception, material accumulates on a white dwarf's surface after being accreted from a companion star (a) and then ignites in hydrogen fusion as a nova outburst (b and c). Part of the surface gas is ejected into space in the form of "bubbles" of hot plasma; the rest relaxes back down onto the accretion disk (d). The real photo in (e) corresponds roughly to the events depicted by the artist in frame (c). This nova is called Nova Persei (1901). (See also the chapter opening photo and art sequence A—D.)

The initial flare-up of luminosity from a nova declines in time, and eventually the star returns to its normal, preexplosion appearance. The luminosity and temperature of a nova are not usually plotted on an H—R diagram, however. Instead, the change in luminosity is plotted in the form of a light curve, like that shown in Figure 21.4. Such curves show the dramatic rise in luminosity over a few days, followed by the much slower decay over the course of several months. The decline in brightness results from the expansion and cooling of the dwarf's surface layers as they are blown into space. Studies of the details of these curves provide astronomers with a wealth of information about both the dwarf and its binary companion.

Figure 21.4 The light curve of a typical nova. The rapid rise and slow decline in the light received from the star, as well as the maximum brightness attained, are in good agreement with the explanation of the nova as a nuclear flash on a white dwarf's surface.

Because of the binary's rotation, material leaving the companion does not fall directly onto the dwarf. Instead, it "misses" the compact star, loops around behind it, and goes into orbit around it, forming a swirling, flattened disk of matter called an accretion disk (shown in Figure 21.2). Due to the effects of viscosity (that is, friction) within the gas, the orbiting matter in the disk drifts gradually inward, its temperature increasing steadily as it spirals down onto the dwarf's surface. The inner part of the accretion disk becomes so hot that it radiates strongly in the visible, the ultraviolet, and even the X-ray portions of the electromagnetic spectrum. In many systems the disk outshines the white dwarf itself and is the main source of the light emitted between nova outbursts. X-rays from the hot disk are routinely observed in many Galactic novae. The point at which the infalling stream of matter strikes the accretion disk often forms a turbulent "hot spot," causing detectable fluctuations in the light emitted by the binary system.

A nova represents one way in which a star in a binary system can extend its "active lifetime" well into the white-dwarf stage. Recurrent novae can, in principle, repeat their violent outbursts many dozens, if not hundreds, of times. But even more extreme possibilities exist at the end of stellar evolution. Vastly more energetic events may be in store, given the right circumstances.