IV.

The Outburst

A nova reaches its maximum brightness as layers of gas expand off of the white dwarf after fusion reactions begin to occur in the layer of accreted material. The explosion that carries away the accreted material is so powerful that most of the gas escapes the gravitational pull of the nova system and drifts off into space. The time that a nova takes to reach its maximum brightness and return to normal levels varies widely. So-called fast novas reach maximum brightness within a few days and stay at maximum brightness from a few hours to a week. They decline in brightness very rapidly, returning to normal levels within a few months. Slow novas rise more slowly to maximum, spend more time at their maximum, and decline more slowly. Slow novas also tend to be more erratic than fast novas in their brightening. For example, Nova Herculis, which exploded in 1934, stayed at or near maximum for almost three months. It then dropped rapidly in brightness for about a month, recovered slightly, and continued in slow decline for a number of years. Another very slow nova, Nova Delphini 1967, stayed near maximum for almost a year.

Astronomers use a technique called spectroscopy to study nova systems. Using spectroscopy, astronomers can separate the light of a star into the wavelengths that make it up. The separated light is called a spectrum. Spectroscopy allows astronomers to learn about the composition and temperature of a star by examining its spectrum. The wavelengths, or color, of light that a star emits are a measure of its temperature. Different chemical elements absorb and emit light at different wavelengths, so astronomers can tell which elements are present by looking for especially bright or especially dark lines in the spectrum.

When first discovered, the spectrum of a nova shows that the expanding layers of gas that cause the brightening have temperatures of 40,000° to 50,000° C (70,000° to 90,000° F)—about eight times as hot as the surface of the Sun. By the time a nova reaches maximum brightness, the temperature of the material has fallen to about 10,000° C (about 20,000° F), or lower.

Just after maximum brightness, the escaping cloud of gas cools and expands enough to become transparent. This transparency allows astronomers to view all of the gas that the star has ejected. They have found that a typical nova outburst blows away about 0.01 percent of the mass of the star. The material that the star loses is very different from the material found in the atmosphere of a normal star. The expanding cloud of gas contains much higher levels of helium and lower levels of hydrogen. Carbon, nitrogen, oxygen, and sometimes neon exist in much higher levels in the nova cloud than in the atmosphere of a normal star. Astronomers have also discovered a relationship between the speed of the nova and the amount of heavier elements, such as carbon and nitrogen, in the cloud. Fast novas generally (but not always) eject clouds that contain larger proportions of carbon, nitrogen, and oxygen than the clouds of slow novas.