ERUPTIVE VARIABLE STARS are 8tare that brighten quickly and unpredictably, then fade. They may return to their previous, normal brightness or they may be so profoundly changed by the eruption that they could never return to their previous state. Some pre-main-sequence variables are eruptive by nature, and some Hubble-Sandage variables have properties in common with eruptive variables. No classification scheme is perfect, however, and we shall restrict our discussion here to those variables which are relatively quiescent before and after their eruptions.

FLARE STARS (also called UV Ceti stars) are cool, faint main-sequence stars which unpredictably brighten by up to four magnitudes in a few seconds then fade in a few minutes Only a few dozen of these variables are known, but that is because of the difficulty of detecting such faint stars much beyond the Sun’s immediate neighbourhood. In fact, about 5 per cent of cool main-sequence stars are flare stars, and since 80 per cent of all stars are cool main-sequence stars, it follows that 4 per cent of all stars are flare stars, and that they are perhaps the most common variables known. A flare star may ‘perform’ several times in one night, and the flares are large enough to be observed visually by patient amateur astronomers. The spectra of flare stars are like those of normal K and M dwarfs, except that emission lines of hydrogen are present; these intensify considerably during a flare.

These stellar flares are quite similar to the flares which occur on the Sun. The amounts of energy released are comparable. This amount of energy is small, compared to the normal energy output of the Sun, but is large compared to the normal energy output of a K- or M-type dwarf. Therefore, solar flares would barely be detect¬able if the Sun were several parsecs away from us. Like solar flares, stellar flares are accompanied by bursts of radio waves. These were first observed by Sir Bernard Lovell at Jodrell Bank, England, and he has continued to study these bursts, in collaboration with observers using optical telescopes (figure 4.17). The flare and the burst arrive almost simultaneously – good evidence that light and radio waves travel at the same velocity! The small difference in arrival times is consistent with observations of solar flares, and suggests that the two kinds of flares are caused by a similar mechanism: a disruption of the magnetic field on the surface of the star sends a supersonic shock wave upward through the chromosphere and corona, accompanied by an expanding cloud of plasma and magnetic field.

A similar process, operating almost continuously, may cause the irregular light variations of the T Tauri stars. If that is the case, then the flare stars (and the Sun) are distant descendants of the pre-main-sequence variables, in more ways than one.

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