Eruptive Variables

Novae: Occasionally, a new star suddenly appears in the sky. After several days, it begins to decline, and the star disappears several months later. In fact, the star is not truly new, but was extremely faint before the outburst. Several types of nova have been observed.

Novae are known to be close binary systems. The interpretation of the events has undergone significant changes in the last 20 years. It is now thought that one of the components fills the roche lobe, whilst the other is a tiny, white dwarf. There is a mass transfer from the component that fills its lobe towards the white dwarf. The gas builds up in a layer on the outside of the small star, until eventually the pressure and temperature become high enough for nuclear reactions to occur. The sudden onset gives rise to the outburst of energy and light that is emitted by the nova over a period of weeks.
The greatest amplitude known for a nova is that observed for the Nova Cygni 1975, which was magnitude 1.8 at maximum. Before its outburst on August 29 1975, the star was not visible on the Palomar Sky Survey plate, which reached down to magnitude 21.

Dwarf novae: Dwarf novae are stars that are intrinsically faint, and which frequently, and fairly regularly, undergo sudden outbursts. They rise by 4 to 5 magnitudes over a few hours, and then decline more slowly back to normal light. Again, they are binaries, and the mechanism has something in common with the brighter novae. In this case, the gas lost form the larger star forms a disk around the compact object. By a mechanism that is still poorly understood, this disk occasionally brightens. After the outburst, the system returns to its normal state, but the mass transfer soon causes yet another outburst to occur.
Dwarf novae of the U Geminorum type have distinct, well-separated outbursts, between which they are at minimum. Stars of the Z Camelopardalis type, on the other hand, are even less regular. They may sometimes remain at an intermediate light, at "standstill" - known as "stillstand" in North America - accompanied by rapid, minor fluctuations.

Supernovae: The explosion of supernova is certainly the most spectacular event that a variable-star observer can follow. Although the majority of stars end their lives as dense white dwarfs, there are certain stars that undergo a violent death, the mechanisms governing which have not yet been fully explained. The explosion of a star is far more violent than any other stellar phenomenon. Quasars are the only relatively small objects that have intrinsic luminosities greater than those of supernova. In our own galaxy, which contains 150 thousand million stars, the last visible supernova was observed in 1604 by Kepler. Nowadays, several supernova are discovered in other galaxies a t distances of several millions parsecs, so the objects are generally quite invisible before they explode. Supernova 1987A in Large Magellanic Cloud was the first object for which the precursor could be identified. This was extremely important for supernova studies, even though it proved not to be a typical supernova.
Observations of extragalactic supernovae have allowed some progress to be made in studying these objects. Two major types are distinguished:

When the explosion occurs, the magnitude may attain -20, and the supernova is sometimes as bright as the whole of the rest galaxy in which it occurs. The amplitude of the explosion is not known accurately: it is of the order of 14 magnitudes for type II, or even up to 22 magnitudes for type I. (Before their explosion, Type I supernovae are much fainter than those that will become Type II supernovae).
After the explosion, the star normally fades rapidly (approximately 2 magnitudes per month), and disappears. A nebula formed from the stellar debris appears some decades later. A small, very compact, central object may remain, as in the case of the Crab Nebula (the 1054 supernova). The youngest supernova remnant known in the Galaxy is the radio source Cas A. Strangely, the explosion, which should have occurred around 1667, was not observed!
Supernova explosions are highly significant. Heavy elements (oxygen, carbon, nitrogen, etc.) which are formed by nuclear reactions within the cores of stars are expelled by the explosion and enrich the surrounding space. Stars that form millions or thousands of millions of years later in the region will contain these elements, and solid planets may exist around these stars. On their surfaces, carbon compounds may arise. All living creatures are carbon based beings, whose very existence is the result of the explosion of supernovae at least 6-8 thousand million years ago.

Symbiotic Stars: The symbiotic (or "Z-Andromedae") stars are binaries,both components of which are unstable. One star is a red giant, semiregular or Mira-type, and the other is a very hot , tiny star, which is often eruptive. The combination of these two variations gives rise to peculiar light-curve. Examination of the spectra of these stars has enabled their nature to be understood.

Recurrent novae: These objects are similar to novae, but they have undergone two or more outbursts during their recorded history. Example: RS Ophiuchi.

U Geminorum stars: These are close binary systems with periods on the order of a few hours. After intervals of quiescence at minimum light, they brighten suddenly. Depending on the star, the eruptions occur at intervals of 10 to thousands of days. The light amplitude of the outbursts ranges from two to six magnitudes, the duration from 5 to 20 days. Example: U Geminorum.

Z Camelopardalis stars: These stars are physically and spectroscopically similar to U Geminorum stars. They show cyclic variations interrupted by intervals of constant brightness (standstills). These standstills last the equivalent of several cycles, with the star "stuck" at a brightness approximately a third of the way from maximum to minimum. Example: Z Camelopardalis.

SU Ursae Majoris stars: These stars are also physically and spectroscopically similar to U Geminorum stars. They have two distinct kinds of outbursts: one is short with a duration of one to two days, faint and more frequent; the other (supermaximum) is long with a duration of 10 to 20 days, bright and less frequent. During supermaxima, small- amplitude, period modulations (superhumps) appear with periods two to three percent longer than the orbital period of the system. Example: SU Ursae Majoris.

The ã Cas stars: These are hot variables with Be spectra. They are probably very young and still slightly eruptive. They have characteristically fast rotation, which broadens the spectra lines. At the equator, the rotation velocity is only slightly less than escape velocity; if there is a slight eruption, a cloud of hydrogen escapes. The type star, ã Cas is the brightest of the class, but it includes other well-known stars, such as Pleione (also known as BU Tau) in the Pleiades cluster. Frequent ejections of material have created a shell around Pleione.
The brightness variations in these stars are small, being less than 0.5 magnitude, but from time to time absorption of light by the shell may cause a deeper decline.

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