The study of Nova Phenomemnon

The study of Nova

Phenomenon

A Report by:

Comm. R.M. Wey

OSR: SFS/SFC

Research into the after math of the death of Nova V1974 Cygni has led to some interesting quandaries. Considered to be a "naked eye" nova [or one that can be seen without artificial aids], its very short life nonetheless left many more questions than answers.

It is generally accepted that a nova system begins its life as a widely separated binary [where one is more massive than its companion]. As the massive star evolves faster, it fuses its hydrogen into helium and so on, until it expends its resources. At this point, the star will begin to mushroom [due to its mass] and swell to that of a red giant, engulfing its companion.

They continue to orbit one another within the envelope, gradually losing orbital energy and angular momentum. As a result, they begin to spiral inwards, toward one another. All the while gases are being expelled into space. Eventually, all the material that formed the envelope is lost, the massive star [having spent all its fuel] is reduced to a compact white dwarf, its companion changing very little.

However, as the companion star continues to orbit the white dwarf, it too begins to lose gas. As it passes the accretion disk, the outer layers of its atmosphere are slowly sucked onto the surface of the white dwarf. The intense gravity of the white dwarf intensifies the accumulation of these gases, and thus their compression. As this occurs, the temperatures rise to several million degrees Kelvin. This process of accumulation also acts to mix the materials of the core of the white dwarf, changing its composition. Thus, hydrogen nuclei fuse into helium and release energy, similar to that of normal stars.

But, unlike normal stars, expansion of the gases resulting from this energy is not possible due to the intense gravity. Carbon and oxygen mixed from the core begin to catalyze, causing the fusion to speed up. With no where to go, the pressures build until an explosion occurs. The actual first few moments of a nova explosion have never been observed, it is only through simulations that speeds of such explosions have been measured.

Initially, the expanding shell consists of hot, dense gas [mostly electrons and ions], as it cools, these electrons begin to recombine with the ions and form an iron curtain which absorbs all the ultraviolet light emitted by the nova. Such outbursts can last for months, and the life span of a nova depends on mass of the white dwarf that hosts it.

Some can repeat the cycle many times before their fuel [and thus their lives] are exhausted. In the case of V1974 Cygni, its short life connotes a mass some 20 to 30 percent greater than the sun; And the amount of mass ejected [expected to be around 10 to the minus 5 solar masses] was more than ten times that.

Many questions remain about nova activity, for while we understand the thermonuclear reactions that produce them, what are not clear are the dynamics. Do the shell and core mix while material is being accreted, or at some point during the explosion? What are the effects of repeated outbursts on the evolution of a white dwarf? Perhaps, in time, we will know the answers.

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