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Novae fall in between the evolutionary stages of stars like our own, a G0 Main-sequence star having (1) one solar mass unit, and those categorized by having masses > 9 solar mass units, which become supernovae. It has been determined by observing the spectral changes that occur during an outburst, that a nova event is the blowing away of the outer (surface) layers in the "onion skin" model of a star at this stage in its evolution.

Stage One is thought to consist of the photosphere bloating into a shape distorted by magnetic lines of force. It appears to grow brighter, yet the internal temperature actually drops This point marks the maximum emission, and the spectrum is continuous, and crossed by dark lines, as it has been so far in the nova cycle of evolution. The photosphere is thought to still be intact at this point. The light from the ejectae shell that are not along the line of sight with the observer will emit a bright line spectrum. Hydrogen and Ionized metals are produced in the portion of the nova that is between the surface of the main expanding cloud and the observer. This "blueshift is due to its apparent relative motion towards you, the observer (Redshift is the reverse). Rates of expansion at the rate of 2000 Km./Sec. are not uncommon.

When a massive star collapses, the outer parts of the star explode at speeds of up to 10,000 kilometers per second and more. A similar amount of energy is released when a white dwarf star undergoes complete disintegration in a thermonuclear detonation.¹³ Novae have long been understood as part of the processes which contribute to element production in our galaxy, and astronomers generally believe that only hydrogen and helium were formed in the Big Bang event which formed the universe; therefore all other chemical elements must have been formed by processes in stars. This could be the reason the proportion of elements such as carbon, oxygen and iron has slowly grown over the lifetime of the galaxy.

As the ejectae expand, it reaches a point where the gases are so rareified as to be transparent, and through this we can see the whole expanding shell. Once they reach this stage, it is possible to observe the high-excitation lines of the permanent gasses that come from a continuously ejected cloud close to the core of the star, all within the tenuous encumberance of its shell.

As this shell continues to expand further still, the bright metalic lines fade, while Hydrogen, Nitrogen, and Oxygen are more visible because they are more difficult to ionize, and their lines remain apparent long after they are physically gone. The most prevalent lines are those of a category known as "forbidden lines." Those lines are emitted from what is now left of the star, at which point it has fallen back to or below pre-nova emissions across the spectrum. If we sum all the physical contributions to this event up to this point, we form a P-Cygni-like profile. It is thought that nova will eventually return to their pre-outburst state, and some actually become recurring nova, with periods from 10 to 40 years. These double-star systems are called Cataclysmic Variables .

Nova List

NICMOS has imaged thick, clumpy gas shells in the remnants of Cataclysmic Variable novae from three systems -QV Vul and QU Vul (both in the constellation Vulpecula), and V1974 Cyg (in the constellation Cygnus). Before this imaging, scientists could only guess how the gas was distributed in space. Astronomers generally believe that only hydrogen and helium were formed in the Big Bang event which formed the universe; all other chemical elements are formed by processes in stars. Thus the proportion of elements such as carbon, oxygen and iron has slowly grown over the lifetime of the galaxy.

Much of the gaseous material in the galaxy comes from supernova explosions involving stars like the sun or more massive stars, and there are certain isotopes that can only be produced by novae. Nova gas is enriched in elements such as carbon, nitrogen, oxygen, neon, magnesium, and aluminum. It is possible that some of the aluminum in our own solar system came from nova explosions - there is evidence that novae eject radioactive aluminum and there is also evidence that radioactive aluminum once existed in our solar system but has since decayed.

Dust grains often condense in the shells ejected by such novae - Their infrared spectral signature shows that dust they make is similar in size to the small dust grains released from comets in our solar system. It is therefore possible that novae are among the stars that produce the solid grains that are the building blocks of planets. Like the gas produced by novae, these grains end up in the gas and dust clouds that produce new stellar and planetary systems. Two of the three novae imaged, QV Vul and QU Vul, formed dust in their gas shells.

During the 1980's, QV Vul was found to produce four types of astrophysical grains at various times during a two-year period following its eruption. The dust that formed contained carbon, silicates, silicon carbide, and hydrocarbons. The earlier models of QV Vul suggested that the carbon dust components formed in fast-moving polar flumes, and that the silicates formed in a slow-moving equatorial ring. The heavy elements ejected by these novae explosions produced bright infrared emission lines - an infrared signature that will allow the Infrared Spectrograph instrument aboard the upcoming NASA Space Infrared Telescope Facility (scheduled for launch in 2001) to discover and study novae in a wide range of galaxies for the first time.