Nova

is the first and the lightest element in the periodic system of elements. The atomic nucleus of this element consists of one particle with a positive electric charge, the proton. The atomic shell contains one particle of with negative electric charge, the electron. Given the temperatures on a white dwarf, the proton is not able to bind the electron and both particles are separated. This situation is called to be the "ionized" state of hydrogen.

is the element following hydrogen in the periodic system. Its nucleus consists of two protons and two neutral particles, the neutrons. This nucleus is surrounded by two electrons in the atomic shell. On the surface of a white dwarf helium also exists only in the fully ionized state. The nucleus of helium is often called 'alpha-nucleus' or 'alpha-particle'.

Lighter elements such as hydrogen or helium can undergo nuclear fusion reactions to heavier elements. In contrast, chemical reactions only change the chemical bindings of the participating elements. Temperatures of several million degrees Kelvin are necessary for fusion reactions to take place. Fusion reactions release typically a million times more energy than chemical reactions. Indeed, the nuclear "burning" of hydrogen to helium in the stellar center is the energy source for most stars like our Sun. Temperatures of several 100 million degrees Kelvin can be achieved during Nova outbursts. This is called a thermonuclear explosion.

A system of two stars which orbit around their common center of mass due to their gravitational attraction is called a binary system. A very large fraction of stars are bound in binary systems. For instance, Sirius is such a system consisting of the Sirius A and Sirius B companions (see picture on the left side).

Red dwarf stars are cool low-mass stars. They burn hydrogen like our sun, but at a much lower rate.

Stars with masses similar to the mass of our Sun evolve to white dwarfs. at the end of their lives. After the Sun has used up its hydrogen in nuclear burning it contracts to a white dwarf of the size of the Earth. The average density will increase a million times. The picture on the left shows an HST image where white dwarfs are marked by circles.

A nova is a thermonuclear explosion on the surface of a white dwarf star in a stellar binary system. The second star in such systems is often a red dwarf which transfers gas to the white dwarf.

Nova Cygni 1992 exploded in the Summer constellation Cygnus (the Swan) in February 1992. It is about 10000 light years away (1 light year = 9.46 trillion km). This means that Nova Cygni 1992 did not really explode in February 1992 but 10000 years before.

In astrophysics like in many other disciplines the use of the most powerful supercomputers is indispensable for some problems. The supercomputer we used for our nova simulations was a CRAY T3E-900 located at the Computer center in Garching. It is a so-called "massively parallel" supercomputer where it is possible to distribute a calculation to a large number of processors simultaneously. This saves time compared to the same calculation being carried out on a single processor only. The T3E in Garching houses 784 processors of the type DEC Alpha eV5 (21164) with a maximum performance of 600 Mflops/s per processor (= 600 million floating point operations per second per processor). Therefore the whole T3E system is as fast as 784 DEC Alpha workstations together. Despite of the enormous computing power it took about 400 hours on the whole system to perform a single nova simulation. The same simulation on a DEC Alpha workstation would have run for about 35 years.

Every now and then a seemingly new star appears in the sky which before had hardly been visible even with the biggest telescopes. Such a star that suddenly changes its luminosity by orders of magnitude is called a nova. It is possible to observe novae still many years after their apparent extinction with the Hubble space telescope, as for instance this nova that appeared in the constellation of the swan in 1992. According to our understanding, novae occur in binary systems consisting of a developed red dwarf star and a burnt out, compact star, a so called white dwarf. It has got the mass of our sun but is compressed to the size of our earth. Because of its enormous gravitational attraction the white dwarf draws hydrogen rich material from its companion into a gas disk around the white dwarf. Subsequently, the material falls onto the white dwarf and gradually forms an ocean of hydrogen on top of it. Under the action of the gravitational force on the surface of the white dwarf the hydrogen ocean gets very dense and hot and hence triggers the nuclear fusion of hydrogen into helium. Since it is not possible to observe the following processes on the white dwarf directly we must simulate them on a computer. Thus, we consider the white dwarf and the surrounding hydrogen ocean shortly before the nova outburst. Making a cut through the star, one can see that the white dwarf is covered by the surrounding ocean. Since computer simulations are very time consuming we only consider the processes within a part of the ocean. To simulate the marked region one still needs a CRAY T3E super computer with 512 processors. Here we show the simulated evolution of the flow velocities within the ocean. Bright colours correspond to high velocities. The plane shown represents a vertical slice through the three dimensional calculation domain. The fusion of hydrogen causes strong turbulence yielding velocities of up to several hundred kilometers per second. In the course of the simulations the flows spread from the bottom shells through the entire hydrogen ocean. At the same time the intensity of the fusion process and the temperatures rise in the ocean. The temperatures can reach 200 Million degrees and more. These processes end up in the explosive ejection of the hydrogen ocean and in a rise of the luminosity of the star for several orders of magnitude. This can be observed as a nova.