Why not to marry a white dwarf *star*.

This is a follow up to the post -Binary Stars Blitzed (http://ya.astroleague.org/?p=491).

Binary star systems can operate normally for billions of years without any problems, the difficulties arise when the more massive star begins to die. As it expands into a red giant the less massive star may be completely enveloped by its partner’s outer layers, whilst this does not significantly affect the star it is a sign of things to come. After the more massive has collapsed into a white dwarf (assuming it didn’t go supernova (Type II) – which would have blown the other star out of the system) the partnership develops into something that can be devastating. Depending on the physical type and the mass of the still main sequence star various interactions may occur.

As the main sequence star expands into its red giant stage it fills its Roche lobe (the area of space in which its gravity is the strongest gravitational force present). If it exceeds the lobe matter transfer will occur and the white dwarf can begin to take on mass from its partner (this process is also detailed in Binary Stars: – Blitzed http://ya.astroleague.org/?p=491). Eventually enough mass falls onto the surface of the white dwarf for a burst of nuclear fusion to occur. This causes a drastic increase in the luminosity of the white dwarf which can go from a luminosity of around 0.02 suns (2% the luminosity of our sun) to over 100 suns in just a few hours (10000% the luminosity of our sun). The fusion does not last as it is only occurring in the uppermost region of the white dwarf which will slowly return to its normal brightness. This ‘flash fusion’ is known as a Nova. The white dwarf and its partner are largely unaffected by this explosion and the process of mass transfer can continue and the cycle of novas can continue many times. There is a large amount of debris created by a nova at this expands outwards after the event occurs. The next two images show the aftermath of a nova created by the binary star system Nova Cygni 1992. The first image was taken in 1993 fifteen months after the initial explosion and the second taken seven months after the first. It is clear that the debris expanded into a wave which travels through the surrounding space heating it and emitting large amounts of EM radiation in the process

The term nova comes from the Greek word for new. This name came about as ancient Greek astronomers thought that a nova was the birth of a new star more recent scientific studies allow us to understand the really nature of these events but the name has stuck.

Some white dwarfs go too far however. The mass transfer between a giant star and a white dwarf can be maintained for several million years as the amount of matter transferred is comparatively small. If the mass transfer is very much larger than normal enough matter can be sucked onto the white dwarf to push it past the Chandrasekhar limit. The resulting runaway nuclear fusion reaction rips the star apart entirely leaving only dust and gas behind in a remnant that expands and cools. As a star has exploded this is classed as a supernova but the force of the explosion is much greater than a ‘normal’ Type II supernova so these events fill the Type IA class of supernovae. The white dwarf’s partner star may be destroyed in the blast wave however it seems more common that the star is ejected from its position in space and travels outward from the explosion site as a rogue star.

These explosions are so powerful and emit so much energy that they can temporarily outshine their host galaxy (the galaxy in which the supernova takes place). One such example is the Type IA supernova that took place in the outer reaches of the lenticular galaxy NGC 4526. The supernova (SN1994D) is clearly visible in the image to the right. The explosion itself shone with the equivalent brightness of 5 billion Sols!

Perhaps the most famous example of a Type IA supernova is the Tycho’s supernova remnant or SN 1572. This supernova was documented by the astronomer Tycho Brahe four hundred years ago and thanks to History detailed and accurate records of the event today’s astronomers have been able to calculate the position of the remnant. The remnant can be seen below. The image has been taken in the X-ray band of the Em spectrum – this helps us understand just how powerful this explosion was and still is today. X-rays are only produced by objects that are at temperatures of several million Kelvin – the fact that the nebula is still emitting large quantities of X-rays today and that it has expanded to a size of 24 light years across in just 400 years (24 light years is a BIG distance) is tantamount to the massive energy released by the initial blast.

An artist’s impression of a Type Ia supernova

Artist’s Impression of a Type IA Supernova Credit for the video is to the ESO Source http://www.eso.org/public/videos/eso0943b/

I prepared a diagram to show the final possible steps in a star’s life cycle. The diagram follows below but please note all copyrights are reserved.