Nando is a professional astronomer at the European Southern Observatory. His passion for astronomy started in 1975, when he looked at the Moon with his father's 6x30 binoculars. That vision has stayed with him ever since! His main scientific interest is supernovae in the local Universe. In his rare moments of spare time, he likes to play the piano and flute.

This article is about one of the most energetic events in the known Universe: Type Ia Supernovae. They are thought to occur when an extremely dense, small and hot star called a white dwarf gathers material from a companion in a binary system. Its greediness for matter comes at a price, though: when the white dwarf reaches a critical stage it undergoes a violent thermonuclear explosion which converts the star into one of the brightest sources in the sky, outshining a whole galaxy. Yet, the exact nature of the binary system is still unclear...

Seeking the origins of thermonuclear supernovae

Many of the stars we see in the Universe are actually part of binary systems; two stars orbiting around each other. Explanations of why thermonuclear explosions take place within a binary system composed by a white dwarf and a donor star were first suggested in the 1970s. We have several ideas, but the true answer remains an unsolved problem.

Thermonuclear explosions — kaboom!

Most stars spend normal lives creating energy through nuclear fusion. Once they run out of fuel they cool down, and quietly fade away. Some others go through extraordinary epiphanies, giving rise to one of the most powerful phenomena we know: a supernova explosion.

There are two main mechanisms that can blow up a star: the collapse of a stellar core and a thermonuclear explosion. The first is supposed to take place in young, massive stars. The second is thought to be the responsible for one particular variety of supernova explosion, which has become a hot topic in science over the last ten years. They are called Type Ia supernovae and are actually very similar to each other, making them useful to astronomers trying to measure cosmic distances. This fiendishly difficult task has always been the torment of astronomers. One of the solutions is the use of so-called standard candles: if you have a type of objects which have the same brightness, then you can deduce their distance from how bright they appear to be from Earth; the dimmer they are, the further away. As it turns out, Type Ia supernovae are one of these standard candles, becoming billions of times brighter than the Sun. This means they are visible even from extremely large distances.

The host galaxy M100 before (left) and after (right) the explosion of SN2006X. The supernova is the bright stellar object in the center of the right image.
Image credit: ESO (VLTFORS1).

In the last ten years the study of these supernovae has gone one step further. Astronomers have used these extraordinarily bright candles to map the geometry of the Universe, coming to the conclusion its expansion underwent a significant acceleration. This has opened a whole new chapter in physics and probably is one of the most exciting discoveries ever made.

The quest for the progenitors

However, not everything is known about Type Ia supernova explosions. Early on we realised that they must come from relatively old, low mass stars. But how can an otherwise quiet star turn into such a catastrophic event? A possible solution was proposed in the 1970s: an old white dwarf belonging to a binary system gathers material from its companion until it reaches a limit, when a thermonuclear explosion is ignited. In fact, a white dwarf is made up of matter which is in a very special condition, called degenerate. Normally, if you heat a gas, its pressure increases. This causes the expansion of the gas which, in turn, produces a decrease in the temperature and acts as a thermostat that self-regulates the system. However, when matter is degenerate, this thermostat is switched off and temperature can rise out of control.

So once a white dwarf reaches the required critical conditions to ignite the thermonuclear burning of carbon, the lack of a self regulating mechanism causes the star to be completely incinerated. The gas is ejected into space at velocities that exceed one tenth of the speed of light — very fast indeed! In principle, different stars can act as donors in the binary system, even another white dwarf.

Chasing the companion star

So far all attempts to detect any matter around Type Ia supernovae have been frustratingly difficult. In the autumn of 2005 my collaborators and I realised that there may be a way to detect tiny amounts of material in the intermediate surroundings of the exploding star, which would help in understanding the nature of the companion.

Hydrogen is the most abundant element in the Universe, but we can only see it at the right temperature: neither too hot nor too cold. Luckily hydrogen is not the only element in space. In particular there are two others, calcium and sodium, which are rather rare (a million times less abundant than hydrogen), but which leave very strong spectral signatures which we can detect. This means we can track down even tiny amounts of those elements. If a supernova shines through these elements we would be able to see — for the technically minded, we actually look for the absorption lines. But how would we tell if the gas is related to the exploding system? For a distant cloud the absorption would remain constant with time, while if the gas is close to the explosion site, then the strong supernova radiation field can modify its physical conditions and produce changes in the amount of absorption. The plan was set: carefully study the next supernova explosion, study the light, see where it came from.

SN2006X and its heritage

The first chance to test our idea came in February 2006, when a new Type Ia supernova, imaginatively named SN2006X, was discovered in spiral galaxy M100 in the Virgo Cluster. We repeatedly observed it with the European Southern Observatory's Very Large Telescope, named Kueyen. Much to our delight, a number of features were indeed seen to change. We believe this was the long sought sign that SN2006X had been generated within a binary system with a red giant star as a donor.

Our results were soon followed by theoretical and observational work, some favouring but some others disputing our idea. However, as often happens when a new discovery is made, we were left with more questions than answers and, as of today, the debate is still open. Many other objects will have to be studied before the matter can be settled. Rather than the final word of a book, our finding can be regarded as the first word of a new chapter. This is what makes science so exciting!

This feature article was written as part of the Cosmic Diary Cornerstone project for the International Year of Astronomy 2009. To find out more, check out www.cosmicdiary.org and www.astronomy2009.org.