Astronomers have made a decisive step to understanding of the nature of supernova stars of Ia type* in the beginning of this year. Works of the leading research assistant of the RAS Institute of Space Studies, Marat Gilfanov, Dr. Sc. (Phys. & Math.), and his postgraduate student Akosh Bogdan from the Institute of Astrophysics of Max Planck Society (Germany), have shown: the most probable reason of their explosions is a merging of two white dwarfs**. This conclusion is extremely important for cosmology as the rate of the Universe expansion is determined according to such space objects. It was based on the data received with the help of Chandra X-ray orbital observatory of the US National Space Agency, published by Gilfanov and Bogdan in the Nature magazine in February 18, 2010.

Astronomers refer to flashes of supernova Ia type stars as "standard candles"—on the basis of practically identical luminosity. Besides, they are extremely bright, therefore they can be found at great distances, comparable with the size of a part of the Universe under observation. Measuring red displacements of heavenly bodies and distances to them, it is possible to determine its expansion rate at different moments of time in the past. For example, on the basis of such data, in 1998, specialists assumed that there existed a certain mysterious "dark energy"***.

However, up till now the mechanism of flashes has remained unclear. It was practically obvious that supernova Ia type stars are a result of a thermonuclear explosion of the white dwarf. The most probable reason-exceeding a critical limit of the mass equal to ~ 1.4 of the Sun's mass—was also mentioned. For more than 20 years the scientists have been arguing about the way the accumulation of the substance leading to explosion is taking place.

Two scenarios were considered. According to the first one, the white dwarf while interacting with a usual star gradually "draws" to itself the substance of the partner (this process is called accretion). According to the second one, the explosion mechanism is started up by the merging of two "dead" stars.

The idea stated and checked up by Gilfanov and Bogdan consists in the fact that these two approaches can be distinguished by X-ray luminosity. The accreting white dwarf is producing powerful X-ray radiation during almost 10 mln years before explosion. And two stars moving relative to each other, deprived of their own sources of thermonuclear energy, behave "silently": they almost do not radiate in any of the ranges of electromagnetic spectrum up to the last moment before merging. In a word,

* The heavenly bodies with an explosion mechanism based on the process of thermonuclear synthesis are related to supernova stars of la type.-Ed.

** White dwarf-a part of the "dead" star, in which nuclear reactions have stopped; its size is approximately 50-100 times less than the Sun, while its density is a million times more.-Ed.

***See: "Dark Matter Puzzles". Science in Russia, No. 1, 2010,-Ed.

The composite picture of M31 Galaxy (Nebula of Andromeda). Yellow color shows a picture in the X-ray range (data of the CHANDRA observatory), blue color-in optical range (DSS survey), red color-in infrared range (data of the SPITZER space telescope). In the inset-the picture received at CHANDRA. It covers only the central part of the Galaxy.

each of the variants predicts a different luminosity of white dwarfs—predecessors of the supernova ones. Due to the studies of X-ray radiation of near galaxies and to the comparison with predictions of the theory, we can experimentally determine the mechanism, which actually works.

Scientists have checked up their hypothesis using the example of five elliptic galaxies and the spiral Nebula of Andromeda (M31). For this purpose they used Chandra observatory's data and also data of the space infrared telescope Spitzer (USA) as well as of the sky survey in an infrared range. It has become clear that the X-ray luminosity of elliptic galaxies are 30-50 times less than was predicted by the accretion scenario. That is why the share of supernova stars, exploding up after achievement of the critical mass limit by a white dwarf as a result of "drawing" of the partner's substance, does not exceed several percent, and merging of two "dead" stars becomes the most probable source of their flashes.

Earlier the majority of astronomers was in favor of the accretion scenario more often as there are only a few systems consisting of two white dwarfs. However, the reason can be not so much their small quantity, but the complexity of observation: after all, we should underline once again that they almost do not radiate electromagnetic waves. So, the result of the carried out research turned out to be unexpected and generated a number of new questions.

Conclusions of Gilfanov and Bogdan concern, first of all, elliptic galaxies, i.e. systems with the old star population. In younger, spiral ones, where up to now there is still going on an intensive star formation, the situation can be different, which requires a further search.

The "standard type" of flashes is natural during escalation of events: white dwarfs explode at one and the same mass equal to critical. The scenario of merging compact stars foretells a scattered nature of masses and, hence, parameters of flashes, observed by the astronomers. The picture will become even more complicated, if in galaxies of different types, contributions of two scenarios are different. It should be considered on carrying out high-precision cosmological measurements using the supernova stars of Ia type. They will be one of the main tasks of astronomical projects of the coming decade.

According to the report of the press-service of the RAS Institute of Space Studies

Prepared by Marina KHALIZEVA