Observations from the Hubble Space Telescope of two globular clusters orbiting the Milky Way suggest that white dwarfs may continue to burn hydrogen in the later stages of their lives, making them appear younger than they are. are actually.This discovery could have consequences for the way astronomers measure the age of star clusters and therefore establish chronologies with galaxies.
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[EN VIDÉO] How do the stars evolve? The stars are born, live and die. Their history is determined by their initial mass, which decides the thermonuclear reactions that will take place there and the types of nuclei they will synthesize before ending their lives as white dwarfs, neutron stars or black holes.
As Futura explained in previous articles from which we take part of the content, the astronomers discovered white dwarfs in the XVIIIe century, despite their weak brightness, and they did not know then to what extent these stars were exotic. They began to realize this at the very beginning of the XXe century with the determination of the extraordinary density of white dwarfs. To the amazement of astrophysicists of the time, a value of the order of the ton per cubic centimeter was indeed derived from the observation ofstars like Sirius B.
Quickly however, the physicist British Ralph Fowler understood that the brand new quantum statistical mechanics discovered by his colleague Paul Dirac in the late 1920s (which theoretically predicted the existence ofantimatter at the same time), describing a gas ofelectrons degenerated in the jargon of physicists, could explain the existence of these stars. This gas could exert a pression large enough to resist that caused by gravitation of a star as dense as white dwarfs.
Taking up the work of Fowler, the very young astrophysicist Subrahmanyan Chandrasekhar (then 20 years old) had the idea of introducing the effects of the theory of Relativity and he asked the foundations of the stellar structure of these strange objects. He thus came to a conclusion which has become famous, there can be no stars of masses greater than about 1.4 solar mass become a white dwarf. This is the famous limit of Chandrasekhar.
Extract from the documentary From the Big Bang to the Living (ECP Productions, 2010), Jean-Pierre Luminet talks about the evolution of solar-type stars, their transformation into red giants and then into white dwarfs. © Jean-Pierre Luminet
White dwarfs, the mass of the Sun in the volume of the Earth
Observational progress simultaneously made it possible to discover several types of white dwarfs, differing in the chemical composition of their atmospheres. White dwarfs were ultimately determined to be the final stage in star evolution, containing at most about 8 solar masses when they ran out of fuel. The thermonuclear reactions no longer exist and no longer release a flow of photons whose pressure balances with that of the gravity, it is the pressure of the matter degenerate who opposes thecollapse of these stars in black holes.
They can then gather the mass of the Soleil in a volume the size of the Earth. They are therefore very dense stars which cool very slowly.
They have been studied by many great theorists ofastrophysics around the middle of the last century, for example Evry Schatzman, who with the progress of post-war nuclear astrophysics, demonstrated that the bulk of a white dwarf must consist of nuclei of carbone and oxygen bathed in a degenerate gas of relativistic electrons. But, it was during the 1960s that various theoretical astrophysicists, including Edwin Salpeter in 1961, realized that the heart of a white dwarf must quickly turn into a huge crystal lattice of carbon and oxygen nuclei.
A large part of the volume of a white dwarf must thus resemble a kind of diamond giant, as explained Jean-Pierre Luminet in the video above, although the crystal structure obtained with carbon is not exactly that of diamond on Earth.
This theory was clarified by Hugh M. Van Horn in 1967, which led him to the conclusion that the crystallization process had to occur on the basis of a theory already advanced in 1934 by the great physicist Eugene Wigner. At the end of its life, a star like the Sun should therefore transform into a white dwarf which, on cooling, will become a Wigner crystal, an object that we also encounter in physique you solid.
Hydrogen envelopes that still burn
White dwarfs are therefore considered inert stellar corpses cooling slowly. However, there is a whole theory relating the mass, the radius, the temperature and the composition of a white dwarf. It predicts a law of cooling and therefore of the evolution of its temperature and its luminosity over time. By studying the composition of the atmosphere of a white dwarf and knowing some of the previous parameters, we can therefore calculate the age of a white dwarf. It is an important tool for making galactic archeology and in particular dating the globular clusters around the galaxies which contain a lot of old stars and which must have formed at or near the same time for each of these clusters.
Astrophysicists therefore study globular clusters and in particular those of the Milky Way for a long time by means in particular of their contents in white dwarfs. A team of researchers has just announced an interesting discovery on this subject. via an article published in Nature Astronomy and which can be read in free access on arXiv.
It all started with studies in the field of radiation ultraviolet by means of the instruments fitted to the télescope spatial Hubble whose gaze had been turned in the direction of the globular clusters M3 and M13. The latter is none other than the cluster of Hercules, made famous because the famous message from Arecibo to eventual extraterrestrial civilizations.
M3 and M13 globular clusters share many physical properties such as age and metallicity, that is to say the content of elements heavier than hydrogen andhelium. The name metallicity comes from the fact that in astrophysics we qualify as metals all the chemical elements “heavier” than these last two elements. Metallicity is an indicator of chemical evolution through nucleosynthesis stellar from the hydrogen and helium of the Big Bang.
But astrophysicists have surprises when they compare more than 700 white dwarfs in the two clusters.
Could dying stars hold the secret to looking younger? To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © ESA / Hubble, ESA, Nasa, N. Bartmann, G. Piotto et al., Nasa’s Goddard Space Flight Center Conceptual Image Lab, Nasa’s Goddard Space Flight Center / Chris Smith (USRA / GESTAR)
They found that M3 contains standard white dwarfs which cool in accordance with our previous conceptions of stellar structure and evolution.
The Hercules cluster, on the other hand, contains two populations of white dwarfs: the standard white dwarfs and those that have demonstrably managed to retain an outer shell of hydrogen surrounding a core of degenerate matter. This envelope was obviously the seat of stable thermonuclear reactions and not catastrophic and explosive, as is the case for white dwarfs in binary systems accreting matter from a companion star to the point of transforming itself into novae, even in supernovae SN Ia.
These white dwarfs therefore appear to be warmer and especially younger than one might naively believe with a classic stellar model for a white dwarf. By comparing their observations with computer simulations of the stellar evolution of M13, astrophysicists have deduced that about 70% of the white dwarfs of M13 burn hydrogen on their surface, thus slowing their speed cooling.
This discovery could have consequences for the way astronomers measure the ages of stars in the Milky Way, and in fine globular clusters by introducing a bias and a dating error that could go up to 1 billion years old. What to review chronologies in the field of galactic archeology, as we said previously.
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