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.
As Futura explained in previous articles from which we take part of the content, thediscovered in the XVIIIe century, despite their weak , and they did not know then to what extent these were . 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 of the time, a value of the order of the ton per cubic centimeter was indeed derived from the observation of like B.
Quickly however, theBritish understood that the brand new quantum statistical mechanics discovered by his colleague in the late 1920s (which theoretically predicted the existence of at the same time), describing a d’ degenerated in the jargon of physicists, could explain the existence of these stars. This gas could exert a large enough to resist that caused by the gravitation of a star as dense as white dwarfs.
Taking up the work of Fowler, the very young astrophysicist(then 20 years old) had the idea of introducing the effects of the theory of and he asked of these strange objects. He thus came to a conclusion which has become famous, there can be no stars of greater than about 1.4 solar mass become a white dwarf. This is the famous .
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. 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 whose pressure balances with that of the , it is the pressure of the degenerate who opposes the of these stars in .
They can then gather the mass of thein a the size of the Earth. They are therefore very dense stars which cool very slowly.
They have been studied by many great theorists ofaround the middle of the last century, for example Evry , who with the progress of post-war nuclear astrophysics, demonstrated that the bulk of a white dwarf must consist of nuclei of and D’ bathed in a degenerate gas of relativistic electrons. But, it was during the 1960s that various theoretical astrophysicists, including in 1961, realized that the heart of a white dwarf must quickly turn into a huge of carbon and oxygen nuclei.
A large part of the volume of a white dwarf must thus resemble a kind ofgiant, as explained 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 inof .
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 thearound the 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 thefor 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 .
It all started with studies in the field of radiationby means of the instruments fitted to the whose gaze had been turned in the direction of the globular clusters and . The latter is none other than the cluster of Hercules, made famous because the to eventual .
M3 and M13 globular clusters share many physical properties such as age and, that is to say the content in element heavier than the and the . The name metallicity comes from the fact that in astrophysics we qualify as all the “heavier” than these last two elements. Metallicity is an indicator of chemical evolution through stellar from the hydrogen and helium of the .
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 inaccreting matter from a companion star to the point of transforming itself into , even in .
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 withof 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 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.