In the early 1930s, the brilliant physicist Paul Diracone of the founders of the Quantum mechanicswhich allowed him to theoretically discover the existence of antimatter, had advised the very young Subrahmanyan Chandrasekhar to continue his training by spending time in Denmark at the Niels-Bohr Institute. It was founded in 1921 for the physicist Danish Niels BohrNobel Prize in Physics in 1922 and another brilliant father of the quantum revolution with Plank and Einstein.
Chandrasekhar was only 20 years old when he used the famous statistic discovered by Enrico Fermi and Paul Dirac to model the behavior of the interior of starsalso mobilizing in passing the theory of Relativity. He then discovered that there was a mass limit for white dwarfs. In 1935, the great astrophysicist Arthur Eddington strongly opposed the existence of this limit, without really realizing that it was potentially discrediting the young doctor by astrophysics by its prestige.
At first glance, a star is a ball of gas, mostly hydrogen. Hydrogen nuclear fusion reactions at the center of the star release energy. In other words, heat that radiates in the form of red, yellow or blue light… Thus, whether they are brown or red dwarfs, the mass of a star determines its luminosity, that is to say the power luminous that it radiates in all directions. © CEA Research
The story about it is well known and despite the feeling of injustice experienced by Chandrasekhar, the two men will respect each other thereafter and Chandraas it is also called, will even write a biography of his eldest. It must also be said that Chandrasekhar had asked the opinion of Niels Bohr who, from the height of his stature as a Nobel Prize winner, had replied that he should not worry and that he absolutely did not understand the reasoning that Eddington opposed Chandra’s calculations, which he and his Belgian collaborator, the physicist Léon Rosenfeld, found perfectly correct. Chandrasekhar would finally publish in the late 1930s a treatise that is still authoritative on the structure of the stars.
All this to say that there is a tradition of excellence in astrophysics associated with the Niels-Bohr Institute and that one can only take seriously the recent communiqué of this institute concerning an article published in The Astrophysical Journal and which can also be consulted freely at arXiv.
The stars of 140,000 galaxies over billions of light years
This is a discovery made by analyzing star populations in about 140,000 galaxies whose observations are archived in the database of the Cosmic Evolution Survey (Cosmos), an astronomical survey designed to probe the formation and evolution of galaxies as a function of both cosmic time (redshift) and the local environment of galaxies. The survey covers an equatorial field of 2 square degrees with spectroscopy and imagery ranging from the domain of X-rays to that of the waves radio thanks to most major telescopes spacecraft and a number of large ground-based telescopes. More than 2 million galaxies are detected, covering 75% of the age of theUniverse.
The explosion of very massive stars in gravitational supernovae enriches the interstellar medium with the chemical elements synthesized by nuclear fusion, while giving rise to a neutron star or a black hole by the collapse of the star’s core. The transition between the collapse of the core and the expulsion of the stellar envelope is a challenge for the theoretical understanding of supernovae. A hydraulic experiment designed and carried out at the CEA made it possible to reproduce by analogy one of the phenomena of hydrodynamic instability which facilitates the explosion. This experimental approach is complementary to numerical simulations. Check out this animation experience. This animated film was produced and co-financed by the CEA and the ERC, and directed by Studio Animea. Scientific and technical design: T. Foglizzo, J. Guilet, G. Durand (CEA). © CEA Research
The galaxies studied by the astrophysicists of the University of Copenhagen and the Niels-Bohr Institute are far from the Milky Waysometimes several billions oflight years. For the first time for such distant galaxies, it was possible to draw up statistics concerning the populations of stars in a given range of masses and especially to compare them with those determined in the Milky Way for decades. We know, for example, that our Galaxy is largely dominated by red dwarfs.
Technically, the published article gate on what is called in astronomy, the initial mass function (initial mass function or IMF, in English) which is the relation which describes the distribution of the masses of the stars for a population of newly formed stars and which was introduced in astrophysics by Edwin Salpeter in 1955. To measure it, researchers looked at the amount of light emitted by galaxies at different wavelengths. Large massive stars are bluish, while small low-mass stars are color more yellow or red. This means that by comparing the distribution of blue and red colors in a galaxy, one can determine the distribution of large and small stars.
An effect of the chemical evolution of galaxies?
It now appears that the IMF of distant galaxies is not the same as that of the Milky Way and that they contain more massive stars, the proportion increasing with distance from our Galaxy. This is not necessarily surprising because the theory of the structure and evolution of stars tells us that very massive stars which explode in supernovae SN II after a few million years produce nuclei heavier than thehydrogen and helium left by the big Bang, nuclei that these explosions disperse in the interstellar medium. But it is in the clouds molecules enriched in these heavy nuclei that will form new stars. Over billions of years, galaxies will therefore evolve chemically and we know that the composition of heavy elements in turn influences the mass of stars that can form.
As looking far is looking early in astrophysics, it could therefore be predicted that the IMF of galaxies billions of years ago would probably not be the same as that measured today in our Galaxy. But we did not yet have the proof for lack of sufficient quality observations.
” The mass of stars says a lot to astronomers. If you change the mass, you also change the number of supernovae and black holes who emerging massive stars. As such, our result means that we will have to revise many things that we had assumed, because distant galaxies look very different from ours. “, explains in the press release from the Niels-Bohr Institute Albert Sneppen, doctoral student at the Niels-Bohr Institute and first author of the published study.
” We could only see the tip of an iceberg and had long known that expecting other galaxies to look like ours wasn’t a particularly good guess to make. However, no one had ever been able to prove that the other galaxies form different star populations. This study allowed us to do just that, which could open the door to a deeper understanding of how galaxies form and evolve. », adds Charles Steinhardt, co-author of the study and stationed at Cosmic Dawn Center (Dawn).
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