Stars in distant galaxies are more massive than those in the Milky Way

A team of astrophysicists from the University of Copenhagen has achieved an important result by studying stellar populations in a large sample of galaxies far beyond the Milky Way. The discovery could change our understanding of a wide range of astronomical phenomena, such as those related to the formation of black holes, supernovae and the death of galaxies.

In the early 1930s, the brilliant physicist Paul DiracPaul Diracone of the founders of Quantum mechanicsQuantum mechanicswhich allowed him to theoretically discover the existence of antimatter, had advised the very young Subrahmanyan ChandrasekharSubrahmanyan Chandrasekhar to continue his education by spending time in Denmark at the Niels-Bohr Institute. It was founded in 1921 for physicistphysicist danish Niels BohrNiels BohrThe Nobel Prize in Physics in 1922 and another brilliant father of the quantum revolution with PlankPlank and EinsteinEinstein.

Chandrasekhar was only 20 years old when he used the famous statistic discovered by Enrico FermiEnrico Fermi and Paul Dirac to model the behavior of the interior of starsstarsalso mobilize in passing the theory of RelativityRelativity. He then discovered that there was one lotlot limit for white dwarfswhite dwarfs. In 1935, the great astrophysicist Arthur Eddington strongly opposed the existence of this limit, not really realizing that it potentially discredited the young doctor by astrophysicsastrophysics by its prestige.


At first glance, a star is a ball of gas, mostly hydrogen. Hydrogen nuclear fusion reactions in the center of the star release energy. That is, heat that radiates in the form of red, yellow or blue light… Regardless of whether they are brown or red dwarfs, the mass of a star determines its brightness, that is, the brightness that it radiates in all directions. © CEA Research

The story of this is well known and despite the sense of injustice experienced by Chandrasekhar, the two men will respect each other thereafter and ChandraChandra, as he is also called, will even write a biography of his elder. It should also be said that Chandrasekhar had asked the opinion of Niels Bohr, who, from the height of his stature as a Nobel laureate, had replied that he should not worry and that he absolutely did not understand the rationale for Eddington’s opposition to Chandra’s calculations, which he and his Belgian collaborator, the physicist Léon Rosenfeld, found absolutely correct. Chandrasekhar will finally, in the late 1930s, publish a treatise which 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 press release from this institute about an article published in The Astrophysical Journal and which can also be freely consulted on arXiv.

The stars of 140,000 galaxies across billions of light years

This is a discovery made by analyzing stellar populations for about 140,000 galaxiesgalaxies whose observations are archived in the database Cosmic Evolution Survey (CosmosCosmos), an astronomical survey designed to investigate 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 images ranging from the domain of X-raysX-rays to the waves radioradio thanks to most major telescopestelescopes spacecraft and a number of large ground-based telescopes. More than 2 million galaxies have been discovered, covering 75% of the age ofUniverseUniverse.


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 black hole upon 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 CEA made it possible to analogically reproduce 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 CEA and ERC and directed by Studio Animea. Scientific and technical design: T. Foglizzo, J. Guilet, G. Durand (CEA). © CEA Research

The galaxies studied by astrophysicistsastrophysicists of the University of Copenhagen and the Niels-Bohr Institute is far from it The Milky WayThe Milky Waysometimes several billion oflight yearslight years. For the first time for such distant galaxies, it has been possible to compile statistics on the populations of stars in a given mass range and, above all, to compare them with those determined in the Milky Way for decades. We know, for example, that our galaxy is largely dominated by red dwarfsred dwarfs.

Technically the published article gategate on what in astronomy is called the initial mass function (initial mass function or IMF), which is the ratio that describes the distribution of stellar masses for a population of newly formed stars and was introduced to astrophysics by Edwin Salpeter in 1955. To measure it, scientists looked at the amount of lightlight emitted by galaxies at different wavelengthswavelengths. Large massive stars are bluish, while small low-mass stars are colorcolor more yellow or red. This means that by comparing the distribution of blue and red colors in a galaxy, you 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 that explode in supernovaesupernovae SN II after a few million years produces nuclei heavier thanhydrogenhydrogen and helium left by big bangbig bang, nuclei that these explosions scatter in the interstellar medium. But it is in cloudsclouds molecules enriched in these heavy cores 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 affects the mass of stars that can form.

Since looking far is early in astrophysics, it can therefore be predicted that the IMF of galaxies billions of years ago would probably not be the same as that currently measured in our galaxy. But we did not yet have the proof of a lack of sufficient quality observations.

The mass of stars speaks volumes astronomersastronomers. If you change the mass, you also change the number of supernovae and black holesblack holes WHO show upshow up 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 Albert Sneppen, PhD student at the Niels-Bohr Institute and first author of the published study, in the press release from the Niels-Bohr Institute.

We could only see the tip of an iceberg and had long known that expecting other galaxies to look like ours was not a very good guess. However, no one had ever been able to prove that the other galaxies form different stellar 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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