Tuesday, August 3

The age of stars, degenerate dwarfs and diamonds | Cosmic Void

Image of stars (NGC 6397) taken by the 'Hubble' space telescope.
Image of stars (NGC 6397) taken by the ‘Hubble’ space telescope.T. BROWN S. CASERTANO / AFP

Someday the universe will stop building stars. The raw material from which they are made is depleted as new stars are created. When the star ages, only part of its material is recycled. In the future, galaxies will use up the gas necessary to form young stars. That’s how the accounts come out, not even the universe has unlimited resources.

We also know that it is impossible for stars to exist before the time when the first hydrogen atoms were formed in the cosmos. We estimate that the first stars formed between 200 and 400 million years after the Big Bang, but determining when the universe saw the first light from a star is one of the goals of the space telescope. James Webb (NASA, ESA and the Canadian space agency), which is expected to launch in late 2021.

The total number of stars that exist today in the universe is expected to increase only 5% from the current value. Taking into account that light was made after the time of recombination (when protons and electrons come together to form hydrogen) and that we know that there is a later in which no more will be born, the conclusion is simple: we live in age of the stars. And what a wonderful age we have been! They provide energy to most living things here on Earth and perhaps in many other places in the cosmos, and they make galaxies shine, because a galaxy only lasts as long as its stars do.

In the age of stars, clouds of cold gas collapse to form stars, planets, and brown dwarfs. The stars process gas by nuclear fusion, enriching it chemically and throughout their lives they recycle part of this material, returning it to the interstellar medium, sometimes in the form of gentle winds and sometimes explosively.

When stars stop forming and conventional stellar evolution has ceased, we will only be left with the remains, the embers or stellar ash: white dwarfs, neutron stars and black holes.

But let’s focus on what the stars don’t return. When stars stop forming and conventional stellar evolution has ceased, we will only be left with the remains, the stellar embers or ash: white dwarfs, neutron stars and black holes. Most of the stars that exist are low-mass stars and these end their days as white dwarfs. This will also be the final destination of the Sun. Therefore most of the mass of the universe will remain in the form of isolated white dwarfs that will be the absolute protagonists of the future. They, the conventional isolated stars, the ones that barely attract attention now, the ones that are not violently extinguished by explosions, nor do they monopolize the imagination of science fiction movies like black holes. They, the white dwarfs, the “degenerates”, will have their moment of glory in the future.

But what exactly is a white dwarf? It is a structure that despite having exhausted the nuclear fuel manages to overcome gravitational contraction thanks to quantum physics. She is a champion. It wins the battle against gravity by exerting pressure to stop the collapse using its electrons and the quantum mechanical property that causes that when they are subjected to high densities they do not tolerate being in the same space with the same clothes as another (or in other words, that two electrons cannot have all four equal quantum numbers), that is what is known as electron degeneracy.

There are many white dwarfs in the vicinity of the Sun, but they are very weak because they are small and they get cold.

There are many white dwarfs in the vicinity of the Sun, but they are very weak because they are small and they get cold. One of the first to be discovered is the companion of Sirius (the brightest star in the northern sky is actually a double system). They are one of the densest structures that exist: in the size of a planet like Mars (now that it is in fashion) the mass of a star like the Sun is concentrated. And now it would come to say that in a white dwarf coffee spoon would fit two elephants, but where will the elephants be by then. A white dwarf is a most fascinating stellar material, made up of a solid of carbon and oxygen ions floating in a sea of ​​degenerate electrons. As their cores cool, they freeze (and not at 0 degrees Celsius like water on Earth), crystallizing and releasing latent heat just as it has been. confirmed with GAIA Recently. A diamond is crystallized carbon, so the structure of a white dwarf resembles a spherical diamond the size of our planet floating in space.

This is the fate of the heaviest ones, those that now have up to eight times the mass of the Sun. However, most of the white dwarfs of the future are currently M stars, red dwarfs, those in which some are centered. from searches for habitable planets like CARMENES. Red dwarfs are so small that they have barely had time to undergo any kind of evolution, they remain like Peter Pan frozen in childhood even though the rest of the stars have already passed through several generations. Their times are different, they go beyond the current age of the universe and will end up as white dwarfs, but this time the structure that will cool will be made of helium instead of carbon-oxygen.

A large amount of what is known as baryonic matter (that which we and the stars are made of) will end up as degenerate remnants

A large amount of what is known as baryonic matter (that which we and the stars are made of) will end up as degenerate remnants. If we allow enough time to pass, the white dwarfs will turn black, the galaxies will evaporate expelling most of the stars, some will collide with black holes and those that are in binary systems, if they are close enough, will merge due to the emission of gravitational waves. Even black holes will evaporate due to the emission of Hawking radiation. What happens next remains in the fog, depends among other things on the stability of the proton. If the proton is unstable, we will have black dwarfs emitting a light of about 400 watts for a time, little more than a streetlight and well under the 110 megawatts per day of the Christmas lights of Vigo. If the proton is stable, we can expect the most massive black dwarfs to explode as isolated supernovae in a dark and cold universe. And these yes, perhaps, are the last lights of the universe.

Eva Villaver She is a researcher at the Astrobiology Center, dependent on the Higher Council for Scientific Research and the National Institute of Aerospace Technology (CAB / CSIC-INTA)

Cosmic Void It is a section in which our knowledge about the universe is presented in a qualitative and quantitative way. It is intended to explain the importance of understanding the cosmos not only from a scientific point of view but also from a philosophical, social and economic point of view. The name “cosmic vacuum” refers to the fact that the universe is and is, for the most part, empty, with less than 1 atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillion atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe. The section is made up of Pablo G. Pérez González, researcher at the Center for Astrobiology; Patricia Sánchez Blázquez, Associate Professor at the Complutense University of Madrid (UCM); Y Eva Villaver, researcher at the Center for Astrobiology

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