Monday, October 25

Forbidden lines | Cosmic Void


Hourglass nebula in an image taken by a Hubble Space Telescope camera.
Hourglass nebula in an image taken by a Hubble Space Telescope camera.Raghvendra Sahai / John Trauger (JPL) / Equipo WFPC2 / NASA / ESA / Raghvendra Sahai and John Trauge

Forbidden lines, it could be a good title for a crime novel. A mysterious paragraph that causes death to whoever reads it. An investigation with some disheveled and perhaps intelligent characters, who try to clarify these strange events. If we adjust the plot a little and accept that Sherlocks are astronomers who also die naturally, we have our story of intrigue type “The name of the rose” in the discovery almost 100 years ago of the mechanism that explains the emission of light of many celestial objects. One last detail, the only corpses are stars and the forbidden lines are written in the astrophysical equivalent of a book: a spectrum.

It was the year 1868 when the English astronomer Sir William Huggins He named what astronomers of the time had on their minds: bright green emission lines that were observed when the light from certain nebulae decomposed in its spectrum. As these lines of light were not observed in the spectra that were taken in the gases in the laboratories here on Earth, the English Sir proposed that it was a new chemical element. He did not eat his head much and called him nebulium. Let us remember that the only way to determine the nature of the substances responsible for the light that we see in the sky, in the absence of theory, is to compare their lines with the spectra produced in the laboratory. It was not until 1927 that it was discovered by Ira S. Bowen that he nebulium it does not exist and that they are simply common elements such as electrically charged oxygen and nitrogen, that is, ionized, which are responsible for producing this light in certain astronomical objects.

It was not until 1927 that it was discovered by Ira S. Bowen that the ‘nebulium’ does not exist and that they are simply common elements such as electrically charged oxygen and nitrogen, that is, ionized, which are responsible for producing this light in certain astronomical objects.

These lines, despite the fact that they are observed in many objects with very different energy sources such as supernovae, planetary nebulae, environments of black holes in active nuclei of galaxies or northern lights, they are lines that we call prohibited. The term is misleading and it would be much more accurate to call them “very unlikely emission lines under terrestrial conditions” (no matter where we look, anthropocentrism always escapes us in one way or another), although we also have to recognize that this is one of Rarely have astronomers given something a catchy name.

The fascinating thing is to descend into the world of the tiny, the quantum world, to understand how that light is produced, so useful to understand the cosmos and that allows us to measure, for example, how much oxygen of it we breathe has a galaxy that is thousands of people away. millions of light years from Earth. Now we know that for this type of light to be emitted we basically need two things: a light source with a lot of energy and a gas with an extremely low density. Suppose we have these two ingredients, let’s turn into something much smaller than ant-man for a moment to see what is happening at the elementary particle level.

All atoms have a compact nucleus with a positive charge where the protons are and surrounding this small structure we would have the electrons in their clouds. One of the four fundamental interactions in nature, the electromagnetic force, holds the two together as long as there is no photon of light through that can give an electron so much energy that it literally rips it out of its atom. We already know then why we needed the energy source: to pull electrons from the control of the protons. This process called ionization occurs in the Earth’s atmosphere on a large scale in the ionosphere.

The photons behave like an elevator, energizing the electrons and taking them up to the upper floors of the atom building or throwing them directly outside as in Willy Wonka’s glass elevator.

Now imagine that these ionized atoms are like skyscrapers inside, with a series of floors where the rest of the electrons live. Well, contrary to common sense, these electrons do not like to have views and live on the high floors, but prefer to be near the nucleus, in the basement. The photons behave like an elevator, energizing the electrons and taking them up to the top floors of the atom’s building or throwing them directly outside as in Willy Wonka’s glass elevator. Most of these towers are those of the most common atom that hydrogen exists and in them the only electron there is rises in a photon (absorbs it) and is placed on a high floor. The fact is that in less than a hundred millionth of a second he decides that he does not want to be there, he pulls himself off the elevator (re-emits the photon) and they return to occupying the ground floors. The key is that there are certain levels or atomic levels in some ions, such as oxygen, sulfur and nitrogen, which can be reached without an elevator (without a photon). In this case, it is other free electrons (the uprooted ones that were loose around there) that give up energy and transport the electrons trapped in the atoms to the upper floors. The levels reached in this way are called forbidden. At these atomic levels, electrons like to stay for up to minutes (millions of times longer than they stay at permitted levels). The sad thing perhaps is that they stay there because they cannot go anywhere else.

Describing what I just had with elevators is quantum mechanics and requires a lot of equations (and years of study) to formalize it. In that technical language we would say that the emission is produced by collisional excitation lines when downward transitions occur from metastable energy levels in ionized atoms with low transition probabilities. But back to Earth, in our environment, the density of particles is always so high that collisions between atoms carry away the energy before a downward transition can occur. That is why this emission could not be reproduced in the laboratory.

In the conditions of low density that the gas has in which we know as HII regions, planetary nebulae and supernova remnants, it is ions from atoms like oxygen, sulfur and nitrogen that emit these unusual lines. Forbidden beauty that not only reveals the structure of objects among which are the most fascinating in the sky, but they are so bright that they allow us to observe them at great distances and reconstruct, for example, the chemical history of galaxies very far away.

Eva Villaver She is a researcher at the Astrobiology Center, dependent on the Higher Council for Scientific Research and the National Institute for 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, Professor at the Complutense University of Madrid (UCM); Y Eva Villaver, researcher at the Center for Astrobiology.

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