The beginnings of universe formation started with the first stellar bodies, which may not have been like those we know today. Observations from the James Webb telescope seem to confirm this hypothesis
What is a star? According to the RAE student dictionary (the first one that came up in my search engine, for whatever reason), it is ‘a celestial body that shines at night, except for the Moon’. That would lead us to say that planets such as Venus or Mars, asteroids such as Vesta and comets such as 3I/ATLAS are stars. As an astrophysicist, I am not convinced, although it is a good definition for understanding the etymology of the word “planet”, which was coined to refer to “wandering stars” because, to the naked eye, they are very similar to stars, but unlike stars, they move across the sky. In the official dictionary of the RAE, the definition adds ‘with its own light’, which could point to the source of energy, which I will discuss later, but which does not convince me either, because everything at a certain temperature emits its own light.
According to Wikipedia, a star is a ‘plasma spheroid sustained by gravity’. This definition is closer to what we astrophysicists use, and even to what most people have in mind. The definition already contains many more details, implicitly. First, plasma is an incandescent gas at a temperature high enough to be ionised. And incandescent means that it ‘emits red or white light’. Secondly, the phrase ‘supported by gravity’ conceals many more concepts. It means that it has reached an equilibrium, which does not have to be eternal —stars also die — and in which gravity is involved.
But there is also something else involved that is not made explicit in that encyclopaedic definition: the very nature of plasma. Plasma, like any gas, has a pressure that tends to expand the volume it occupies towards areas of lower pressure. This physical phenomenon explains the wind in our atmosphere, among many other things. In a star, the expansive pressure of the gas is counteracted by the contraction imposed by gravity. And if a balance is achieved, we have a star.
As you can see, neither the RAE nor Wikipedia mention something that will come to mind if you are interested in astrophysics: nuclear fusion. This is a very important point, because if gravity is counteracted by plasma pressure, it is essential to maintain equilibrium over long periods of time so that the pressure, and therefore the temperature, does not change. But all plasma, everything at a certain temperature, emits light and loses energy in the process. So the plasma would cool down and the equilibrium would be broken. Since stars, including the Sun, emit light constantly — the Sun only changes its brightness by 0.1% — this means that there is a mechanism that heats the gas and gives it the energy it needs to prevent it from cooling down, so that its pressure remains in equilibrium with gravity, assuming that gravity does not change because the mass of the star does not change either; which is not generally true, but that is another story.
The source of energy that keeps the Sun in balance is nuclear fusion in universe .
Of course, we did not always know about this phenomenon. Only 200 years ago, the theory most accepted by leading physicists, such as Hermann von Helmholtz, was that the Sun was contracting; and, curiously, any gas that contracts due to the effect of its gravity loses energy, which can be used to heat it. It is a concept that is not very intuitive, but very important for planets such as Jupiter or Neptune, which emit more energy than they receive from the Sun, something that does not happen with Earth. But the theory did not work very well, because we do not see the Sun changing size. Furthermore, this way of gaining energy meant that the Sun could only be 20 million years old, which is a long way from the age of many rocks on Earth.
A hundred years ago, another famous physicist, Ernest Rutherford, proposed that the plasma in the Sun was heated by the radioactive decay of certain elements inside it. This phenomenon was also dismissed, although it is, again, tremendously important: it explains the structure of the Earth with a hot mantle and core, which has very relevant consequences for us, such as the existence of plate tectonics.
Just under 100 years ago, Albert Einstein and Arthur Eddington, two other giants of physics, concluded that the temperatures and pressures inside the Sun are high enough for hydrogen atoms to fuse to form helium atoms. This is a process in which energy is released, equivalent to the difference in mass between the sum of the masses of hydrogen and helium, which is slightly less than that sum. Just over 50 years ago, nuclear fusion was confirmed as the source of energy that keeps stars in equilibrium, along with gravity, and is responsible for synthesising most, if not all, of the elements around us, especially some as important as oxygen, carbon and nitrogen.
We then return to our original question: what is a star?
We may ask ourselves whether fusion is necessary for a celestial body to be called a star. If we do not consider it necessary, then we can call certain stars neutron stars, or consider white dwarfs to be stars, which are ‘celestial bodies that shine at night’ and also ‘plasma spheroids sustained by gravity’. In both cases, gravity is not counteracted by gas pressure maintained by nuclear fusion, but by other phenomena—very curious, I would say, and quantum!—less classical than gas pressure, but which I do not want to dwell on today.
If we consider that a star does not necessarily have to undergo fusion to be called such, there is another object whose existence was hypothesised two decades ago, and which we may have just discovered with the James Webb space telescope. These objects would be similar to our Sun, in the sense that they would be a large ball of ionised gas forming an envelope around a radiant core where energy is created. But instead of fusion, these objects would have a black hole at their core. The black hole, thanks to its extreme gravity, could heat the nearest gas to millions of degrees, enough for there to also be an outer atmosphere with temperatures similar to what we see in stars like our Sun, around 5,500 degrees. This is what is known as a black hole star — BH* for short — or a quasi-star. The latter name has nothing to do with a quasar, which comes from another quasi-stellar object: astrophysicists often get terribly confused with names. The James Webb Space Telescope may have discovered this type of object among the so-called little red dots, some of which have all the characteristics expected of BH*. But we are checking the alternatives and trying to learn how these objects are formed, because the process should be similar to that of supernovae, but without an explosion! And perhaps these BH* are essential for the beginning of galaxy formation in the early universe.
Cosmic Void is a section that presents our knowledge of the universe in a qualitative and quantitative way. It aims to explain the importance of understanding the cosmos not only from a scientific point of view but also from a philosophical, social and economic perspective. The name ‘cosmic void’ refers to the fact that the universe is, for the most part, empty, with less than 1 atom per cubic metre, even though, paradoxically, there are quintillions of atoms per cubic metre in our environment, which invites reflection on our existence and the presence of life in the universe.
