For an inhabitant of Earth, with the naked eye, our galaxy appears as a faint strip of diffuse light that crosses the night sky. The name given to it in ancient Greek civilization and which we still retain, the Milky Way, refers to its appearance: a “milk-white area” as described by Claudius Ptolemy (c. 90 – 170 CE). It was easy to see, and I say it was, because now it is only possible to appreciate it from the few privileged places that remain with access to a dark sky.
Over time and a few telescopes, we have learned that this strip of light is actually a concentration of stars that is shaped like a thin disk with a radius of about 97,200 light years and a thickness of about 2,000 light years. That is why it is classified as a disk galaxy, it is much larger in one direction than in the perpendicular, very similar to a vinyl record, a factor of 100 between diameter and thickness. The Sun resides in that galactic disk, about 50 light years above the median plane and 27,700 light years from the center.
In the center of the galaxy lives a supermassive black hole called Sagittarius A* and all the stars of the Milky Way rotate, with almost circular orbits, in that disk with a rotation speed that depends on their distance from the center. The rotation speed at the Sun’s position is about 220 kilometers per second. This means that our star takes 240 million years to complete an orbit around the galaxy and that since its birth it has barely had time to orbit 19 times.
Although the stars are the most famous, we do not know the true shape of our galaxy from the stars, but from observations of the gas found between them. It is the simplest element, hydrogen in its neutral state and its line at 21.1 centimeters, which shows us with radio telescopes that the Milky Way has a spiral structure, similar to that observed in numerous external galaxies. Although we are always learning from the details of its shape, especially because it is very difficult to determine the structure of something when you are inside it. Imagine, for example, making a map of your city without having the possibility of leaving your home to layout the streets. Well, that is the job of a person dedicated to this specialty of astronomy.
A galaxy, therefore, is a very complex system, it is not a simple aggregate of stars united by gravity; There exists between them a common reserve of gas and energy that we know as the interstellar medium. And now let’s not fool ourselves, the interstellar medium is not a mere diffuse space that fills the gaps between stars. It could be said that it is one of the most important components that a galaxy has, and although its density is very low when compared to that of the air we breathe, it controls most of its properties. Without him there would be no stars.
But what is the interstellar medium made of? Well, above all, gas in all its phases (ionized, atomic and molecular), dust (tiny solid particles), high-energy particles and magnetic fields and is in continuous exchange of matter and energy with the stars and the gravitational potential of the galaxy. It is highly heterogeneous and very dynamic.
Without going any further, in the vicinity of the bubble we inhabit, in the solar vicinity, there are five phases of interstellar gas that is mainly hydrogen. On the one hand, there are clouds of cold dense molecules (at temperatures between -253 and -263 degrees); These are the places where stars form. Then we have the atomic gas (at -173 degrees) that is almost transparent to the background starlight, except at a series of specific energies that give rise to traces in the form of absorption lines that allow us to detect it. The presence of nearby stars or high-energy particles can strip electrons from atoms and give us two other gas components, the warm ionized medium (at 10,000 degrees) and the hot ionized medium, which can be said to be like a galactic water bath. millions of degrees that surrounds everything and is fueled by the brutal energy of supernova explosions.
The entire interstellar medium is also peppered with magnetic fields, high-energy particles, and the solid particles we call dust. And if this were not already complicated enough, in order to determine the structure of each of its components we need observations that cover different energy ranges: the 21 centimeter line of hydrogen and the cold, molecular clouds are observed in radio ; the hot medium in X-rays or optical and for dust and molecular gas we also need infrared.
Furthermore, a galaxy is not a closed system, it lives in an environment of clusters and superclusters. We have determined that to maintain the rate of star formation since its formation, the Milky Way has had to receive fresh fuel almost continuously. And it seems that it has been able to obtain it in the form of high-speed clouds identified for the first time in the emission of atomic hydrogen from the 21-centimeter line at anomalous (non-galactic) speeds. Some of that gas originates in satellite galaxies, but some is ancient disk material that “rains” like a galactic source, and some may be part of the hot material that, after being fed by supernova explosions, simply condenses and returns. to fall to the plane of the galaxy.
Regarding the interstellar medium, we still have big questions: How much gas flows in and how much is lost, at what speed does it do so, and what forces act on it along the way? As you see, the apparent nothingness is also full. You just need to know how to look and have the instruments to be able to do it.