The vast majority of stars that we observe in the universe, including those close to Earth, were formed billions of years ago . The Sun, specifically, about 5 billion years ago . And all these stars each formed from a gigantic cloud of gas and dust that ended up contracting due to its own gravity. But why did it take the Sun’s cloud of gas and dust about 8.7 billion years from the beginning of the universe to start contracting? And why have other clouds waited until today , forming stars that are yet to be born? Ultimately, why weren’t all stars born in the first billion years of the universe’s existence, and why are some still being created today ?
First of all, let’s make it clear that the universe is a place in constant renewal. Right now in the Milky Way there are millions of stars running out of fuel and with it their days and millions more being born from just as many clouds of gas and dust scattered throughout the galaxy. If this is not apparent to us by looking at the night sky, it is because the time scales required for these processes to take place are incredibly long compared to a human lifetime . Even the most massive and short-lived stars take several million years to use up all their fuel.
We know of many star-forming regions in our galaxy and beyond. One of the most prominent, due to its size and the number of stars it contains, is the NGC 604 nebula. This nebula lies on the outskirts of the Triangulum galaxy, a spiral galaxy that is the third largest member of our galactic neighborhood, known as the Local Group . NGC 604 is a real monster, about 1,500 light-years in diameter , containing about 200 stars of the spectral type O and WR in its central region alone. These stars reach masses of up to one hundred thousand times that of the Sun. These stars in the center of the nebula, the ones that illuminate that central region, are less than three and a half million years old. We don’t know what caused it, but several million years ago that inconceivably large cloud of gas and dust began to contract , forming thousands of stars in the process.
During this contraction, some packets within the main cloud end up concentrating large amounts of matter and contracting faster than the rest and heating up in the process. When the central region of these packets contracts enough, temperatures of several million degrees are reached and the pressures necessary for the hydrogen nuclei to overcome the electromagnetic repulsion and fuse , giving rise to helium nuclei and releasing large amounts of energy in the process. It is then that we can say that a star is born.
However, if the nebula in NGC 604 began to contract a few million years ago, what prevented the remaining 99.9% of life in the universe from doing so? Inside any gas, even a cold gas like the one that occupies the interstellar medium, the particles that compose it will have a certain temperature and therefore, a certain energy . For sufficiently small or sufficiently scattered clouds, this energy may be enough to oppose the contraction of gravity . If the gas is hot, because it is in the vicinity of a star that emits large amounts of energy, its particles will move too fast and the agglutinations that, over time, will give rise to a star and a complete stellar system will not form . .
If, in addition, the cloud has a certain rotation , this may also hinder the contraction of the material. If a small contraction occurs, the material in the cloud will rotate faster (because it must conserve the total angular momentum of the cloud ), sometimes rotating so fast as to expel the material located in the outermost regions. Also magnetic fields can prevent this whole process . In the same way that the Earth, the giant planets or the Sun have magnetic fields, a cloud of gas and dust will have them. After all, a magnetic field can be created by moving charges, and that abounds in these clouds.
All of these factors can compete with gravity and prevent or at least slow down the contraction of the clouds that will give rise to stars. Many cases have been observed in which a cloud has not started to contract until it has received an external disturbance , such as a supernova explosion or simply the birth of a nearby star . These events create shock waves that compress the gas in its path and accelerate this contraction process. Furthermore, they can enrich the material in those clouds with chemical elements heavier than hydrogen and helium, making the formation of rocky planets like Earth possible .
Reference:
J. Maíz-Apellániz, 2004, NGC 604, the Scaled OB Association (SOBA) Prototype. I. Spatial Distribution of the Different Gas Phases and Attenuation by Dust, The Astronomical Journal. 128 (3), doi:10.1086/422925
E. Chaisson, S. McMillan, 1993, Astronomy Today, Prentice Hall
