The Birth of Stars

Ever looked up at the sky, that beautiful black sky filled with the wondrous tiny specks of light we have come to call stars – and previously worshipped as gods – and wondered how they come about? If you haven’t, you should! For they are mighty and powerful, dangerous but fasinating but, most of all, just mesmerising. Thus, I shall today fulfill that wish of yours.

In order to gain any understanding about stars at all, we need to first clear out a myth that many, especially the younger ones – because this is actually what they are taught in school! – and those without a background in Physics believe: that gravity if the pull of the Earth on an object. Wrong! Gravity is a force of attraction between any two objects which have a mass. As such, there is a force of attraction between you and the chair you’re sitting on, the monitor you’re using to look at this post and everything else you can imagine, from the tiniest specks of dust to the Earth itself. In most cases, this is not noticeable but when you have a mass as large as the Earth and yourself, with there being a relatively tiny distance between the two bodies, you can clearly see the force playing its part, which is why we are stuck to the surface of the Earth and not floating and why astronauts are seen jumping around on the moon (it has a smaller mass than the Earth so while they are still pulled back to the surface, the force is nowhere near as strong as the pull of the Earth). Thus, the very first point to understand is that the Earth’s pull is only an example of gravity at work, it is not the very definition of gravity, just as Messi is an example of a footballer; he isn’t the very concept of being a footballer (though, I’m sure some of his fans might disagree…).

I'm flying!
I’m flying!

With that done, we can now proceed to the formation of stars. Millions of years ago, in a nebula far, far away, there existed a cloud of gas and dust, tiny particles in the millions of bazillions, all attracting each other due to the force of gravity. Gas being gas, however, was, as always, rebellious and wanted to move out. And, so we see the ultimate game of tug-of-war, with the gravity pulling the particles in toward each other and the gas pressure pushing outwards. As long as the two are equal, the cloud will stay in an equilibrium (a state called hydrostatic equilibrium) but, often, the combined mass of the cloud is so huge that the gas pressure is no longer able to sustain it and we experience gravitational collapse, whereby the cloud collapse on itself and we see many spherical bobs of gas – baby stars, if you will. As these bobs collapse and the gas partciles are pulled further in toward the core, there is, due to a number of processes which would be too technical to discuss here, an increase in temperature. And, whenever temperature increases, what else does? Gas pressure, of course! Thus, we have a new lease on life, with the greater pressure being able to counteract the gravitational pull a bit better, though still not entirely.

S106, The Late Stages of Star Formation, Credit: NASA & ESA
S106, The Late Stages of Star Formation, Credit: NASA & ESA

This tug of war continues, with gas pressure mostly losing until the temperatures in the core are hot enough for fusion to occur and the temperature of the gas is high enough for the kinetic pressure of the gas to counteract further collapse. And, thus, we see the beginning of the life of a star (and, a return to hydrostatic equilibrium)!



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