By Chris Fellows, Serenity Mobile Observatory
Last we met we talked about the smallest bits of the universe: the comets, asteroids, planets and other detritus that make up solar systems. This time we are going to look at the stars themselves. Stars are the dynamos of our universe and provide the temperature gradients and light sources that make the cosmos an interesting place.
So, where do the stars come from and why are they important to us? A star is born when a massive cloud of gas and dust becomes dense enough for gravitational forces to take over and pull matter together. It all starts with a perturbation within the Interstellar Medium (ISM) when a slightly denser region of baryonic matter is large enough to pull more matter into itself. There are several possible triggers for this, such as two clouds colliding with each other, a nearby super nova sending shock waves of hyper velocity particles into the cloud, and even galactic collisions!
The ISM is about 70% hydrogen, about 29% helium, and trace amounts of heavier elements like oxygen and the other building blocks of the periodic table. As this clump gets bigger and heavier, more and more matter is pulled in from the surrounding cloud in a cascade like water going down a drain. Also like water, any angular momentum of the in-falling matter will nudge the protostar into rotational motion and start the entire system spinning.
As this process continues, more and more gas and tiny dust grains fall into the gravitational well and things start to heat up. There are some interesting mathematical problems with this model (that are beyond the scope of this article) having to do with magnetic fields disrupting the process, that you can read more about at the Physics.org Web page. Let us ignore that for the moment and continue building our new star.
As more and more matter falls into the gravity well, the temperature rises, density increases, and a core is formed that starts to radiate energy. This loss of energy allows the core to collapse even further and the temperature continues to rise until the outward pressure reaches a state called hydrostatic equilibrium. At this point the outward pressure equals the inward pull of gravity. Accretion of material onto the protostar continues partially from the newly formed circumstellar disc. When the density and temperature are high enough, deuterium fusion begins, and a protostar is born.
At this point several more very geeky steps take place allowing the protostar to accrete more mass from the surrounding disk and continue to heat up even more until finally, hydrogen begins to fuse in the core of the star and the rest of the enveloping material is cleared away. This ends the protostellar phase and begins the star’s main sequence phase.
Depending on its mass, a main sequence star can burn its hydrogen from a few hundred million years to tens of billions of years or possibly more. Paradoxically, the smaller or less dense a star is the longer it burns. Larger stars burn much hotter or bluer and therefore go through their fuel much faster.
Our star, the Sun, is in its main sequence and has been burning for about 4.5 billion years. It is estimated this is about middle age for the Sun so we have about another 4-5 billion years of main sequence left. This process happens all the time and everywhere in the universe. Our galaxy alone has between 100 to 400 billion stars with new ones being born all the time. Keep that in mind and then realize that there are more than 1 trillion galaxies in the observable universe. That’s crazy talk!
At the end of the main sequence there are several possibilities for a star to pass through. When low mass stars run out of fuel, the pressure from the core tends to puff up the outer layers and you get a red giant. Higher mass stars can burn through the hydrogen and then ignite the helium, burn through that and then move on to burn lithium, carbon, neon, oxygen, silicon and finally reach iron. At this point it is doomed because when iron fuses it releases less energy than was put in and the outward pressure is no longer great enough to hold off gravity. The star collapses in a fantastic event we call a Supernova and explodes its guts into the surrounding space. It’s during this process that the heavier elements, those with more protons and neutrons than iron, are formed. This entire process is called Stellar nucleosynthesis and if it wasn’t for it, we wouldn’t be here because elements heavier than iron wouldn’t exist. We are star dust, we are in the universe and the universe is in us.
Well, I think that is as good a place as any to finish this article up. Take a moment and think about this process the next time you look up at the night sky and ponder your existence. It is truly an awe-inspiring and humbling experience. Next time we will move to the next larger structures in the universe, galaxies, and our best understanding of where they come from and where they are going.
Till next time!
Chris Fellows, Serenity Mobile Observatory
Find Chris on Facebook (or, if you’re lucky, at your campground). (Editor: Check out his amazing photos on his Facebook page!)