“Man alone is born crying, lives complaining, and dies disappointed.” -Samuel Johnson
But the stars, as opposed to humans, are born shining, with hundreds (or more) of brothers and sisters, shine ever more brightly over their lifetimes, and die in spectacular fashion. As far as we can tell, here’s the past, present and future story of all the Sun-like stars in our galaxy.
At some point in the far distant past, every star in our galaxy was once no more than a molecular cloud of gas, with gravity attempting to contract the cloud down into a point.
But gravity typically can’t do this on its own; the cloud needs to be cool enough and the internal pressure needs to be low enough. Simply waiting around for long enough will do it, if you wait for the internal temperature and pressure to drop. But — unless you’re some of the very first ones to form — it helps to get a nudge.
Once that collapse starts, you’ll inevitably get one region that begins to accumulate more mass than the other regions around it. Because of the way that gravitation works, that region will start to attract more and more mass, and you’ll eventually get runaway gravitational growth. But the cloud is still going to be dark, and it won’t yet be visible to your eyes.
But you can see something, if you look just right.
Inside these dark molecular clouds where gas and dust is collapsing, the pressures and temperatures are rising faster than they can radiate that heat away. While the outer layers of gas and dust continue to block the visible light, the infrared light coming from the stars formed inside can pass straight through. Thanks to infrared space telescopes (like Spitzer), we can see the newly formed star clusters that are still in the earliest stages of infancy, as shown in yellow (with the red halo) above.
The thing is, these molecular clouds are far bigger than just a single solar mass.
Instead, these clouds range from many thousands to hundreds of thousands of solar masses. Maybe 10% of each cloud will contract to form stars before the radiation from the newly formed stars blasts the remaining cloud apart into the interstellar medium, where they will someday find new molecules and begin gravitational contraction. But the high mass of those clouds means that there are literally many hundreds to up to hundreds of thousands of stars in a new star cluster.
Our Sun formed in a star cluster much like this — the Pleiades — some 4.5 billion years ago. The brightest, bluest stars are the most massive and will die too quickly to be anything like our Sun. The Sun-like stars are longer-lived, and will by-and-large outlive even the cluster they were born into.
Over time, gravitational passes between this star cluster and other objects in the galaxy, as well as close passes between the individual stars, will cause the cluster itself to dissociate over time, with individual stars being flung across space. (Break out your red-green glasses to see what’s happening to the Hyades, our closest star cluster.)
Most star clusters dissociate within the first few hundred million years, while Sun-like stars typically live much longer, with lifetimes in the billions or even trillions of years, depending on what their mass is.
Over most of their life, Sun-like stars burn at a relatively even rate, turning hydrogen into helium at a very even pace. The only variation is that, as a star burns through its fuel, the interior core region in which fusion can occur gets slightly larger, meaning that over its entire lifetime, it eventually and very gradually gets hotter and more luminous.
Eventually, it will have burned through so much fuel in the core — and it does so faster than new fuel can fall in from the outermost layers — that the core will run out of hydrogen, so that fusion only occurs in a shell around the core. This causes the star to become significantly more luminous, resulting in our star (and other Sun-like stars) becoming a subgiant star.
Procyon (above), the 7th brightest star in the sky, is a subgiant star, a phase of stellar evolution that lasts a few hundred million years, on the star’s way to becoming a true red giant, when it begins fusing heavier elements (like helium, carbon, oxygen, etc.) in its core!
At this point, the star becomes many, many times its original size, so large that the Sun will likely engulf Mercury, Venus, and possibly even Earth when this happens.
Eventually, all the material that can be fused in the star’s core will be used up, while the outer layers of hydrogen and helium will be blown off. This happens slowly and in pulses at first, creating a protoplanetary (or preplanetary) nebula,
followed by a full-blown planetary nebula, where maybe 50% of the star’s original mass (and 97% of that will be pristine, unburned hydrogen) is returned to the interstellar medium,
and a white dwarf star, a degenerate core of carbon, oxygen, and in some stars, sulphur, silicon and even iron, will be left behind. While it may be 50% of the mass of the original star, it’s thousands of times dimmer and over a hundred times smaller in diameter.
It will take many trillions of years for this white dwarf to eventually radiate its heat away and cool to become black, and that’s the eventual fate of all Sun-like stars.
But given that 95% of all the mass that it took to form these stars was eventually returned to the interstellar medium as burnable fuel, we’ll still have stars lighting our night sky for trillions upon trillions of years, and the atoms from our Solar System will be a part of countless future generations of them.