Gravity Wins

Though outside the purview of my academic background or professional interests, astronomy and astrophysics have always fascinated me.  I find the life cycles of stars to be a particularly entertaining subject (primal id desire for explosions through the veneer of a cultured, intellectual ego).

Gravity in everyday life is an understated necessity - with the exception of the folks who remain convinced that the spoiler on their pick-up truck actually produces down-force.  Since we are sitting on a nice stable sphere of rock and metal, gravity is boring.  For incandescent balls of gas and plasma, gravity makes things interesting.  

To predict the life of a star, all we need to know is the mass of the star and the fact that gravity exists and nuclear fusion is possible.  Given a large amount of hydrogen in space, gravity will begin squishing all the hydrogen together until the hydrogen atoms begin fusing with each other.  When the outward pressure from the fusion of hydrogen equals the force of gravity, we have a star.  

At this point, science documentaries often dumb down this phenomenon to a near-fallacious point.  They may say that a battle is fought between the star and gravity.  The star desperately attempts to fuse matter in order to counter gravity's onslaught.  Though energy generated by fusion in a star's core does prevent gravity from squishing it into oblivion, if one took gravity out of the equation, then no fusion would be possible.  Also, stars are not living things and do not get existential.

For featherweight red dwarf stars, this equilibrium of gravity and pressure could go on for trillions of years.  These stars eventually just fizzle-out in anticlimactic fashion.

Big stars, on the other hand, live more interesting albeit brief lives.  Since there is a lot of matter composing big stars, gravity is much more forceful.  As a result, the star fuses hydrogen very fast and runs out within a couple billion years.  Gravity compacts the star's core until fusion restarts - this time helium atoms begin fusing.  At this point, different stellar masses will produce varying results:

-  Stars like our Sun will expand (Goodbye Earth) and start sloughing-off their outer layers until nothing but the core is left over.  A final equilibrium is established because gravity cannot compact the star any further.  This time, the electrons in the remnant prevent further collapse.  Basically, as long as a star has less than 1.38 solar masses (mass of our Sun = 1 solar mass) worth of matter, the pressure generated between the electrons in very dense places maintains equilibrium with gravity.

-  Stars that exceed the Chandrasekhar limit (1.38 solar masses as cited above) will form neutron stars once the helium supply is exhausted.  The rebound force of the sudden implosion of the core causes a supernova (kablooie!).  Picture dropping a basketball with a tennis ball on top: after impact, the tennis ball goes flying much further than it would just by itself.  

Once again, a new equilibrium is established.  The neutrons (all that is left at this point) prevent further collapse because the Pauli exclusion principle tells us that a neutron can't occupy the same space as another neutron.

-  For the true behemoths (ten times more massive than the Sun) of the cosmos, gravity wins in the end.  When the helium runs out, these stars are able to start fusing carbon.  Sadly, once the carbon runs out, the star is incapable of fusing iron.  Once again, we get the "kablooie!"  Now we are left with a really big neutron star.  The Telman-Oppenheimer-Volkoff limit says that if the neutron star's mass is greater than 0.7 solar masses (since we lack some of the tools to predict and quantify extremely dense matter, some estimates say 1.5-3 solar masses), gravity collapses the neutron star into a singularity (a point of infinite density) also known as a black hole.


So what are the implications for humanity? Minimal.  Since we live in a relatively safe part of the Milky Way, we are out of range of any nearby supernova candidates and any other fates that might befall us (wandering black holes, gamma ray bursts from neutrons stars, etc.) are highly unlikely.  The Sun will indeed consume the Earth in about 5 billion years, but it will render the Earth uninhabitable (assuming we don't render it so beforehand) in about 1 billion years due to its increasing luminosity.  The clock is ticking nonetheless, and there could be some alternatives for us.