Astronomy 101: Stellar Life Cycles
This is the first in a series of posts I plan to make explaining basic astronomy concepts. I am not a professional astronomer, but I have a B.S. in Astronomy and Astrophysics, experience performing professional astronomical research (primarily photometry), and a lifelong amateur interest in astronomy. I am committed to accuracy in these posts (though some simplification is necessary) and welcome questions/comments.
(Image source)
Nearly every aspect of a star’s evolution is governed by a single characteristic: the star’s mass. Stellar masses are enormous, and so astronomers find it more convenient to measure them in solar masses (M☉, M being mass, and ☉ being the astronomical symbol for the sun, pronounced “M-sol”). 1 M☉ is equivalent to the sun’s mass; 2 M☉ is twice the sun’s mass; 0.5 M☉ is half the sun’s mass; and so on. Our sun masses approximately two quintillion kilograms—that’s 30 zeroes!—so it’s easy to see why this convention makes comparisons between stars easier.
Stars are composed primarily of hydrogen, with non-trivial amounts of helium (ex. our sun is about 74% hydrogen and 25% helium). They fuse the elements within themselves to produce an outward pressure from radiation that balances the inward pressure imposed by their own gravity (gravity wants to collapse the star; radiation keeps this from happening). The lowest stellar mass that can sustain a fusion reaction is theorized to be 0.08 M☉. It’s not physically possible for a star to be smaller than this. On the other end, the most massive stars range up to 250 M☉. (It should be noted that as the universe ages, stars are getting smaller; it is hypothesized that new stars cannot exceed 150 M☉.)
Stars are born from the gravitational collapse of giant molecular clouds (GMC), which are large, cold, and relatively dense pockets of interstellar gas. It’s theorized that several mechanisms can trigger GMC collapse, including the shock wave from nearby supernovae, collisions with other clouds, or passing through the spiral arm of a galaxy (which is denser than other areas). These all disrupt the balance within the cloud. As it collapses, it begins to fragment, forming individual stars. One GMC will produce numerous protostars.
The protostar phase lasts a relatively short time, about 500,000 years for stars the size of the sun. During this time, the protostar continues to accrete mass from the surrounding nebula (its own pocket of the GMC). Gravitational collapse is countered by gas pressure and magnetic pressure rather than by nuclear fusion as the protostar grows. Eventually, this is not sufficient and the protostar begins to collapse and enters a new phase: it reaches sufficient density to ignite nuclear fusion in its core, and becomes a proper star.
Initial fusion in the core fuses hydrogen into helium. It may seem counter-intuitive, but the lower a star’s mass, the slower it burns through its fuel and the longer it remains in this phase of its life. Massive stars are not only much hotter than small stars, but they need to produce more energy to stave off gravitational collapse. This causes them to burn through their supply of hydrogen very quickly. Very low mass stars born early in our universe have not existed long enough to burn through their hydrogen supplies. Models predict stars of 0.1 M☉ will take 6-12 TRILLION years to exhaust their hydrogen, and our universe is only 13.8 billion years old. These stars are barely getting started. These stars, known as red dwarfs, will never become red giants. Instead, they will continuously mix their original hydrogen stores with their newly produced helium until the entire star is composed of helium, at which point fusion will shut down. As they reach the end, they are predicted to become blue dwarfs, and then eventually white dwarfs. No blue dwarfs presently exist because the universe is still young.
Stars more like our sun, ranging from 0.6 to 8 M☉, have lives more consistent with what many people are taught in high school science classes. When these stars exhaust the hydrogen in their core, they still contain a large volume of hydrogen in their outer layers. They enter the subgiant phase, where they fuse hydrogen in a shell surrounding the helium core, and the star begins to expand and cool. This new helium adds to the volume of the core, the hydrogen-fusing shell moves outwards. This phase lasts several million to two billion years, again with lower-mass stars spending longer in this phase.
(You may have read that the sun will get hotter as it ages. This is true, and it’s due to the fact that as the fraction of hydrogen in the core decreases, the core temperature and rate of fusion increase. The sun will contract and become hotter as it approaches the subgiant phase. It’s theorized that this will make the Earth uninhabitable in about one billion years. However, the effects of plate tectonics and specifically the subduction of water from the oceans and the gradual slowing of tectonic activity is likely to play as large a role if not larger.)
This hydrogen-burning shell supports the star against gravitational collapse until the helium core grows too large. The core will contract, and the outer layers will further expand and cool as the star becomes a red giant. In the case of our sun, its radius will expand beyond the orbit of the Earth, in about six billion years. Shell burning continues at an increasing rate, but the star is no longer in equilibrium, and the core continues to contract even as it increases in mass, until eventually the star begins fusing helium into carbon. This can happen very suddenly for stars at the lower end of this mass range, or more gradually for more massive stars. The growth of the star can also temporarily create new habitable zones, primarily for the moons of gas giants, lasting several hundred million years.
Stars in this mass range do not have sufficient mass to fuse carbon, and so when the helium is largely consumed in the core, the core begins to collapse, and this time, only a strange state of matter called electron degeneracy will stop it. The Pauli Exclusion principle states that no two electrons can occupy identical states, and electron degeneracy occurs when a star has collapsed to the point that all the electrons of the star have been forced to occupy all the lowest-available energy states within the atomic structure of the star. The energy of the gravitational collapse is insufficient to overcome this electron pressure. White dwarfs are very small and incomprehensibly dense; similar to the mass of the sun, they are similar in size to the Earth. A teaspoon of white dwarf would weigh about 15 tons. Fusion has ceased, and the star is an inert ball of slowly cooling degenerate matter.
This is the end of the road for stars in this mass range, which is most of the stars that presently exist in the universe. Very, very slowly, solo white dwarfs will continue to cool. White dwarfs are predicted to outlast the lifespan of galaxies as structures in our universe. They may still be extant when enough time has passed for proton decay to become a significant force and ultimately devour the white dwarf. If proton decay takes significantly longer than predicted or turns out to not exist, the ultimate fate of a white dwarf is to be devoured by a black hole in the very, very, very far future of our universe when other forms of stars have all ceased to exist. (The black hole era is predicted to begin 10^43 years after the Big Bang, but it’s dependent on a lot of poorly-understood factors. Safe to say, this is not anything anyone needs to worry about.)
More massive stars have a more interesting if shorter life. They blow through their hydrogen quickly, ranging from 1 billion years to as few as 10,000 years. (You can calculate here.) As these stars enter their supergiant phase, they lose mass rapidly due to strong stellar winds. These stars do possess enough mass for more complex fusion reactions, and will gradually develop shells fusing various elements, and finally in the core, ultimately producing iron via fusion. All elements aside from hydrogen, some helium, and small amounts of lithium and beryllium are created by stellar nuclear fusion. Iron, however, is special, because it is the lightest element on the periodic table that requires more energy to fuse than it produces through fusion. A star attempting to fuse iron is going into an energy deficit, rather than producing energy through radiation that can stave off gravitational collapse. A star cannot fuse iron.
The final days of such stars come on quickly. The core turns to iron within a few hundred years, so rapidly that there is little change in the outward appearance of the star, which continues shell burning right up to the end. Eventually, this iron core reaches the effective Chandrasekhar mass, a little larger than the mass of our sun, and the core can no longer support itself. The core collapses.
It’s easy to say the star then explodes in a supernova, but the truth is more complicated. A supernova is triggered by the release of gravitational potential energy from the core collapse. If this is insufficient, instead the core becomes a neutron star or a black hole with very little fanfare. Some supernovae (pair-instability supernova) do not leave behind any stellar remnant despite originating from supermassive stars. Other supernovae do not involve supermassive stars, but white dwarfs in binary pairs accreting matter from their stellar companion.
The core collapse takes less than a quarter of a second. Within a few hours, the shockwave reaches the surface of the star, which will brighten tremendously over the next few months. At peak, supernovae can outshine their host galaxies. Perhaps the most famous supernova, SN 1987A, was visible to the naked eye and wasn’t even located in our own galaxy, but in a satellite galaxy. This shockwave is incredibly high-energy, to the point that it not only ejects outer material from the star, but ignites fusion reactions in that material. Here, we have an excess of energy, and nuclear fusion reactions that require more energy than they create are possible. This is how all elements heavier than iron are created.
The core becomes a neutron star or a black hole. Which depends on a large number of variables, perhaps strangely only two of which are core mass and stellar mass. A neutron star is similar to a white dwarf, in that it is composed of degenerate matter, but in this case electron degeneracy was not strong enough and it is held up by neutron degeneracy. In this state, electrons have combined with protons to produce neutrons, leaving behind a highly dense star composed of nuclear matter. While a neutron star is again slightly more massive than our sun, it has a diameter of only about 20 kilometers. A teaspoon of neutron star weights approximately 4 billion tons. Many neutron stars rotate rapidly, giving off jets of radiation from their poles. These stars are known as pulsars. The fate of neutron stars is similar to that of white dwarfs; they will eventually merge into black holes in the far distant future.
Other stars become black holes. In this case, even neutron degeneracy cannot stave off gravitational collapse, and the core collapses into a singularity. A singularity is a poorly-understood object where spacetime curvature becomes infinite. We can study singularities mathematically, but we can’t observe one, because it is hidden behind an event horizon—a boundary surrounding the black hole where gravity is too strong for even light to escape. And if light can’t escape, we can’t observe anything inside the event horizon. Our current understand of physics also breaks down here; our theories of gravity aren’t sufficient to fully understand singularities. We can, however, study event horizons, accretion disks (matter falling into the black hole), and jets of particles and radiation some black holes exhibit. This has given us a good understanding of the external properties and behavior of black holes, and we detect them based on these characteristics and the effects they have on other objects (ex. an orbiting companion star).
So if everything eventually becomes part of a black hole, what happens to a black hole? The answer is they evaporate through a process called Hawking radiation. Essentially, according to quantum mechanics, space has a quality called vacuum energy that causes virtual particle pairs to continuously pop in and out of existence. The pair consists of one particle and one anti-particle, which annihilate each other and return the energy of their creation to the vacuum. Therefore, no energy is created or destroyed. However, every so often, one of these pairs pops into existence with one particle on the outer side of the event horizon, able to escape, and the other trapped inside the event horizon. The virtual particle that escapes becomes a real particle, and that energy has to come from somewhere. It takes it from the black hole. Because mass and energy are functionally equivalent, this drains a tiny amount of mass from the black hole. (Please note: this is a dramatic oversimplification, but gets the basic idea across.) On long enough timelines, this will cause even the largest of black holes to evaporate. Unlike just about everything else involving stars, smaller black holes evaporate more quickly than large ones.