Dead Stars

A month ago, we looked at the varieties of stars that can be found in the universe. But stars have a limited amount of fuel, and when it runs out, stuff happens. Exactly what happens depends on the mass of the star, and some of them are among the weirdest and most interesting things in the universe.

The most common stellar remnant by far is a white dwarf. White dwarfs are what we get when a star stops fusing its atoms and its matter settles down. In a white dwarf, the gravity is so strong and the pressure is so high that it runs into a physical limit called electron degeneracy. You may have learned in chemistry class that atoms have electron orbitals, sometimes called electron shells, which can only hold a certain number of electrons each. This is an example of electron degeneracy. In a white dwarf, the atoms' outer layers of electrons are unbound, moving freely around the material, and they are degenerate because they have the maximum density allowed by the laws of physics. Because of their variety of temperatures, white dwarfs are not necessarily white, but can also be yellow, orange, red, and brown.

Once our sun goes through all of its phases, it will become a white dwarf, slowly cooling down until the end of time.

There are ways to increase a white dwarf's pressure beyond the electron degeneracy limit. One of them is for it to be made of denser material. Some white dwarfs are made of helium, which has two protons and two electrons, neither of which are bound to it. Other white dwarfs are made of various mixtures of carbon, oxygen, neon, and magnesium, each of which is more dense and has more bound electrons not contributing to degeneracy.

The other way to increase the pressure is for gravity to be so strong that the electrons combine with the protons, making neutrons. When this happens, there is no more electrical repulsion, and all of the matter collapses to the density of an atomic nucleus, where neutron degeneracy once again makes things stable. You can probably guess what neutron degeneracy is; if electron degeneracy is what we get when we have the maximum density of electrons, neutron degeneracy is what we get when we have the maximum density of neutrons. A stellar remnant made of neutron degenerate matter is called a neutron star.

White dwarfs do not turn into neutron stars. Instead, we start out with a very massive star, which has enough pressure to fuse its atoms beyond the elements mentioned above. The higher the element number, the faster the fusion happens, until it reaches core collapse, which causes a supernova and leaves behind a neutron star.

When neutron stars are young, they shoot high-energy gets of light and other radiation from their magnetic poles. They are also spinning, and their rotational poles are not lined up with their magnetic poles. This means their jets spiral around in a pair of cones. If Earth is in the path of one of these jets, the neutron star appears to pulsate in the night sky. Because of this, we call this kind of neutron stars pulsars. Most pulsars rotate once every few seconds, but some are as fast as a few milliseconds.

By Kevin Gill on Flickr

Some neutron stars have the strongest known magnetic fields in the uinverse, strong enough to deform atoms. These neutron stars are called magnetars.

If a relatively small supernova makes a neutron star, what happens if we turn up the mass? If we explode progressively heavier supergiant stars, we get heavier and heavier neutron stars, until suddenly . . . there is nothing. The star explodes, leaving only empty space behind. According to supernova theory and observational data, there is a mass gap between neutron stars around three times the mass of the sun and our next type of stellar remnant at five times the mass of the sun. This heavier next type is what I’m sure you have been waiting for this whole blog post: black holes.

An ancient philosophical question goes like this: what happens when an immovable object encounters and unstoppable force? Well it turns out that there is no such thing as an immovable object, but gravity can get strong enough to become an unstoppable force. If enough mass gets crammed into a small enough space, not even neutron degeneracy can prevent it from collapsing down to an infinitesimal point called a singularity. A certain distance away from the singularity, called the event horizon, gravity switches between weak enough to escape from and too strong for anything to resist. The event horizon is the black ball we picture when we think about black holes. Black holes are so mind-bendingly fascinating that they deserve a whole discussion to themselves.

These are all of the stellar remnant types we have evidence for (and as of last month, I might add with pride, we have pictures to back all of them up). However, there are still more which are theorized to exist, either presently unobserved or far in the future.

It may be that between neutron stars and black holes, there is another stopping-off point. Neutrons are made of quarks, so perhaps quark degeneracy can stop the formation of a black hole when neutron degeneracy is not enough. Such an object would, unsurprisingly, be called a quark star. It is unknown whether this is possible; the only hint we have is the small amounts of quark-gluon plasma made in particle accelerators under completely different conditions than we would expect in quark stars.

Remember how we left white dwarfs cooling off indefinitely? One day, many eons in the future, white dwarf stars will have cooled so much that they no longer give off any visible light. Then, they will be called black dwarfs. The time it will take for white dwarfs to cool down this much is orders of magnitude longer than the current age of the universe.

Yet even black dwarfs are not the end state of stellar remnants. To find out why, we have to talk about nuclear fission and fusion. Radioactive materials break apart into lighter elements, each with its own half-life, the time it takes for roughly half of the atoms to decay. This is natural nuclear fission. Fusion happens when atoms fuse together into heavier elements, releasing energy. Now you might notice that I said both fission and fusion release energy. This is only true when the product has less mass per nucleon (less energy density) than what we started out with. Iron has the lightest mass per nucleon of all, so the elements lighter than iron fuse, and the elements heavier than iron fission.

When we talk about radioactivity, we say that some atoms heavier than iron are radioactive, and some are stable. When we talk about fusion, we imagine we need the pressures and temperatures at the core of a star. These are both not entirely true. High temperatures and pressures raise the probability of fusion, but that probability never goes to zero. Similarly, the “stable” heavier-than-iron elements have an extremely long, but not infinite, half-life. For us humans, it is true enough to say fusion requires enormous temperatures and pressures, and many heavy elements are stable.

But if we look ahead in the future, and I mean so far ahead that it might as well be infinite, we get a different story. Given an infinite amount of time, anything with a static non-zero probability is guaranteed to happen, no matter how small that probability is. On a large enough timescale, black dwarfs will fuse their atoms together into iron, and any elements heavier than iron will break apart into iron. Long after even the biggest black holes have evaporated, iron stars will be the last objects left in the universe.