Sirius A/B in x-ray (left) and optical (right) observations. The x-ray observations show the white dwarf Sirius B much more prominently than in the optical. (Credit: NASA/SAO/CXC, McDonald Observatory)

In many ways, I am indebted to Sirius, the brightest star in the northern hemisphere, located very near the recognizable constellation of Orion. This is in part because Orion and its neighbors have kept me company in the sky, the spot where I rest my gaze for much of a long winter's night. More specifically, though, I should honor Sirius B, the companion to Sirius A that is 10,000 times fainter and can only be seen with the aide of a telescope. It was one of the first white dwarf stars discovered, and posed a rigorous challenge to early physicists.

As with many discoveries in astronomy, Sirius B was first inferred before directly observed, when Friedrich Wilhelm Bessel detected a wobble in the position of the bright Sirius A in the 1840s. This wobble was caused by the then-unseen white dwarf, which astronomers were eventually able to directly observe by the end of that century. There are many objects in astronomy that are still merely wobbles (black holes perhaps the most famous among them), unwilling to share enough light for our Earthly detectors. For now, we are forced to only infer their existence.

By the 1920s, astronomers had a good grasp of some fundamental properties of Sirius B, but the results were unsettling. The object had a mass comparable to that of the Sun, packed into a volume roughly the size of Earth. Such a result required an immensely dense star, a million times more dense than the Sun. There was simply no way to imagine a Sun's mass worth of even the smallest atom, hydrogen, packing into such a small volume without being ionized. Sir Arthur Eddington was especially troubled at what would happen if such an ionized star cooled and the energy to keep the atoms ionized was no longer there; he famously referred to white dwarfs as "impossible" stars in his seminal work on the stellar mass-luminosity relation.

Fortunately, physicists like Eddington pondered impossible stars at the time of the quantum revolution, which provided a way to reach such high densities via the uncertainty principle. This was first shown by R. H. Fowler, who posited in 1926 that quantum mechanics could indeed pack material into such a compact state. Specifically, Fowler showed that the degenerate electrons could have a pressure much higher than the ions. We now refer to this as electron degeneracy pressure: As the space between free electrons gets smaller the average momentum of the electrons gets higher, providing a powerful source of pressure, enough to hold up a star 100,000 times denser than lead.

For a stable star to exist it must satisfy hydrostatic equilibrium and maintain a steady balance between the strong gravity pulling inward and the internal pressure pushing outward. A main-sequence star like the Sun is able to support itself by fusing hydrogen to helium, which releases enough energy to hold a solar mass of material at bay. Electron degeneracy pressure is at work to hold up the white dwarf stars.

We know now that a white dwarf star is a star at the end of its life cycle, and it is essentially the burnt-out core of a star like the Sun, the ashy byproduct of previous epochs of nuclear fusion. White dwarf stars no longer fuse elements in their interior to generate energy; for the significant majority of a white dwarf's existence it is simply cooling, like a coal ember removed from a fire.