The 1800s were a big time in astronomy. Telescopes were becoming bigger and better. Scientists were starting to notice smaller details about the heavens, and people were asking bigger questions. Now with a firm mathematical background to fall back on and basic understanding of the motions of the heavenly spheres, a German astronomer named Friedrich Wilhelm Bessel noticed something strange.
In 1844 he announced that Sirius—the brightest star in sky—was wobbling back and forth (Bartusiak, 2018). Having studied binary stars in the past, Bessel made the logical leap that Sirius must have a heretofore unseen companion whose gravitational tug was responsible for the wobbling motion of the giant star. Based on the periodicity of Sirius’s wobbling, Bessel estimated that the companion likely had an orbital period of about fifty years.
The hunt was on! Challenge accepted, many astronomers swung their telescopes in the direction of Sirius to try and find its hidden companion. Over the intervening decade, there was no such luck. It was suspected that the companion must have been very small or in conjunction with the larger star and was therefore obscured.
The hunt was officially ended on January 31st, 1862, when Alvan Graham Clark—testing his father’s new batch of telescopes—looked in the direction of Sirius and clearly saw a much smaller star just off to the side. This star was comparatively very dim and astronomers were at first perplexed as they thought it must be too small to have the kind of mass it would need to pull Sirius around. Over the following decades though, astronomers confirmed through the orbital do-si-do of Sirius A and Siruis B that Sirius B must have about the same the same mass as our sun. The matter remained settled for several decades.
But then in the early 1900s, things got strange. When Sirius B’s spectrum was finally analyzed, it was found to be white hot. This made no sense since it was so small and so dim! At the time, astronomers thought that all stars followed a straightforward set of rules: the hotter you are, the brighter you are.
Hertzsprung Russell Diagram showing relationship between luminosity and temperature (color)
But here was an object that clearly was not following the rules. It was tiny, dim and white hot. How could this be? Based on the luminosity of Sirius and the distance to the system, it was deduced that Sirius B must be no bigger than the planet Earth to have this combination of apparent magnitude and temperature. But based on its gravitational effects on Sirius A, it weighed as much as the sun. That meant this odd little ball of light was incredibly dense—it was estimated that a matchbox-sized sample of its material would weigh a ton.
It took more time and study for the scientific community to piece it all together. Sirius B was what is now called a white dwarf. It is what happens to a roughly sun-sized star at the end of its life after it has blown all of its bloated outer layers away into space in what is known as a planetary nebula. The result is a naked white-hot core that has exhausted all of its fusion.
At this density of matter, the material has reached a stage of “degeneracy.” This means that all of the atoms are squeezed as close together as they possibly can be and that all of the electrons in the matter are similarly occupying orbitals as close together as they can without violating the laws of quantum mechanics—a completely new field of study at the time. Without nuclear fusion to sustain it, the crushing weight of the white dwarf must rely on this degeneracy pressure to maintain its structural integrity.
But what if it was even more massive? It turns out that when the naked white-hot core of a star is greater than about 1.4 solar masses, not even the electron degeneracy pressure can keep it from collapsing further. At this point, the electrons and protons in the dense ball recombine to form a material that is made of pure neutrons. This so-called “neutron star” is the densest stuff in the universe. Typically no bigger than a city, a neutron star still weighs as much as a star.
But what if it is even MORE massive? If a left-over star’s core is so massive and so dense that the escape velocity—the hypothetical speed an object needs to have going radially outward at an object’s surface in order to escape its gravitational pull—becomes greater than the speed of light, then something even stranger happens. With not even light able to escape its gravitational pull, all electromagnetic radiation from the star ceases to radiate. The star becomes hidden within a black spherical curtain known as the “event horizon.” Inside such a black hole, nothing can escape. Does the star within collapse further? Is it just sitting there? What kinds of bizarre behaviors does the matter within exhibit under such exotic conditions? No one knows. This is where the limits of our current understanding of physics ends and speculation begins.
Since the dawn of time, human beings have looked up into the heavens and tried to piece things together. And just when people think they have things worked out, they observe an outlier. Finding this glowing object that did not seem to fit led to further questions about the fate of stars and many other discoveries that challenged orthodoxy at the time followed. And just like Sirius B, it’s these outliers—the rule breakers—that lead to innovation and discovery. In science, it’s not the data that fits the theory nicely that leads to a breakthrough, but rather the data that makes one say, “that’s strange…”
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