Small Black Holes Haunt Spacetime

The Cosmic Zoo is filled with exotic inhabitants that frequently challenge the limits of the human imagination. Of these strange beasts, black holes certainly rank as some of the most mysterious, as well as enticing, entities. As a result, they have captivated scientists for years with their irresistible Sirens’ Song of bewildering secrets. These gravitational monsters do not come in only one size, but so far scientists have clearly recognized only two distinct classes. There are black holes of stellar mass that form when an especially massive star runs out of its necessary supply of hydrogen fuel and blasts itself into oblivion in a core-collapse (Type II) supernova explosion. There are also supermassive black holes that haunt the dark hearts of perhaps every large galaxy in the Universe, including our own Milky Way, and these extremely massive entities weigh-in at an incredible millions to billions of times more than our Sun. In October 2019, a team of astrophysicists announced their discovery that reveals the census of black holes might be incomplete, and that they may just have discovered a new class of these bizarre beasts.

Black holes play an important role in how astrophysicists make sense of the common-sense defying Cosmos. Indeed, these gravitational monsters are so important that scientists have been trying to create a census of these entities inhabiting our own Galaxy for a very long time. The new research shows that the scientific hunt for these strange beasts may have been missing an entire class that astrophysicists didn’t know existed in the Cosmos. In a study published in the October 31, 2019 issue of the journal Science, astronomers offer a new way to hunt for these mysterious entities, and demonstrate that it is possible there is a class of small gravitational beasts that exist–and that they are smaller than the smallest known black holes in the Universe

“We’re showing this hint that there is another population out there that we have yet to really probe in the search for black holes,” commented Dr. Todd Thompson in an October 31, 2019 Ohio State University Press Release. Dr. Thompson is a professor of astronomy at the Ohio State University and lead author of the study.

“People are trying to understand supernova explosions, how supermassive black stars explode, how the elements were formed in supermassive stars. So if we could reveal a new population of black holes, it would tell us more about which stars explode, which don’t, which form black holes, which form neutron stars. It opens up a new area of study,” Dr. Thompson added.

Neutron stars are the extremely dense relics left behind by massive stars after they have perished in a supernova blast. Although the stellar progenitors of neutron stars are very massive, they are not as massive as the progenitors of stellar mass black holes. Neutron stars are about the size of a city, but they are so dense that only a teaspoon full of their material can weigh as much as the combined weight of all of the alligators in Florida. In a way, neutron stars are really giant atomic nuclei.

Imagine a census taken in the city you live in that counted only people with red hair–and imagine that the person taking the census didn’t even know that there is such a thing as people without red hair. The data obtained from that census would be incomplete–and it would also be invalid because it would give an inaccurate portrayal of the population. That is essentially what has been occurring in astronomers’ hunt for black holes.

Exotic Inhabitants Of The Cosmic Zoo

Even though astrophysicists have only confirmed the existence of two types of these exotic gravitational beasts, theoretically black holes can form whenever matter is squeezed into a small enough space. In the 18th century, the English scientist John Michell (1724-1793) and the French physicist Pierre-Simon Laplace (1749-1827) were already considering the possibility that there could really exist in the Cosmos strange gravitational monsters like black holes. Albert Einstein, in the Theory of General Relativity (1915), also made the prediction that there could be entities lurking in Spacetime that possessed such unimaginably strong gravitational fields that any object wandering too close to their gravitational grip would be doomed. However, the concept that such exotic entities could actually exist in reality seemed so preposterous at the time that Einstein rejected what his own calculations indicated–although he later commented that “Black holes are where God divided by zero.”

The German astronomer and physicist Karl Schwarzschild (1873-1916) formulated the first modern solution to Einstein’s Theory of General Relativity that describes a black hole. However, Schwarzschild’s interpretation of these entities as a regions of space, from which nothing, nothing, nothing at all can escape once captured, was not understood for another half-century. Up until that time, black holes were thought to be only mathematical oddities that could not really exist in nature. Indeed, it was not until the 1960s that theoretical work showed that black holes are a generic prediction of General Relativity.

Astronomers have known for years that it is likely every large galaxy in the observable Universe hosts a central supermassive black hole in its secretive heart. Our own Milky Way Galaxy houses just such a gravitational beast that has been dubbed Sagittarius A*–or Sgr A* (pronounced saj-a-star), for short. Sgr A* is of relatively low mass–as supermassive black holes go. These extremely hefty galactic dark hearts weigh-in at millions to billions of times more than our Sun–and Sgr A* is only millions, as opposed to billions, of solar-masses. Our Galaxy’s resident supermassive black hole is a quiet old beast now, and is usually dormant. Billions of years ago, when Sgr A* and the Universe were both young, it lit up the Cosmos (like others of its kind) in the form of a fiery and brilliant quasar. Quasars were glaring accretion disks that surrounded the young black holes haunting the ancient Cosmos. In its youth, Sgr A* was voracious and greedy, dining on any blob of matter that had tragically wandered too close to its intense gravitational pull. This fiery infalling feast, composed of shredded stars, clouds of gas, and other doomed goodies, tumbled down into the waiting maw of the young black hole from the glaring, swirling accretion disk–the quasar.

Supermassive hearts of darkness, inhabiting the centers of galaxies, grow to their enormous sizes by devouring their surroundings. They are messy eaters, and “bite off more than they can chew”. The tattered, shredded remains of what they were unable to swallow is sent outward into surrounding space.

In contrast, stellar mass black holes are born when an especially massive star reaches the end of the stellar road and runs out of its necessary supply of nuclear-fusing fuel. Stars that are still “living” are kept bouncy as the result of radiation pressure counteracting the crushing squeeze of their own gravity. Radiation pressure pushes the stars material outward, while gravity pulls everything inward. This creates a delicate balance that continues for as long as the star “lives”.

Radiation pressure results from nuclear fusion–the formation of increasingly heavier and heavier atomic elements out of lighter ones (stellar nucleosynthesis). When an elderly massive star has finally succeeded in fusing its necessary supply of lighter atomic elements into heavier things, it forms a core of iron. Iron cannot be fused. As a result, the old star’s core collapses, and it goes supernova. If the progenitor star was massive enough, all that it will leave behind is a black hole of stellar mass.

Black holes are compact, dense areas of space, and they can be large or small. These strange inhabitants of the Cosmic Zoo can be defined as regions of Spacetime where the pull of gravity has become so strong that not even light can escape once it has been captured.

A New Class?

Both stellar mass black holes and neutron stars could reveal some fascinating new information about the atomic elements on our own planet and about how stars “live” and “perish”. But in order to uncover that important information, astronomers first have to determine where the black holes are hiding. In order to solve that particular mystery, they need to know what they are hunting for.

Astronomers know that black holes frequently dwell in binary systems, which means that a duo of stars are close enough to each other to be bound together by gravity in a shared orbit. When one of those stars reaches the end of its hydrogen-burning “life” and “dies”, the other still-“living” companion star can remain–still orbiting the space where its now “dead” companion exists as either a stellar mass black hole or neutron star.

For a very long time, the only black holes that astronomers knew about weighed-in at about five to 15 times the mass of our Sun–while the known neutron stars generally weighed-in at approximately 2.1 times solar-mass. This is because, if they weighed-in at more than 2.5 times our Sun’s mass, they would collapse to a stellar mass black hole in the fiery rage of a brilliant core-collapse (Type II) supernova.

A new discovery in 2017 changed the way that astronomers view black holes. This is because a survey called the Laser Interferometer Gravitational Wave Observatory (LIGO) discovered a duo of these strange entities in the process of merging together. This celestial waltz occurred in a galaxy about 1.8 million light-years away. One member of the duo was about 31 times solar-mass, while the other was approximately 25 times the mass of our Sun.

“Immediately, everyone was like ‘wow’, because it was such a spectacular thing. Not only because it proved that LIGO worked, but because the masses were huge. Black holes that size are a big deal–we hadn’t seen them before,” commented Dr. Thompson in the October 31, 2019 Ohio State University Press Release.

Dr. Thompson and other scienctists had long considered the possibility that black holes might come in sizes outside the known range. LIGO’s discovery clearly demonstrated that they could be larger. However, there remained a gap in size between the largest neutron stars and the smallest black holes, and so Dr. Thompson decided to try and solve the enticing mystery. As a result, he and other scientists started combing through the data obtained from the Apache Point Observatory (APOGEE) Galactic Evolution Experiment, which gathered light spectra from about 100,000 stars across our Galaxy. The spectra, Dr. Thompson realized, could reveal whether a star might be orbiting around an unseen companion. Changes in spectra–a shift toward bluer wavelengths, followed by a shift to redder wavelengths–can reveal if a star is orbiting an unseen companion. A shift to bluer electromagnetic wavelengths means that an object is moving closer, while a shift to redder wavelengths means that it is traveling away.

Next, Dr. Thompson began to sift through the data, on the hunt for stars that exhibited that change. This would indicate that they might be in orbit around a black hole. After he had narrowed down his search to 200 stars that were the most interesting, he gave the accumulated data to Tharindu Jayasinghe, a graduate research associate at Ohio State. Jayasinghe then compiled thousands of images of every potential binary system taken from the All-Sky Automated Supernovae Survey (ASAS-SN). ASAS-SN , which is run by Ohio State University, has discovered about 1,000 supernovae.

The data revealed a giant red star that appeared to be in orbit around something. However, that something, based on the scientists’ calculations, was probably much smaller than the known black holes in our Milky Way Galaxy–but considerably larger than most known neutron stars.

After more calculations and additional data obtained from the Tillinghast Reflector Echelle Spectrograph and the Gaia satellite, the scientists came to the realization that they had discovered a low-mass black hole, that was only about 3.3 times solar-mass.

“What we’ve done here is come up with a new way to search for black holes, but we’ve also potentially identified one of the first of a new class of low-mass black holes that astronomers hadn’t previously known about. The masses of things tell us about their formation and evolution, and they tell us about their nature,” Dr. Thompson explained in the October 31, 2019 Ohio State University Press Release.