Yet another recent find is challenging the fields of science with what is considered known and established. Astronomers may have found both the closest and the smallest black hole ever discovered: a small black hole locked in orbit with a red giant in what is known as a binary star system.
The black hole is a companion to a red giant star in a binary star system known as V723 Mon, and is nicknamed the “Unicorn”—named as such because of the system’s location in the sky as seen from Earth—inside the constellation Monoceros, which is Greek for “unicorn.” The “Unicorn” is pretty small in black hole terms, falling in their smallest category in terms of mass—the so-called stellar mass black holes. The most massive black holes ever found are categorized as supermassive black holes. The “Unicorn” in V723 Mon is a measly three times the mass of our Sun, and is roughly 1,500 light years away (a light year is around 9.5 trillion kilometers (5.9 trillion miles)—the distance light travels in a year); for comparison, the supermassive black hole powering the compact radio source Sagittarius A* in the center of our own Milky Way galaxy is around 4.1 million times the mass of our Sun, and is around 26,000 light years away.
What is a black hole?
Black holes are regions of spacetime where gravity is very strong—so strong that not even light can escape its grasp. Their existence has been predicted by the theory of general relativity, published by Albert Einstein back in 1915, wherein it was predicted that a sufficiently compact mass can deform spacetime to form one.
Stellar mass black hole formation, the ones possibly responsible for the “Unicorn,” is directly related to the stellar life cycle. Stars generally form upon the sufficient gravitational compression of a gas cloud (clouds of mostly hydrogen and helium), with the resulting gravity strong enough to initiate nuclear fusion reactions within the gas cloud’s center, starting the engines that power the resulting star and generating starlight. The outward radiation pressures in stellar cores are balanced out by the inward gravitational forces resulting from its own mass, resulting in hydrostatic equilibrium between the two and generating the spherical appearance of a star. This is the beginning of the main sequence phase of a star’s life, the phase our own Sun is in right now. Stars consume the fuel inside their cores, consuming heavier elements as nuclear fusion fuel in the process (like first fusing hydrogen to form helium, then consuming helium to form even heavier elements, and so on). However, stars progress differently in their life cycle depending on how massive they are.
Less massive stars, like our own Sun, are predicted to eventually start fusing hydrogen in a shell surrounding the now-predominantly-helium core. This causes the star to expand, forming what is known as a red giant, much like the companion of the “Unicorn” in V723 Mon. This goes on—consuming helium in the process as well, and so on—until it cannot sustain its internal nuclear fusion any longer, in which case it “sheds” its outer shell as a gaseous cloud known as a nebula, with a small remnant of a stellar core remaining—an entity known as a white dwarf.
More massive stars, however, die in a more spectacular fashion. Their increased mass allows them to fuse elements heavier than helium. Heavier elements sink further towards the center of the star, so lighter elements form shells around them, similar to Russian nesting Matryoshka dolls. This, of course, also causes the star to increase in size—more drastically so compared to its less massive counterparts, thus their apt names like red supergiants. The star increases in size, forming more and more shells of elements around its core, until it starts fusing iron. The nuclear fusion of all elements lighter than iron releases energy, which feeds the star. Attempting to fuse elements like iron and beyond, however, consumes energy instead. Once a star generates iron in its core, its fate is sealed.
The hydrostatic equilibrium is disrupted as the outward radiation pressure weakens due to the star losing its fuel, as it cannot fuse iron. The star, now greatly enlarged, then collapses inward onto its iron core. The resulting explosion of a star’s outer shell from its accelerating implosion towards its core is called a supernova. What remains in the middle of it all, however, can differ. For some stars, the resulting energy can be enough for the stellar remnant to go past a white dwarf stage, with its protons and electrons forced to combine into neutrons, leaving behind a singular superdense entity known as a neutron star. (They’re so dense that a teaspoon of neutron star material would weigh in at around 10 million tons!) For some stars around 3 to 4 solar masses or heavier, however, the collapse is so great that not even the formation of neutrons are enough to stop it. It progresses further, collapsing to finally form a black hole.
The details
The “Unicorn” falls into a “mass gap” described by astronomers and astrophysicists—the gap between the largest known neutron stars, around 2.2 times the mass of the Sun, and the previously-known smallest black holes, at around 5 times the mass of the Sun. Said Tharindu Jayasinghe, one of the researchers responsible for the find: “The ‘Unicorn’ is truly one of the smallest black holes possible.”
Finding black holes are a very difficult process, too; as not even light escapes its gravitational grasp, it can remain invisible to the electromagnetic spectrum, rendering them elusive to even the most brilliant of astronomers. In fact, what led the team of researchers to the discovery of the “Unicorn” is the gravitational pull it exerts on its red giant companion, causing what is known as tidal distortion and changing the shape of the star. The change in shape due to the presence of the black hole changes the light we see back on Earth as it revolves around its orbit, revealing to the team the presence of the sought-after black hole.
Jayasinghe, however, remains hopeful that the discovery of this black hole will only be one of many in the foreseeable future. In his own words: “I think the field is pushing toward this, to really map out how many low-mass, how many intermediate-mass and how many high-mass black holes there are, because every time you find one it gives you a clue about which stars collapse, which explode and which are in between.”
Bibliography
- Arenschield, L. (2021, April 21). Black hole is closest to Earth, among the smallest ever discovered. Ohio State News. Retrieved September 2, 2021, from https://news.osu.edu/black-hole-is-closest-to-earth-among-the-smallest-ever-discovered/
- Dunham, W. (2021, April 23). A black hole dubbed ‘the Unicorn’ may be galaxy’s smallest one. Reuters. Retrieved September 2, 2021, from https://www.reuters.com/lifestyle/science/black-hole-dubbed-the-unicorn-may-be-galaxys-smallest-one-2021-04-22/
- Lumen Learning. (n.d.). Stellar Life Cycle. Lumen Learning. Retrieved September 2, 2021, from https://courses.lumenlearning.com/earthscience/chapter/stellar-life-cycle/
- Jayasinghe, T., Stanek, K. Z., Thompson, T. A., Kochanek, C. S., Rowan, D. M., Vallely, P. J., … & Murphy, S. J. (2021). A Unicorn in Monoceros: the 3M. dark companion to the bright, nearby red giant V723 Mon is a non-interacting, mass-gap black hole candidate. arXiv preprint arXiv:2101.02212.
