What might be expected if you asked, à la a Jay Leno “Jaywalking” segment, some random people this question: “Where’s the nearest black hole?” Well, tallying those that know what a black hole is  … those that know what the Milky Way is … those that know our Sun is the nearest star … 
Anyway, how did we determine that there’s a supermassive black hole in the center of our Milky Way? Black hole tropes have been around for decades. I’ve taken it for granted that almost every galaxy has a massive black hole in its center; so, why not ours? This post was inspired by recent articles about three scientists receiving the 2020 Nobel Prize in Physics for this very discovery.
• NPR > “3 Scientists Awarded Nobel Prize In Physics For Discoveries Related To Black Holes” by Geoff Brumfiel (October 6, 2020)
The prize was awarded to Roger Penrose of the University of Oxford, for demonstrating that the general theory of relativity leads to the formation of black holes; and to Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics and Andrea Ghez of the University of California, Los Angeles, for the discovery of a compact object at the center of the Milky Way galaxy that governs the orbits of stars, for which a black hole is the only known explanation.
… for years after the prediction [shortly after Albert Einstein unveiled his general theory of relativity in 1915], researchers remained unsure whether black holes could form in the real universe, where conditions were often much more complicated than Einstein’s rarefied equations. It was Penrose who found a more complex mathematical description of black hole formation that matched with the natural world. Published in 1965, his work “is still regarded as the most important contribution to the general theory of relativity since Einstein,” according to the Nobel Prize committee, which awarded him half of the prize for his work.
Genzel and Ghez won the other half for painstaking observations of the supermassive black hole at the center of our own galaxy. Known as Sagittarius A, it is more than 4 million times the mass of our sun. Sagittarius A is shrouded behind a cloud of gas at the very core of the Milky Way, but undeterred, Genzel and Ghez used infrared telescopes to look through the gas. They painstakingly developed technologies to remove distortions caused by the gas and by Earth’s own atmosphere to track objects orbiting very close to the black hole. 
• Scientific American > “How Andrea Ghez Won the Nobel for an Experiment Nobody Thought Would Work” by Hilton Lewis [W. M. Keck Observatory Director] (October 8, 2020) – She provided conclusive evidence for a supermassive black hole at the core of the Milky Way.
Standing in my office 25 years ago was an unknown, newly minted astronomer … She had come with an outrageous request … to carry out an experiment that was basically a waste of time and couldn’t be done – to prove that a massive black hole lurked at the center of our Milky Way.
It was my first encounter with … Andrea Ghez, one of three winners of this year’s Nobel Prize in Physics, for her work on providing the conclusive experimental evidence of a supermassive black hole with the mass of four million suns residing at the center of the Milky Way galaxy.
For 25 years she has focused almost exclusively on Sagittarius A* – the name of our own local supermassive black hole. It is remarkable that an entire field of study has grown up in the intervening quarter century, of searching for and finding evidence of these monsters thought to lie at the heart of every large galaxy.  And Andrea is without question one of the great pioneers in this search.
Andrea’s co-prizewinner Reinhard Genzel has been involved in the same research from the outset—and it is the work of these two teams, each led by a formidable intellect and using two different observatories in two different hemispheres that has brought astronomy to this remarkable result – the confirmation of another of the predictions of Einstein’s more than century-old theory of general relativity.
Andrea is a great scientist … In addition to doing research, she has created the UCLA Galactic Center Group to coordinate research and technical developments.
Today, Andrea sits at the pinnacle of scientific recognition for her achievements. But as she would be the very first to acknowledge, this triumph represents the combined efforts of so many. … the product of the work of thousands.
• The Caltech Weekly > “Two Caltech Alumni Win 2020 Nobel Prizes” (Oct 8, 2020)
Andrea Ghez (MS ’89, PhD ’92) wins Nobel Prize in Physics for research demonstrating the presence of a supermassive black hole at the heart of the Milky Way galaxy, and virologist Charles M. Rice (PhD ’81) receives the 2020 Nobel Prize in Physiology or Medicine for his work on curing hepatitis C. Both are Distinguished Caltech Alumni.
• Caltech > News > “Alumna Andrea Ghez Awarded 2020 Nobel Prize in Physics” (October 6, 2020)
Andrea Ghez (MS ’89, PhD ’92), the Lauren B. Leichtman and Arthur E. Levine Professor of Astrophysics at UCLA, has won the 2020 Nobel Prize in Physics for pioneering research that helped reveal a supermassive black hole lurking at the center of the Milky Way galaxy. She shares half the Nobel Prize with Reinhard Genzel of UC Berkeley and the Max Planck Institute for Extraterrestrial Physics. Together, Ghez and Genzel are being honored “for the discovery of a supermassive compact object at the centre of our galaxy.”
The other half of the Nobel Prize goes to Roger Penrose of the University of Oxford, “for the discovery that black hole formation is a robust prediction of the general theory of relativity.”
At Caltech, Ghez’s PhD advisor was the late Gerry Neugebauer (PhD ’60), formerly the Robert Andrews Millikan Professor of Physics, Emeritus, and a founder of the field of infrared astronomy. Ghez’s PhD thesis looked at the frequency of multiple-star systems and stellar evolution using Caltech’s Palomar Observatory. She was named a Caltech Distinguished Alumna in 2012.
At UCLA, where Ghez joined the faculty in 1994, she and her team began mapping stars in a region at the center of our galaxy known as Sagittarius A*, around which all the stars in the Milky Way orbit.
• UCLA > Newsroom > “Andrea Ghez wins 2020 Nobel Prize in physics” by Stuart Wolpert (October 6, 2020) [includes video below] – UCLA professor is honored for her pioneering research on the Milky Way’s supermassive black hole.
In July 2019, the journal Science published a study by Ghez and her research group that is the most comprehensive test of Albert Einstein’s iconic general theory of relativity near the monstrous black hole at the center of our galaxy. Although she concluded that “Einstein’s right, at least for now,” the research group is continuing to test Einstein’s theory, which she says cannot fully explain gravity inside a black hole.
Ghez studies more than 3,000 stars that orbit the supermassive black hole. Black holes have such high density that nothing can escape their gravitational pull, not even light. The center of the vast majority of galaxies appears to have a supermassive black hole, she said.
The National Science Foundation funded Ghez’s research for the past 25 years. More recently, her research has also been funded by the W.M. Keck Foundation, the Gordon and Betty Moore Foundation and the Heising-Simons Foundation, Lauren Leichtman and Arthur Levine, and Howard and Astrid Preston.
Ghez earned a bachelor’s degree in physics from MIT in 1987 and a doctorate from Caltech in 1992, and she has been a member of the UCLA faculty since 1994. When she was young, she wanted to be the first woman to walk on the moon.
• YouTube > UCLA > “Andrea Ghez reacts to winning the Nobel prize in physics” (Oct 6, 2020) – “A full view of how this dance really works …”
• YouTube > UCLA > “Testing Einstein’s theory of relativity near a black hole” (Jul 25, 2019)
 As in: “Yes, Virginia, there is a Santa Claus.“
 Does Sgr A* have a companion?
• UCLA College > “That Supermassive Black Hole in our Galaxy? It has a Friend” by Smadar Naoz, associate professor of physics and astronomy in the UCLA College (December 20, 2019)
Almost every galaxy, including our Milky Way, has a supermassive black hole at its heart, with masses of millions to billions of times the mass of the sun. Astronomers are still studying why the heart of galaxies often hosts a supermassive black hole. One popular idea connects to the possibility that supermassive holes have friends.
The supermassive black hole that lurks at the center of our galaxy, called Sgr A, has a mass of about 4 million times that of our sun. A black hole is a place in space where gravity is so strong that neither particles or light can escape from it. Surrounding Sgr A is a dense cluster of stars. Precise measurements of the orbits of these stars allowed astronomers to confirm the existence of this supermassive black hole and to measure its mass. For more than 20 years, scientists have been monitoring the orbits of these stars around the supermassive black hole. Based on what we’ve seen, my colleagues and I show that if there is a friend there, it might be a second black hole nearby that is at least 100,000 times the mass of the sun.
 Is there any relationship between the growth of black holes and their host galaxies? In particular, supermassive black holes (SMBHs). Here’s another example of research using sophisticated sets of (cosmological) simulations.
• Daily Galaxy > “‘Written in the Stars’ – Galaxies Supermassive Black Holes Linked to Stellar Growth” (Oct 2, 2019) – Astrophysicists continue to theorize about the origins of black holes, how they grow and glow, and how they interact with host galaxies in different astronomical environments.
“There has been a lot of uncertainty regarding the SMBH-galaxy connection, in particular whether SMBH growth was more tightly connected to the star formation rate or the mass of the host galaxy,” said Yale astrophysicist Priyamvada Natarajan, senior investigator of the new study, which appears in the journal Monthly Notices of the Royal Astronomical Society. “These results represent the most thorough theoretical evidence for the former – the growth rate of black holes appears to be tightly coupled to the rate at which stars form in the host.”
 So, what’s the answer? How far away is the nearest black hole? Regardless of mass – stellar class, massive, supermassive, …
These references (below) discuss the answer. Note that there is some debate about the status of HR 6819.
• Wiki > List of nearest black holes (within our Milky Way galaxy)
• Forbes > “How Close To Earth Is The Closest Black Hole?” by Ethan Siegel (May 11, 2020)
First predicted in 1916 in General Relativity, the first one wasn’t discovered in space until 1964: Cygnus X-1.
The second-largest black hole as seen from Earth, the one at the center of the galaxy M87, …
Sagittarius A*, at the center of the Milky Way, is the closest supermassive black hole, some 25,000 light-years distant.
A smaller one — just 6.6 solar masses — orbits a Sun-like star just 3,500 light-years away: V616 Monocerotis.
That distance record was shattered last week, by trinary system HR 6819: two stars and a black hole 1,000 light-years distant.
As our methods and surveys continue to improve, closer black holes will inevitably be discovered.
• BBC > “‘Nearest black hole to Earth discovered’” by Jonathan Amos (May 6, 2020)
Astronomers have a new candidate in their search for the nearest black hole to Earth. It’s about 1,000 light-years away, or roughly 9.5 thousand, million, million km, in the Constellation Telescopium. [HR 6819]
Astronomers have spotted only a couple of dozen black holes in our Milky Way Galaxy to date, nearly all of which strongly interact with their accretion discs.
“In the Milky Way, the idea is that there should be about 100 million black holes. So there should be perhaps a couple more that are closer by still,” Marianne Heida, a postdoctoral fellow at ESO, told BBC News.
 So, what is a black hole? Here’s an answer, in 5 levels of understanding.
WIRED has challenged NASA’s Varoujan Gorjian (Research Astronomer, NASA Jet Propulsion Lab) to explain black holes to 5 different people; a child, teen, a college student, a grad student and an expert.
• Wired > “Astronomer Explains One Concept in 5 Levels of Difficulty” (Season 1 Episode 6, Released on 07/05/2018).
… in the AGN [active galactic nuclei] community, because we don’t know how the millions to billions solar mass black holes came to be. But it’s, at least we’re building up, or hopefully that at some point, and by understanding these lower mass, how these lower mass black holes came to be, then we can see where there are a large scale number of mergers can potentially give us this, or you really need something, other, another corridor to fundamentally get us something that’s a million solar masses, you know, on the minimum side, but definitely, you know, we’ve gotten those which are billion.
So we know we can merge the million solar mass black holes to get the bigger ones, but how do you get to those in the beginning, particularly so early in the universe, when you get quasars at really high red shifts, so they’re really early on.
Yeah, it is odd, it is very odd. I mean, the other thing that’s a little odd, now we’re going back to stellar mass black holes is, so we look at a lot of supernova remnants, and we, so we see them, we can only really see them in our own galaxy, and so we have a lot of supernova remnants, and so how we see them is, you see the expelled mass from the star as it died, so that creates an extended source, and then you look for the compact object that was left behind.
And what’s interesting is that you see, you quite often see the neutron stars because they pulse, so they’re easy to see, but so far, we’ve not found a single black hole at the center of a supernova remnant. And so, which is interesting, so you say, you should see them, you know, you ought to see some fallback, you know, you need some matter, you need something, but no, never, not yet been detected.