# Statistically, what would the average distance of the closest black hole be?

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The closest confirmed black hole is several thousand light years away from earth. Our galaxy has about 100 billion stars. I didn't find any reliable information on the black hole count of ratio versus stars for our galaxy. Some sources say about one in a thousand.

I would like to estimate how close a black hole would be given the data we have. If I did the calculation myself, I would use the volume of the galaxy and the number of stars in it. This would give the average number of stars per some volume, and out of that, the average number of black holes per volume. The distribution of stars would also be have to taken into account, since our part of the galaxy isn't as densely populated. Out of this the statistically approximate distance of the closest black hole could be obtained.

Is anyone equipped with enough information to give me an estimate?

Basically what is needed is the ratio of black holes to stars for our galaxy and then list of x nearest stars and their distances, where x is the ratio.

(I'm asking this because black holes are object of interest, and will have to be visited in the future. Given that our space capabilities might be limited to a distance of a couple of dozens of light years in the near future, this number is interesting.)

I have found a database of the nearest stars within 25 parsecs. The database contains 2608 stars, given the not very accurate estimate of 1 black hole per 1000 stars, would make 2.6 black holes within 81.5 ly( 1 parsec = 3.26 lightyears ).

Taking only the closest 1000 stars from the database, then the maximum distance is 50.9 ly, so there is on average one black hole within that distance. The average distance of all 1000 stars is 35.8 ly, and that is the average distance to that probable black hole.

A more accurate ratio would make this much more interesting. Imagine a ration of 1 to 100. Then the average distance becomes only 14.3 ly.

This is only a rough estimate. But the number of supernovae in the Milky Way is approximately 2-5 / 100 years (here and here). There is approximately $3 imes 10^{11}$ stars in the Milky Way. Provided the supernovae rate did not change much through the history (which is a big if that probably doesn't work), the total number of supernovae would be $2$-$5 imes 10^8$. I do not know how much supernovae will result in a black hole, but if we estimate 10% - 30%, it might not be that wrong. This would lead to $2 imes 10^7$ - $1.5 imes 10^8$ stellar black holes in the Milky Way. By other words, one star from 2000 - 15000 should be a black hole.

If there is 1000 stars in 50.9 ly around the Sun, with this density there would be one stellar black hole per 100 - 200 ly.

Let us assume that $N$ stars have ever been born in the Milky Way galaxy, and given them masses between 0.1 and 100$M_{odot}$. Next, assume that stars have been born with a mass distribution that approximates to the Salpeter mass function - $n(m) propto m^{-2.3}$. Then assume that all stars with mass $m>25M_{odot}$ end their lives as black holes.

So, if $n(m) = Am^{-2.3}$, then $$N = int^{100}_{0.1} A m^{-2.3} dm$$ and thus $A=0.065N$.

The number of black holes created will be $$N_{BH} = int^{100}_{25} Am^{-2.3} dm = 6.4 imes10^{-4} N$$ i.e 0.06% of stars in the Galaxy become black holes. NB: The finite lifetime of the galaxy is irrelevant here because it is much longer than the lifetime of black hole progenitors.

Now, I follow the other answers by scaling to the number of stars in the solar neighbourhood, which is approximately 1000 in a sphere of 15 pc radius $simeq 0.07$ pc$^{-3}$. I assume that as stellar lifetime scales as $M^{-2.5}$ and the Sun's lifetimes is about the age of the Galaxy, that almost all the stars ever born are still alive. Thus, the black hole density is $simeq 4.5 imes 10^{-5}$ pc$^{-3}$ and so there is one black hole within 18 pc.

OK, so why might this number be wrong? Although the number is very insensitive to the assumed upper mass limit of stars, it is very sensitive to the assumed lower mass limit. This could be higher or lower depending on the very uncertain details of the late stellar evolution and mass-loss from massive stars. This could drive our answer up or down.

Some fraction $f$ of these black holes will merge with other black holes or will escape the Galaxy due to "kicks" from a supernova explosion or interactions with other stars in their dense, clustered birth environments (though not all black holes require a supernova explosion for their creation). We don't know what this fraction is, but it increases our answer by a factor $(1-f)^{-1/3}$.

Even if they don't escape, it is highly likely that black holes will have a much higher velocity dispersion and hence spatial dispersion above and below the Galactic plane compared with "normal" stars. This is especially true considering most black holes will be very old, since most star formation (including massive star formation) occurred early in the life of the Galaxy, and black hole progenitors die very quickly. Old stars (and black holes) have their kinematics "heated" so that their velocity and spatial dispersions increase.

I conclude that black holes will therefore be under-represented in the solar neighbourhood compared with the crude calculations above and so you should treat the 18pc as a lower limit to the expectation value, although of course it is possible (though not probable) that a closer one could exist.

## 'Nearest black hole to Earth discovered'

It's about 1,000 light-years away, or roughly 9.5 thousand, million, million km, in the Constellation Telescopium.

That might not sound very close, but on the scale of the Universe, it's actually right next door.

Scientists discovered the black hole from the way it interacts with two stars - one that orbits the hole, and the other that orbits this inner pair.

Normally, black holes are discovered from the way they interact violently with an accreting disc of gas and dust. As they shred this material, copious X-rays are emitted. It's this high-energy signal that telescopes detect, not the black hole itself.

So this is an unusual case, in that it's the motions of the stars, together known as HR 6819, that have given the game away.

"This is what you might call a ⟚rk black hole' it's truly black in that sense," said Dietrich Baade, emeritus astronomer at the European Southern Observatory (ESO) organisation in Garching, Germany.

"We think this may be the first such case where a black hole has been found this way. And not only that - it's also the most nearby of all black holes, including the accreting ones," he told BBC News.

One of the fascinating aspects of this story is that it's possible see HR 6819 with just the naked eye - assuming you have access to the southern sky. No telescope or binoculars are needed, although conditions are tricky at the moment because the star system is only now emerging from behind the Sun.

Scientists had begun the study of HR 6819 many years ago when looking for what's termed a Be star. This is a star that rotates so rapidly that it nearly tears itself apart, and the outer object in this pairing is a good example.

But a series of circumstances meant the investigation was never carried through to completion - until very recently.

Studies using the 2.2m telescope at La Silla Observatory in Chile reveal the inner of the two visible stars to be orbiting an unseen object every 40 days.

Presumed to be a black hole, this object has a likely mass of at least four times that of our Sun.

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.

But statistics tell us there must be many, many more out there.

"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.

A paper describing the discovery is published in the journal Astronomy & Astrophysics.

## Statistically, what would the average distance of the closest black hole be? - Astronomy

Although radial distances will be given from the black hole in both Schwarzschild radii (R_S) and kilometers (km), please note that one cannot simply measure this distance with a series of meter sticks. This is because, for one reason, any meter stick closer than the event horizon could not be seen by an observer outside the event horizon. A better way of visualizing radial distance is to picture orbiting the black hole at a fixed distance, measuring the circumference of the orbit, and dividing by 2 pi.

Far from the black hole an undistorted night sky is visible with a very small patch of fuzz in the center. As the viewer nears the black hole the fuzzy patch becomes discernable as an unusual conglomeration of stellar images. Fig. 2a depicts the view visible from 1000 R_S (4200 km) away.

Fig. 2b shows the black hole from a distance of 100 R_S (420 km). From this distance the viewer begins to notice that no light comes from a circular patch in the direction of the black hole. The only light that could possible come to the viewer from this area would be from the black hole itself. Since here it is considered that the black hole emits no light [28], this area is dark. The angular size of the filled black circle is the angular size of the photon sphere of the black hole mass and can be found from the discussion in the Appendix.

Fig. 2c shows the black hole from a distance of 25 R_S (105 km). Here the angular size of the black hole has increased and the secondary images, which are inside the first sky Einstein ring, are now quite clearly discernable.

Fig. 2d shows the black hole from a distance of 10 R_S (42.0 km). Here the viewer should notice that the placements of stellar images near the black hole have changed greatly when compared to Fig. 2a. Stars nearest to behind the black hole from the observer now have two bright images. The brightest star in the illustration (and the sky: Sirius) can be seen to have two bright images: the brightest primary image in the field of view on the lower left and a secondary image 180 degrees across the face of the black hole from it. Primary and secondary images can always be matched up by connecting them with a Great Circle (a line on these figures) through the center of the black hole. Sirius is not the only star to have two distinct images, however. Notice that Betelgeuse and each star in the belt of Orion also has two bright images. Sirius and the stars in the belt of Orion have been labelled in Fig. 2d. In fact, all bright stars visible in the field have two bright images. Some dim stars that previously could not be seen now have been amplified by the gravitation of the black hole to exhibit observably bright images. In Fig. 2d, the first sky Einstein ring has been drawn in with a dashed line.

The first sky Einstein ring, shown in Fig. 2d, is an invisible circle centered on the black hole and dividing the first complete set of images (those angularly furthest from the disk of the black hole which lie between the zeroth and first Einstein rings) from the second complete set of images. Each image in the first set is always brighter than the corresponding image in the second set. The second sky Einstein ring appears in the conglomeration of stellar images near the apparent photon sphere position, just outside the photon sphere. A complete image of the sky can be seen between these two Einstein rings.

Note that typically stellar images get much dimmer as one looks closer to the apparent photon sphere position, but the average surface brightness of the sky there remains unchanged. In other words, if Fig. 2d was spun about the center of the black hole smearing all the star images into a blur, the inner regions near the apparent position of the photon sphere would appear to have the same average brightness as the outer regions near the edge of the illustration. This is a consequence of conservation of surface brightness discussed above.

The viewer now does an orbit around the black hole at the radius of 10 R_S (42 km). The distortions the viewer would see are shown in Figs. 2d - 2j. These figures depict viewing angles for relative angular positions of 0 degrees, 5 degrees, 10 degrees, 90 degrees, 180 degrees, 270 degrees, and 360 degrees around the orbit. A complete orbit would encompass, of course, 360 degrees and so Fig. 2j is the same as Fig. 2d.

Fig. 2e, showing a relative 5 degrees orbital angle compared to Fig. 2d, has several interesting differences with this figure. Stellar images nearest the first sky Einstein ring have shifted the most. These images represent stars that are closest to directly behind the black hole from the viewer. These images appear to move with the highest angular speeds. This is because a small angular (unlensed) step of the star from just to the left of behind the black hole from the observer to just to the right causes all of its images to move from one side of the Einstein ring to the other. Apparent angular speeds have no maximum limit. If one attributes a distance to the images they can even appear to exceed the speed of light. Note that images of the same star still appear 180 degrees across the face of the black hole from each other, and that the brighter image is outside the first Einstein ring, while the dimmer image is inside.

Remember, an entire single image of the sky is contained between the zeroth and first sky Einstein rings. It is therefore impossible for an image to leave this region - it cannot just "go" across this ring and end up between the first and second Einstein rings. Stars (in reality) approaching the nadir point below the black hole from the viewer (moving slowly) have images that appear to approach the Einstein ring and get very bright (moving rapidly), eventually receding from this Einstein ring and dimming.

After a complete orbit with the viewer always facing the black hole, the distortions are depicted by Fig. 2j, which is the same as Fig. 2d. This figure is included to provide continuity in the presented sequence.

Now the viewer will go even closer to the black hole. Fig. 2k shows the visible distortions from 3 R_S (12.6 km): at twice the distance of the photon sphere. Here the viewer is looking 45 degrees away from the black hole. Note the great number of clearly resolved secondary images visible near the black hole's limb.

The viewer now reaches the photon sphere and looks up from the black hole to peer directly along the photon sphere. Fig. 2l shows the distortions from this distance: 1.5 R_S (6.3 km). The viewer looks north. The self Einstein ring where viewers could see the backs of their heads is the photon sphere horizon line dividing the light captured by the black hole from the the light coming from the sky: it is a horizontal line across the middle of the figure. The first sky Einstein ring would be an invisible line about 2/9 of the way toward the top of the plot above the photon sphere. Since the viewer's location and orientation does not allow the whole face of the black hole to be visible, both of the bright images (the primary and secondary image) of a single star are not visible at the same time. Those stellar images highly amplified above the Einstein ring are different than those that appear highly amplified just below the Einstein ring. The primary images just above the Einstein ring in one direction will have their secondary image appear just below the Einstein ring in the opposite direction.

The viewer now starts along an orbit at the photon sphere, 1.5 R_S (6.3 km) from the black hole. The position of the first sky Einstein ring becomes more evident when comparing Figs. 2l, 2m, and 2n which have relative orbital angles of 5 degrees and 10 degrees.

The viewer now descends and looks directly away from the black hole. Fig. 2o shows the distortions from 1.1 R_S (4.62 km). All of the sky images are now compressed into a hole in the direction opposite the black hole.

Fig. 2p shows the distortions visible from 1.01 R_S (4.242 km) while looking directly away from the black hole. The black hole now encompasses almost the complete observer sky. The small hole at the top is what remains visible of the outside universe. In this hole there could appear, theoretically, an infinite number of complete images of the outside universe. The angular amplification A_angular of the vast majority of these images is, however, much less than unity: they are greatly deamplified.

Fig. 2p, as shown, is not an accurate depiction of the distortions a viewer would see in this position. It is included because part of it is correct and the part that is not is informative. The part that is correct is the depiction of the relative amounts of black hole and background sky that are visible. The thin fuzzy annular ring is not realistic, however, as the program plotted mostly just the positions of the secondary images. Only a handful of primary sky images are visible as most of them have suffered large angular deamplifications. As stated above, the secondary images are plotted by the program regardless of the amount of deamplification. The outer radial limit of the dim ring marks the position of the second sky Einstein ring. Stellar images that would be seen between there and the apparent photon sphere limb of the black hole would be even higher order images. This is the only figure where these images are noticeable by their absence. The programs were not set up to track these higher order images, and so they are not shown. References
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## This new stat measures which PGA Tour players REALLY hit it closest

Frank Nobilo and I were chatting earlier this year and he mentioned the pushback he received for quoting a proximity stat on air. I’m not sure of the exact circumstance, but it’s not hard to find an example that illustrates the point.

Paul Casey won the 2019 Valspar Championship by one stroke on a tough Innisbrook Copperhead course. Inside the top 10 were major winners Louis Oosthuizen, Bubba Watson and Dustin Johnson. For the week, Casey ranked 9th in driving accuracy, 6th in strokes gained off the tee and 9th in SG around the green. Yet he ranked 43rd in SG putting and, strikingly, 65th out of 70 players who made the cut in proximity to the hole (a measure of approach play). It’s hard—if not impossible—to win with poor putting and even worse approach shots. So how did Casey do it? More importantly, how could he have ranked 7th with such poor proximity to the pin numbers?

The answer is that his approach play was very good, but proximity as a measure of approach play was not. For the week, Casey’s average proximity was 41 feet 11 inches. The leader, Jim Furyk, clocked in at 31 feet. Ten feet farther from the hole on 72 approach shots is an enormous difference.

The problem with the proximity stat is that it doesn’t account for the initial distance of the approach. In round 4 on the par-5 12th, Furyk hit his third shot from 101 yards in the fairway to a cozy eight feet from the hole. On the same round and hole, Casey hit his second shot from 271 yards in the fairway to 98 feet. Casey’s proximity was recorded as 90 feet worse than Furyk’s, but he was putting for eagle! Players who go for more par-5 greens in two tend to look terrible in the proximity stat compared to players who lay up.

## Astronomers spot 'missing link' black hole - not too big and not too small

WASHINGTON (Reuters) - Scientists have detected a mid-size black hole - considered the “missing link” in the understanding of these celestial brutes - eviscerating an unfortunate star that strayed too close.

Using data from the Hubble Space Telescope and two X-ray observatories, the researchers determined that this black hole is more than 50,000 times the mass of our sun and located 740 million light years from Earth in a dwarf galaxy, one containing far fewer stars than our Milky Way.

Black holes are extraordinarily dense objects possessing gravitational pulls so powerful that not even light can escape.

This is one of the few “intermediate-mass” black holes ever identified, being far smaller than the supermassive black holes that reside at the center of large galaxies but far larger than so-called stellar-mass black holes formed by the collapse of massive individual stars.

“We confirmed that an object that we discovered originally back in 2010 is indeed an intermediate-mass black hole that ripped apart and swallowed a passing star,” said University of Toulouse astrophysicist Natalie Webb, a co-author of the study published this week in Astrophysical Journal Letters.

The star was probably roughly a third the mass of the sun, Webb said.

Webb said scientists have searched for intermediate-mass black holes for four decades and fewer than 10 good examples are known, though large numbers may exist.

“So finding a new one is very significant. Also, a black hole swallowing a star happens on average only once every 10,000 years or so in any particular galaxy so these are rare occurrences,” Webb added.

The supermassive black hole at the center of the Milky Way is 4 million times the mass of the sun and located 26,000 light years from Earth. The closest stellar-mass black star is about 6,000 light years from Earth. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

Webb called intermediate-mass black holes the “missing link” in understanding the range of black holes. Scientists know how stellar-mass black holes - roughly three to 100 times the mass of our sun - form. They do not know how intermediate-mass black holes form but suspect that supermassive black holes arise from their mid-size brethren.

“Without finding such objects, it was impossible to validate this theory,” Webb said.

Intermediate-mass black holes have remained elusive.

“The best explanation is that they are mostly in an environment that is devoid of gas, leaving the black holes with no material to consume and thus little radiation to emit - which in turn makes them extremely difficult to spot,” said University of New Hampshire astronomer and study lead author Dacheng Lin.

## Astronomers See Distant Eruption as Black Hole Destroys Star

For the first time, astronomers have directly imaged the formation and expansion of a fast-moving jet of material ejected when the powerful gravity of a supermassive black hole ripped apart a star that wandered too close to the cosmic monster.

The scientists tracked the event with radio and infrared telescopes, including the National Science Foundation’s Very Long Baseline Array (VLBA), in a pair of colliding galaxies called Arp 299, nearly 150 million light-years from Earth. At the core of one of the galaxies, a black hole 20 million times more massive than the Sun shredded a star more than twice the Sun’s mass, setting off a chain of events that revealed important details of the violent encounter.

Only a small number of such stellar deaths, called tidal disruption events, or TDEs, have been detected, although scientists have hypothesized that they may be a more common occurrence. Theorists suggested that material pulled from the doomed star forms a rotating disk around the black hole, emitting intense X-rays and visible light, and also launches jets of material outward from the poles of the disk at nearly the speed of light.

“Never before have we been able to directly observe the formation and evolution of a jet from one of these events,” said Miguel Perez-Torres, of the Astrophysical Institute of Andalusia in Granada, Spain.

The first indication came on January 30, 2005, when astronomers using the William Herschel Telescope in the Canary Islands discovered a bright burst of infrared emission coming from the nucleus of one of the colliding galaxies in Arp 299. On July 17, 2005, the VLBA revealed a new, distinct source of radio emission from the same location.

“As time passed, the new object stayed bright at infrared and radio wavelengths, but not in visible light and X-rays,” said Seppo Mattila, of the University of Turku in Finland. “The most likely explanation is that thick interstellar gas and dust near the galaxy’s center absorbed the X-rays and visible light, then re-radiated it as infrared,” he added. The researchers used the Nordic Optical Telescope on the Canary Islands and NASA’s Spitzer space telescope to follow the object’s infrared emission.

Continued observations with the VLBA, the European VLBI Network (EVN), and other radio telescopes, carried out over nearly a decade, showed the source of radio emission expanding in one direction, just as expected for a jet. The measured expansion indicated that the material in the jet moved at an average of one-fourth the speed of light. Fortunately, the radio waves are not absorbed in the core of the galaxy, but find their way through it to reach the Earth.

These observations used multiple radio-telescope antennas, separated by thousands of miles, to gain the resolving power, or ability to see fine detail, required to detect the expansion of an object so distant. The patient, years-long data collection rewarded the scientists with the evidence of a jet.

Most galaxies have supermassive black holes, containing millions to billions of times the mass of the Sun, at their cores. In a black hole, the mass is so concentrated that its gravitational pull is so strong that not even light can escape. When those supermassive black holes are actively drawing in material from their surroundings, that material forms a rotating disk around the black hole, and superfast jets of particles are launched outward. This is the phenomenon seen in radio galaxies and quasars.

“Much of the time, however, supermassive black holes are not actively devouring anything, so they are in a quiet state,” Perez-Torres explained. “Tidal disruption events can provide us with a unique opportunity to advance our understanding of the formation and evolution of jets in the vicinities of these powerful objects,” he added.

“Because of the dust that absorbed any visible light, this particular tidal disruption event may be just the tip of the iceberg of what until now has been a hidden population,” Mattila said. “By looking for these events with infrared and radio telescopes, we may be able to discover many more, and learn from them,” he said.

Such events may have been more common in the distant Universe, so studying them may help scientists understand the environment in which galaxies developed billions of years ago.

The discovery, the scientists said, came as a surprise. The initial infrared burst was discovered as part of a project that sought to detect supernova explosions in such colliding pairs of galaxies. Arp 299 has seen numerous stellar explosions, and has been dubbed a “supernova factory.” This new object originally was considered to be a supernova explosion. Only in 2011, six years after discovery, the radio-emitting portion began to show an elongation. Subsequent monitoring showed the expansion growing, confirming that what the scientists are seeing is a jet, not a supernova.

Mattila and Perez-Torres led a team of 36 scientists from 26 institutions around the world in the observations of Arp 299. They published their findings in the 14 June online issue of the journal Science. Data from the NSF’s Very Large Array (VLA) and Green Bank Telescope (GBT) were used for some of this work.

The Long Baseline Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

## Supermassive black hole

A supermassive black hole (SMBH or less often SBH) is a black hole with a mass that is between 10 5 and 10 10 the mass of the Sun. Scientists are confident that almost all galaxies, including the Milky Way, have supermassive black holes at their centers.

How supermassive black holes get started is not yet known. Astrophysicists agree that once a black hole is in place in the center of a galaxy, it can grow by attracting matter and by merging with other black holes. Formation of ordinary (star-sized) black holes from the deaths of the first stars has been extensively studied and supported by observations.

However, there appears to be a gap between stellar-mass black holes, and supermassive black holes.

Stellar-mass black holes, formed from collapsing stars, range up to perhaps 33 solar masses. The minimal supermassive black hole is in the range of a hundred thousand solar masses. Between these extremes there appear to be few intermediate-mass black holes. Such a gap would suggest that the two types were formed by different processes.

Observations show that quasars were much more frequent when the Universe was younger. Supermassive black holes of billions of solar masses had already formed when the Universe was less than one billion years old. This suggests that supermassive black holes arose very early in the Universe, inside the first massive galaxies.

Astronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center. [4] It is 26,000 light-years from the Solar System, in the direction of the constellation Sagittarius. The region is called Sagittarius A*, and the evidence for its being a black hole is:

1. The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 light hours ( 1.8 × 10 13 m or 120 AU) from the center of the central object. [5]
2. From the motion of star S2, the object's mass can be estimated as 4.1 million solar masses. [6]
3. The radius of the central object must be significantly less than 17 light hours, because otherwise, S2 would either collide with it or be ripped apart by tidal forces. In fact, recent observations indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit. [7]
4. Only a black hole is dense enough to contain 4.1 million solar masses in this volume of space.

The Max Planck Institute for Extraterrestrial Physics, and the UCLA Galactic Center Group, [8] have provided strong evidence that Sagittarius A* is the site of a supermassive black hole. [4] This is based on data from the ESO, [9] and the Keck telescope. [10] Our galactic central black hole is calculated to have a mass of approximately 4.1 million solar masses, [11] or about 8.2 × 10 36 kg.

## What is a black hole made of?

What do we know about these rather intense gravitational regions?

The simple answer is that we don’t know. A black hole is defined as a region of spacetime from which extremely strong gravity prevents anything, including light, from escaping.

We know that matter falling into black holes is no different from the matter which can be found lurking around the rest of the Universe. However, the closer we get to the centre of a black hole, the faster our understanding of physics breaks down. Thanks to General Relativity, we think we understand what happens in this extreme gravity and with the help of Quantum Mechanics, we can make an intelligent estimate as to what happens at smaller, microscopic scales. But if the two theories are combined – like they would be at the centre of a black hole – they break down, leaving us with no idea as to what’s going on!

To get around the problem, astrophysicists need a theory of gravity that is compatible with Quantum Mechanics that might just describe the physics inside a black hole. At the moment though, no such model exists but physicists are working on it.

Image credit: Alain Riazuelo / CC BY-SA 2.5

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They weren’t even looking for the black hole when they first, er, didn’t see it. They were studying double-star systems and noticed that one of them was orbiting an unseen object every 40 days, while the other orbited a large distance away.

They estimated the mass of the invisible object by observing its visible companion. At four times the mass of the sun, it could only be a black hole.

The discovery so close to Earth – though not near enough to pose any threat, it should be added – opens up the possibility of other black holes even closer to home.

We expect that there are about 100 million of these small black holes, and we’ve only found less than 100 of them.

“There must be a bunch of them closer … that we haven’t found yet,” said the ESO’s Dr. Marianne Heida. “Based on the number of stars in the Milky Way, we expect that there are about 100 million of these small black holes, and we’ve only found less than 100 of them.”

## Nearby black hole is feeble and unpredictable

IMAGE: The large image here shows an optical view, with the Digitized Sky Survey, of the Andromeda Galaxy, otherwise known as M31. The inset shows Chandra images of a small region. view more

Credit: X-ray (NASA/CXC/SAO/Li et al.), Optical (DSS)

For over 10 years, NASA's Chandra X-ray Observatory has repeatedly observed the Andromeda Galaxy for a combined total of nearly one million seconds. This unique data set has given astronomers an unprecedented view of the nearest supermassive black hole outside our own Galaxy.

Astronomers think that most galaxies - including the Milky Way - contain giant black holes at their cores that are millions of times more massive than the Sun. At a distance of just under 3 million light years from Earth, Andromeda (also known as M31) is relatively close and provides an opportunity to study its black hole in great detail.

Just like the one in the center of the Milky Way, the black hole in Andromeda is surprisingly quiet. In fact, Andromeda's black hole, known as M31*, is ten to one hundred thousand times fainter in X-ray light that astronomers might expect given the reservoir of gas around it.

"The black holes in both Andromeda and the Milky Way are incredibly feeble," said Zhiyuan Li of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass. "These two 'anti-quasars' provide special laboratories for us to study some of the dimmest type of accretion even seen onto a supermassive black hole."

The decade-long study by Chandra reveals that M31* was in a very dim, or quiet, state before 2006. However, on January 6, 2006, the black hole became more than a hundred times brighter, suggesting an outburst of X-rays. This was the first time such an event had been seen from a supermassive black hole in the nearby, local universe.

After the outburst, M31* entered another relatively dim state, but was almost ten times brighter on average than before 2006. The outburst suggests a relatively high rate of matter falling onto M31* followed by a smaller, but still significant rate.

"We have some ideas about what's happening right around the black hole in Andromeda, but the truth is we still don't really know the details," said Christine Jones, also of the CfA.

The overall brightening since 2006 could be caused by M31* capturing winds from an orbiting star, or by a gas cloud that spiraled into the black hole. The increase in the rate of material falling towards the black hole is thought to drive an X-ray brightening of a relativistic jet.

The cause of the outburst in 2006 is even less clear, but it could be due to a sudden release of energy, such as magnetic fields in a disk around the black hole that suddenly connect and become more powerful.

"It's important to figure out what's going on here because the accretion of matter onto these black holes is one of the most fundamental processes governing the evolution of galaxies," said Li, who presented these results at the 216th meeting of the American Astronomical Society meeting in Miami, FL.

These results imply that the feeble, but erratic behavior of the black hole in the Milky Way may be typical for present-day supermassive black holes.

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