# What is a black hole's “shadow” and the best angle to view it to measure the spin?

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This excellent answer mentions several ways to try to observationally measure the spin of a rotating black hole. The third one is intriguing, but I don't understand how this works:

• The spin of a black hole also affects how it deflects light. Consequently, the pictures of a black hole's shadow such as taken by the event horizon telescope can be used to determine the spin of the black hole (if we happen to view it under the right angle).

Question: What is a black hole's "shadow" and the best angle to view it to measure the BH's spin?

The black hole shadow is basically the image of the event horizon. As you know the event horizon is the geometric locus of the points from which a light ray pointing in the opposite side of the singularity (the center of the black hole) can't no longer escape from it. Any other light ray emmited in any other direction from those points would never reach an external observer since the one pointing directly outside the black hole was the one with the best chances to do so.

So we might think that because of the absence of light coming from these region inside the event horizon an external observer might see a black ball of 1 Schwarzschild radius (which is the physical size of the even horizon), but this is not correct. Even if the event horizon marks the physical boundary between both regions the fact is that due to the extreme light bending the actual image of the event horizon (its "shadow") is a distorted view of this surface. For a classical non-rotating black hole this "shadow" (we call a shadow the image created by the absence of light rays but we can trace it with hypothetical "dark rays" that behave in the same way) looks like a ball with 2.6 Schwarzschild radii in size. Much larger than the actual event horizon! To visualize this I can only point to this fantastic explanation by Derek Muller from whom I took this animation.

As you can see we are throwing light rays from infinity into the black hole (that's why they are all parallel in the beginning). Our light rays fall reach the event horizon even if they were not directly pointed there, because they curve. Since light paths can be reversed and the physics still hold we can talk about light rays coming from different parts of the event horizon and reaching the outside observer in the reversed paths. So as you can see not only light rays emmited just barely outside the event horizon pointing towards us will reach us but also rays coming from "the back" of the event horizon can reach us. And as you can see those rays would create an enlarged image of the event horizon since they seem to come from a region located farther than the event horizon itself. So when you look to a real black hole from outside you will see this "black shadow", which is a map reprojection of the surface of the true event horizon where you can see even the 100% of the surface of it from a single vantage point.

This phenomenon (which is called relativistic light deflection) is also noticiable in other compact objects like Neutron stars. The intense gravitational distortion around them allows for light rays coming from its bright surface to deflect when going outwards and reach your eye even if they where emmited in regions close to "the back of the neutron star". Even if that region of the star shouldn't be accesible to an observer if light rays moved in straight lines (since those parts of the surface lie behind the curve of the star) you still can see them (which is something that can mess with the calculations of their true brightness).

You can make a square grid on top of the surface of the neutron star and see how much of it you can see from far away in this representation:

As you can see we are able to see more than an hemisphere (more than 50% the surface of the star). In fact you can see both polar regions and their surroundings. Well this is the same thing happening in a black hole but in that case the reprojected map is all black (since the event horizon is a uniform featureless surface all around) and you can see the 100% of that surface not only a small extra percent.

Now, this all changes if the black hole is rotating. Why? Because of relativistic frame-dragging. According to Einstein field equations mass-energy does not only curve spacetime but is also able to "twist it" if the object is rotating. We have measured this "twisting" of the surrounding spacetime in our own planet using exquisite instruments onboard the GRACE satellites.

In our case the important thing is that a non-rotating black hole (a Schwarzschild black hole) has a region outside the event horizon where orbiting the black hole in a stable manner is possible, we call it the innermost stable circular orbit (or ISCO). Getting closer to ISCO makes your orbit unstable and you end up falling into the event horizon. But if the black hole is rotating (a Kerr black hole), then ISCO is different if your orbit is prograde (orbit in the same direction as the black hole rotation) or retrograde (goes in the opposite direction around the black hole) because frame-dragging alters the solution. If you orbit prograde the fact that spacetime is been dragged in the same direction allows you to have some push by the black hole and your orbit can be mantained even much closer to the event horizon in a stable manner. On the contrary if you orbit the black hole in the opposite direction you are fighting against the drag of spacetime and thus you will decay more easily, making the ISCO for retrograde orbits a lot higher than the ISCO for prograde orbits.

If you apply this reasoning to photons you can start to notice something interesting. Light coming from far away stars behind the black hole as viewed from an outside observed gets bent in different ways if it is coming from one side or the other due to this frame-dragging effect. If the light ray is coming parallel over the rotating surface of the black hole it is going to be helped by the black hole itself, and some of the angular momentum is going to be transfered to that light ray from the Kerr black hole. If instead the light ray comes anti-parallel to the rotation (which is going to happen in the other side of the black hole), then it might never reach the observer. This reasoning can be applied to the "dark rays" (which do not exist but is a way to trace the shadow which is the absence of light rays) coming from the event horizon and thus the shadow of the black hole is no longer a perfect black disk but an asymetric D-shaped black region, that tells you if the black hole is rotating clockwise or counter-clockwise.

In this animation you can see the appearance of the shadow of the black hole when we increase its rotational speed, as you can see it goes of-center and asymetric as we increase it.

Since frame-dragging goes as the mass rotates you can't notice it so strongly if you watch the black hole from another inclination. In fact the projected rotational speed if you watch a black hole from the poles ($$i = 0^circ$$) is zero, and thus the black hole would look just like a non-rotating one. Here you can see the dependance of the shape of the shadow (in red) of a Kerr black hole with a fixed rotational speed when you see it from different inclinations (from the equator to the poles). The event horizon is represented in blue (but remember, you don't see that, you only see the shadow)

As you can see the effect is the same in both cases (changing view angle for a fixed rotational speed vs. changing rotational speed for a fixed inclination), which means that you can't tell the real rotational speed of a black hole just by measuring the shape of the shadow (a disk like shadow could mean a non-rotating black hole or a Kerr black hole as viewed from above for example), but at least it gives you a minumim estimate for the rotation. To study the exact rotational speed we need some independent measurements like for example the inclination of a disk of material around it. In those cases you would have complete information about the angular momentum of the black hole.

Finally here you have a beautifull simulation on what you would see from orbit around a Kerr black hole (I don't know how to embed YouTube videos so… ).

## What is a black hole's &ldquoshadow&rdquo and the best angle to view it to measure the spin? - Astronomy

The Black Hole Engine. Note the black hole diagram to the left. The event horizon is the point of no escape. It is a theoretical mathematical sphere of zero width because in real black holes there is nothing there to see. It would be like entering a maximum speeding zone in a car, there is no hard boundary. But in this case, there is no way to turn back.

The "singularity" is at the center of the black hole where all inside materials head towards and whose density and gravity approaches infinitely. The appearance of a singularity in General Relativity indicates the limit of this theory. It illustrates places and/or conditions where the theory breaks down and one can not know the details of exactly what is happening.

All "rotating" black holes pull along a region of spacetime with it due to a phenomenon called Frame Dragging. General Relativity predicts that a fast rotating object will drag any nearby object along with it. The region where it is not possible for an object to remain stationary is called the "ergosphere". Its shape is that of an oblate spheroid - bulging at the equator, and flattened at the poles of the rotating black hole. Since the ergosphere is outside the event horizon, it's possible for an object within it to escape the black hole. When it does, it will leave with extra energy accumulated from the spinning black hole. Not all scientists believe that the ergosphere is real, it may be just a mathematical entity.

The red ring above labeled the photon sphere is also called the photon ring. The photon sphere is also called the innermost stable circular orbit (ISCO). This is another mathematical theoretical boundary. As the gas and other material in the accretion disk spin closer and closer to the black hole, they travel faster and faster approaching the speed of light. The photon sphere is the point where only light photons can can travel fast enough at the full speed of light to continuously orbit the black hole. Again, this is a theoretical radius equal to one and a half times the event horizon radius - the Schwarzschild Radius.

In real life even photons would have a tough time orbiting the ISCO for any length of time because they would eventually collide with another photon and either travel off into the future or fall into the event horizon and be lost. Therefore the photon sphere is in real life an unstable reference point. However it is very useful for mathematical calculations of the innermost ring of the accretion disk. The important take away here is that there is "space" between the black hole and the accretion disk which we shall discuss further below. See Inner Edge Accretion Spacing .

Model Of A Supermassive Black Hole. Plasma gas (electrons and protons), falling very close to a black hole, orbits it and accumulates into a flattened Accretion Disk. The gas in the disk spirals inward, becoming compressed and heated as it nears the center. Ultimately reaching temperatures up to 20 million degrees Fahrenheit (12 million C), the gas shines brightly in low-energy "soft" x-rays.

Over 40 years, observations show that black holes also produce "hard" x-rays with energy tens to hundreds of times greater than the soft x-rays. This implies the presence of a much hotter gas above and below the disk, the Corona, with temperatures reaching billions of degrees. The Corona is believed to be the source of the "hard" x-rays and gamma rays. This very hot gas is a phenomenon similar to the hot corona that surrounds our sun.

Keep in mind though, a Corona has never been directly detected and we have no real knowledge of what a Corona is shaped like or what its composition is. Most scientists believe Coronas are real because of the "hard" x-rays and gamma rays that are somehow being generated. Many astro-physicists think the Corona is the source of jets, but at this point it is speculation. Only about 10% of black holes have jets.

Around the edge of a super-massive black hole there is a huge ring of very dusty gas called a "Torus". The Torus is very dense and lumpy. If one is looking at a black hole edge-wise, all you can see is the Torus as it is so large it blocks the view of the actual black hole. One has to be located on quite an angle to see over the Torus to witness the black hole itself. See the NASA artists' rendition of NGC 1068 to the left below as a result of NuSTAR, the very high x-ray/gamma ray satellite, taking very detailed images of the NGC 1068 Torus. The Torus prevents many black holes from being seen from earth as they are edge on to us. However, they can still be detected by x-rays.

Between the Torus and the Accretion Disk lies a region called the Broad Line Region (BLR). It consists of clouds of swirling gases that are not part of the Accretion Disk, but are dense enough to radiate broad optical emission lines. These come from cold material close to the Accretion Disk. The lines are broad because the emitting material is revolving around the black hole with high speeds causing a range of Doppler shifts of the emitted photons.

Outside of the Torus there exists more cold swirling dust clouds. This area is called the Narrow Line Region (NLR) because they emit narrow optical lines.

There is no single signature of a black hole. The features described here cover the most important features that characterise a black hole. Top

## How are Black Holes Used in the Movies?

I mean, is the spaghettification of John Cusack using awesome 2012 doomsday graphics too much to ask? Instead of an improbable alien spacecraft appearing over the White House, why not use a black hole, producing so much tidal shear that it rips the building apart brick-by brick? Oh, and then have all the matter being sucked into the black hole accelerate to relativistic velocities, creating an X-ray belching accretion disk, lighting up the solar system with our planet’s regurgitated mass-energy? Movie audiences will have a total doomgasm over that!

Or we could just use it as a nifty time travel device.

*I just saw this on Graph Jam, had a giggle. More sci-fi black holes please!

## 2 Summary of Millimeter-VLBI Observations

Figure 1: Locations in the u – v plane of the visibilities observed during the 2007, 2009.95, 2009.96, 2009.97 epochs. Also shown are the combined set of observations. Finally, for references the combined set is compared to the potential baselines from existing and upcoming sub-mm telescopes. Each baseline is color-coded according to the associated two sites. In all plots, detections are denoted by green circles and upper-limits by red triangles.

In the analysis presented here we make full use of the recent observations described in Fish et al. (2010) and Doeleman et al. (2008) . In both cases, observations targeting Sgr A* were made at 1.3 m m using the Submillimeter Telescope (SMT) on Mt. Graham in Arizona, 10 m dishes in the Combined Array for Research in Millimeter-wave Astronomy (CARMA) at Cedar Flat, California, and the James Clerk Maxwell Telescope (JCMT) located on Mauna Kea, Hawaii.

### 2.1 April 2007

Doeleman et al. (2008) report upon measurements obtained on the nights of the April, 11 & 12, 2007, using the JCMT, SMT and a single CARMA dish. 19 visibility amplitudes were obtained on the CARMA–SMT and JCMT–SMT baselines, with an upper limit on April 11th, 2007 along the JCMT–CARMA baseline. The locations of these observations on the u – v plane are indicated in the lower-left panel of Figure 1 , labeled 2007. Signal-to-noise ratios typical of the short and long baselines are 8 and 4, respectively.

During this time, observations the single-dish flux was estimated via the full CARMA array, operating as a stand-alone instrument, to be 2.4 ± 0.25 J y . This is similar to the visibility amplitudes obtained on the CARMA–SMT baselines and consistent with a single, compact gaussian component (Doeleman et al., 2008) . This flux is anomalously low in comparison to the typical 1.3 m m flux of ∼ 3 J y , and was taken as evidence for Sgr A* appearing in a quiescent state. This interpretation is supported by the lack of a significant difference between analyses of each day separately (Broderick et al., 2009) .

Full details of the observations, calibration and data processing can be found in Doeleman et al. (2008) .

### 2.2 April 2009

Fish et al. (2010) report upon more recent observations performed on the nights of April, 5–7, 2009, corresponding to the 95, 96, and 97 days of 2009. These made use of the JCMT, SMT, and two CARMA dishes, operated as independent VLBI stations. 54 visibility amplitudes were obtained on JCMT–SMT and CARMA–SMT baselines on all days, and to both of the JCMT–CARMA baselines on days 96 and 97. Positions of the observations on each day are indicated in the upper panels of Figure 1 , labeled 2009.95, 2009.96, and 2009.97. Signal-to-noise ratios typical of the short and long baselines are 17 and 5, respectively. Thus, this second data set represents a significant improvement in both the number and precision of the data obtained.

In addition to the VLBI baselines, the presence of two independent CARMA dishes in the array allowed the measurement of very-short baseline visibilities, probing angular scales ∼ 10 ′ ′ . These found substantially more correlated flux density than the CARMA–SMT baselines did, inconsistent with a single compact gaussian component. The interpretation of the difference in correlated flux density between the inter-CARMA baselines and the CARMA–SMT baselines is presently unclear, and it may be possible for multiple geometric models (e.g., annular rings, extended double source) to fit the data. Within the context of our analysis, we will assume that this difference is due to a separate large-scale component not present during the 2007 observations. This is indirectly supported by the fact that the source sizes inferred from the mid and long baseline data are unchanged despite the variations in the visibility magnitudes (Fish et al., 2010) . Therefore, we do not consider the inter-CARMA data further here, restricting ourselves to modeling the compact component observed with the longer baselines.

On days 95 and 96 the short-baseline flux densities are consistent with each other, with inferred single dish fluxes of 2.15 ± 0.06 J y , which while somewhat lower than those obtained in 2007, justify treating these as a similar quiescent period. This is not the case for day 97, which exhibited a 30 % – 40 % increase in the luminosity of the compact component. Note that during the 3 h r observing periods on days 96 and 97 there is no evidence for rapid changes in the CARMA–SMT visibility amplitudes, implying that during each Sgr A* was stable i.e., the process responsible for the brightening occurred between observing periods and is stable on timescales of hours. As a consequence, we will treat the visibilities obtained on each day as due to a stationary source, though with properties that vary from day to day.

Full details of the observations, calibration and data processing can be found in Fish et al. (2010) .

### 2.3 Combined Data Set

Combined, the 2007 and 2009 mm-VLBI measurements may be separated into 4 observational epochs: that containing the entire 2007 observations (2007), those on day 95 of 2009 (2009.95), those on day 96 of 2009 (2009.96), and those on day 97 of 2009 (2009.97). The combined coverage in the u – v plane is shown in the lower-middle panel of Figure 1 . The long baselines (JCMT–CARMA and JCMT–SMT) are oriented primarily in the east-west direction, extended roughly 3.6 G λ . Nevertheless, the combined data set also extends roughly 2 G λ in the north-south direction, providing substantial angular coverage in the u – v plane for the first time.

In Section 7 we will discuss the implications out analysis has for future observations. However, we note here that the baselines considered in the 2007 and 2009 mm-VLBI experiments are a small subset of the baselines that are possible with existing mm and sub-mm telescopes. Figure 1 shows the combined visibility data set in comparison to baselines associated with other potential mm-VLBI stations. These include stations in Chile (e.g., the Atacama Pathfinder EXperiment, Atacama Submillimeter Telescope Experiment, and Atacama Large Millimeter Array APEX, ASTE, and ALMA, respectively), Mexico (Large Millimeter Telescope LMT), Spain (Pico Veleta PV), France (Plateau de Bure PdB), and at the South Pole (South Pole Telescope SPT). These both, extend the region covered in the u – v plane, and provide additional complementary short and intermediate baselines, primarily along the north-south directions. To date, visibilities on only a handful of potential baselines have been measured.

## The electromagnetic counterparts of compact binary mergers

### 2.3 Ejection processes and properties of the sub-relativistic ejecta from BH–NS mergers

The dynamics of a BH–NS merger starts very differently than that of a BNS merger, with the following sequence of events. At the tidal radius of the BH, the NS is totally disrupted by tidal forces to form a single tidal arm, a small fraction of which is ejected. If the tidal radius is outside of the BH innermost stable circular orbit (ISCO), then part of the ejected mass is unbound, and it forms the dynamical ejecta. The other part is sent into bound orbits and falls back after some time. Almost the entire NS material falls into the BH within a few ms and a small fraction of the material remains outside of the BH, forming a disk within ∼ 10 ms . The BH-disk system and its secular evolution are rather similar to those of the disk that is formed following a BNS merger.

The main parameter that determines the properties of the ejecta is the location of the tidal radius with respect to the ISCO. When the tidal disruption of the NS takes place within the ISCO almost all of its material falls directly into the BH and there is almost no dynamical ejecta and no disk. When the disruption takes place outside the ISCO, a significant amount of mass is ejected. The location of the tidal radius depends mostly on the NS radius (and thus on the NS EOS), where a larger NS radius implies a larger tidal radius (there is also a weak dependence on the BH/NS mass ratio). The location of the ISCO increases with the BH mass and decreases with its spin if it is aligned with the orbital angular momentum. Thus, for a given NS radius there is a maximal BH mass above which there is no significant ejecta. The larger the BH spin component that is aligned with the orbital angular momentum of the binary, the larger the value of this maximal mass. Fig. 4 shows the results of Foucart (2012) for the dependence of the total mass that remains outside of the BH after ∼ 10 ms (i.e., tidal ejecta + disk) as a function of the NS radius, BH spin and BH/NS mass ratio.

The properties of the ejecta as a function of the NS EOS, the mass ratio, and the spin magnitude and orientation, were explored numerically in GRMHD simulations (e.g. Foucart, 2012 Foucart et al., 2013, 2014 Kawaguchi et al., 2015 Kawaguchi et al., 2016 Kiuchi et al., 2015 Kyutoku et al., 2015 Foucart et al., 2018 ). These simulations follow the merger and the post-merger evolution until the formation of the disk (at least 10–20 ms after the merger). The results of the various studies are in general agreement, probably because the evolution during the first 10 ms depends mostly on gravitational forces with minor effects of magnetic fields and neutrinos. The main result is that when the tidal disruption takes place well outside of the ISCO, the mass of the dynamical ejecta is ∼ 0 . 01 − 0 . 1 M ⊙ and the disk mass is ∼ 0 . 1 − 0 . 3 M ⊙ . When the tidal disruption takes place within the ISCO, both the dynamical ejecta and the disk mass are very small < 1 0 − 3 M ⊙ .

Fig. 4 . Left: The BH spin parameter and BH–NS mass ratio for which the remnant mass (i.e., disk mass + dynamical ejecta) is 10% of the NS mass. The BH spin is fully aligned with the orbital angular momentum of the binary. Each line represents a different NS radius assuming a NS mass of 1 . 4 M ⊙ . Right: The mass of the remnant as a function of the BH spin parameter and the BH–NS mass ratio from a NS with a radius of 11.5 km (consistent with the constraints found based on observations of GW170817, see Section 6.6.2 ).

The dynamical ejecta has a highly anisotropic distribution. It is thrown out in the shape of a thin fan concentrated along half of the equatorial plane, namely with an azimuthal opening angle of about 180° and a vertical opening angle of about 20°. The ejected material is highly neutron rich, Y e ≲ 0 . 1 and therefore it contains almost only heavy r -process elements. The ejecta velocity depends on the depth of the potential well at the location of the disruption. Thus, high-spin, high-mass, BHs produce the fastest ejecta (as long as the disruption takes place outside of the ISCO). Typical velocities are in the range of 0 . 1 − 0 . 4 c.

The secular evolution of the disk is expected to be similar to the one that follows a BNS merger when the central object collapses to a BH. At first, during the phase of efficient accretion, up to 20% of the disk mass is ejected in a high latitude wind at a velocity of about 0 . 1 − 0 . 15 c. Later, during the inefficient accretion phase, the mass that remains in the disk, which is about 20% of the disk’s original mass, is expelled as a roughly spherical wind at a velocity of about 0.05 c. Simulations of a wind from a BH-disk system typically find an outflow that is less neutron-rich than the dynamical tidal outflow, but the electron fraction is still low, Y e ∼ 0 . 1 − 0 . 3 (e.g., Siegel and Metzger, 2018 Fernández et al., 2019 Christie et al., 2019 ), although some high Y e mass may be ejected at high latitudes ( Miller et al., 2019 Christie et al., 2019 ) (see the discussion in Section 2.1.2 for more details).

An astronomical unit of measurement equivalent to the distance light travels in a year, which is approximately 9.461 x km.

A group of more than 54 galaxies, including the Milky Way, surrounding a gravitational center located in a region between the Milky Way and the Andromeda Galaxy.

When the moon passes into the Earth’s shadow. A total lunar eclipse occurs when the moon passes into the umbra, or inner region of the Earth’s shadow. A partial lunar eclipse occurs when the moon passes into the penumbra, or the outer region of the Earth’s shadow.

## The world gets its first look at a black hole. Here’s how we got there

First image of a Black Hole | European Southern Observatory (eso.org)

Bengaluru: An international team of scientists unveiled the first-ever image of the silhouette of a black hole in a press conference Wednesday. The black hole that was imaged using the Event Horizon Telescope radio telescope network is of the ‘supermassive’ type, located in the centre of the Messier 87 galaxy. It is 6.6 billion times the mass of the sun, at 53.5 million light years from Earth, and is 40 billion kilometres in diameter.

As even light doesn’t escape from a black hole, directly capturing a photo of it is impossible. Instead, what we have is an image of the glowing disc of light wrapping itself around the black hole, revealing the outline of its structure. While this isn’t a true photograph of the actual black hole, it is as close as one can get to calling it that.

The astronomers published their results in six papers in a special edition of The Astrophysical Journal Letters.

“We have taken the first picture of a black hole,” said EHT project director Sheperd S. Doeleman of the Center for Astrophysics, Harvard & Smithsonian in an accompanying press release. “This is an extraordinary scientific feat accomplished by a team of more than 200 researchers.”

### The insides of a space vacuum

Black holes are often depicted as giant monstrous vacuum cleaners of the universe — they suck everything and not even light can escape. This is true when matter approaches the boundary of a black hole, called the ‘event horizon’. The radius of the event horizon is called the Schwarzschild radius, and is effectively the radius of the black hole. Once something crosses the event horizon and falls into a black hole, it cannot escape. This is because black holes are singularities: Known laws of physics break down inside them.

According to Einstein’s theory of relativity, black holes spin so fast and are so massive that they distort the space-time fabric around them, as if everything was divided by zero. These relativistic effects play out only when matter approaches a black hole physically. Should a black hole be swapped out for a star of equal mass, nothing that’s in orbit around it would change, although the physical size of the black hole would be much more compact than that of the star.

A black hole is essentially a lot of matter packed into a very small area, thus carrying an enormous amount of gravity. It is so densely compact that if a human should stand at the event horizon, the gravitational force on their feet would be millions of times greater than that felt at their heads. If this human should fall in, she will undergo an almost comically terrifying process: The difference in gravitation will slowly stretch her feet out, then her legs, then her torso, neck, and then head. This is scientifically called ‘spaghettification’ and would all happen in a matter of microseconds. And one wouldn’t know what happens afterward.

### Different sizes

Black holes come in different ranges of mass and size. The smallest are the theorised micro black holes, with a mass approximately the same as that of the Earth’s moon, and a pin-point sized radius of 0.1 millimetre. These were primordial black holes that are thought to have existed in the moments just after the Big Bang.

Then come ‘stellar’ black holes, which are created by the collapse of stars into themselves. These have a mass of about 10 to 100 times the sun’s, and a radius of about 30 km.

Beyond this, there are ‘intermediate-mass black holes’, with masses ranging from 100 times to 100,000 times that of the sun packed into about the same radius as Earth.

Then there are the most massive kind, called the supermassive black holes (SMBH). One SMBH can have a mass of thousands to billions of times that of the sun, all held tightly together within a radius of few thousands to millions and up to billions of kilometres. Such SMBHs are not uncommon pretty much every galaxy is thought to have one in its centre, around which everything in the galaxy revolves, much like the sun in the solar system.

Our Milky Way galaxy has an SMBH right in the centre too, around which all stars and their planetary systems revolve. It is present in the general direction between the constellations Sagittarius and Scorpio in the sky. When it was discovered, it was classified as an “exciting” source of radio waves, and thus named Sagittarius A* — excited states of atoms are denoted with an asterisk. It is commonly shortened to Sgr A*, pronounced ‘Sagittarius A star’.

Sgr A* is 25,000 light years from Earth, has a mass about 4.3 million times that of the sun, and a radius of 30 million km. For comparison, Mercury, the nearest planet to the sun, is about 46 million km away from it.

### The M87 black hole

The image released Wednesday is of a much farther black hole, located inside one of the most massive galaxies near us, Messier 87. Shortened to M87, this bright galaxy can be seen in the Virgo cluster of galaxies as a nebulous cloud. It is 53.5 million light years away from us, and is known for the large jets of material it ejects, spreading out for a distance of 5,000 light years.

M87 is bright and easy to spot in radio wavelengths, and is also a popular target with both amateur and professional astronomers for being one of the largest, most massive observable galaxies.

Bang in the centre of the M87 galaxy is an SMBH that has a mass 6.6 billion times that of the sun, unofficially nicknamed M87* at the press conference by EHT Science Council chairperson Heino Falcke of Radboud University. While it is nearly 2,000 times farther away from us as Sgr A*, it is also 2,000 times more massive. Thus, both black holes appear approximately to be the same size to the EHT telescopes.

However, the distance meant that M87 also spun slower compared to Sgr A*, thus providing for more stable observation.

Sgr A* was also observed by the EHT, back in 2016, and the direct radio observations made seem to still be under processing. The EHT had started observing the M87 black hole later, in 2017.

There are other black holes of various masses scattered across our galaxy. Thankfully, the nearest black hole to us is 1,600 light years away and doesn’t concern us.

### Peering at dark giants

The M87 black hole looks less like something out of Interstellar, and more like something out of another science fiction film, Arrival. The coffee-stain pattern actually perfectly depicts the black hole.

Because black holes are so elusive, owing to no electromagnetic radiation escaping from them, they have been hard to find and locate through optical telescopes. However, because they exert gravitational pull, their effect on the matter surrounding them is visible.

Credit: ESO, ESA/Hubble, M. Kornmesser/N. Bartmann

Several SMBHs are surrounded by an accretion disk or matter that is being pulled in towards the gargantuan black hole. This is a disk of swirling dust and gas, orbiting outside the event horizon and feeding the black hole. The disk is an extremely energetic environment with strong magnetic fields, which cause the gas in the region to heat up as it spins around the black hole on its way in. The hot accretion disk emits light (or photons) of varying wavelengths, making the whole disk glow white hot as the light dances around the black hole.

“Thus, even though a black hole itself cannot be seen, its outline or silhouette can,” explains Abhijeet Borkar, astrophysicist at the Czech Academy of Sciences. Borkar’s research is on accretion around SMBHs, mainly Sgr A*, using radio interferometry. He is not connected with the EHT findings.

Photons can actually orbit much closer to the event horizon than the accretion disk does because they do not have mass. However, their orbits are almost always unstable. They eventually either fall into the black hole or are ejected outwards at great speeds, sometimes traveling towards us, like in this case.

The photons coming from the accretion disk are still affected by the gravity of the black hole despite not falling in. This causes the light to warp and distort as it emanates from the outer regions near the black hole. Such a process is called gravitational lensing. This kind of bending of light is one of the ways the presence of gravitationally strong bodies like SMBHs can be detected.

Observing the bending of light is possible only when traveling photons are far enough away from the event horizon to escape the deathly pull of the black hole and warp around it. So the ‘shadow’ or the dark area inside the lit image is actually much bigger than the black hole itself — 2.6 times bigger to be precise.

When observed dead-on, as in the case of the M87 black hole, the accretion disk is perpendicular to our line of sight,like a mystical halo around a deity’s head, and we get a perfect outline of the black hole’s shape as light travels around it and comes to us. When observed at an angle, where the accretion disk is probably angular or even on the same plane as our line of sight, light will still travel around the black hole, illuminating the accretion disk behind the black hole and showing it in a sort of bent manner above and below, like the Saturn-like black hole from Interstellar.

Caption: Gargantua, the black hole from the movie Interstellar. Credit: Interstellar/Paramount Pictures

Observing the entirety of the accretion disk face-on means that the M87 SMBH image shows a clean shadow in the centre. The scientists also calculated that the radius of the black hole is 20 billion km.

Light also undergoes the Doppler effect. So when photons travel towards us as they spin around a black hole, they move faster and thus appear much brighter. As they go around to the other side, they appear dimmer. This is why the coffee-mug stain is asymmetric and brighter on on the bottom as the material and light travel in our direction while revolving around the black hole.

Sometimes, active black holes even emit ‘relativistic jets’ — jets of supercharged ions expelled outwards close to the speed of light. Scientists still have no idea what causes these jets and how they behave. These jets are characteristic of the M87 galaxy, but were not observed in this session.

### Event Horizon Telescope

This diagram shows the location of the telescopes used in the 2017 EHT observations of M87.

The EHT is not one telescope but an array of eight networked telescopes spread across the globe. It is an international collaboration involving multiple countries and institutions. The purpose of the project was to obtain the first image of a black hole.

All of the telescopes combined effectively act as a single earth-sized telescope, which now has enough power to peek at the event horizon of a black hole.

The EHT telescopes are all radio telescopes, as a black hole and its surroundings cannot be optically photographed. Radio signals arrive in the form of waves and numbers to them. The telescopes looked at the M87 SMBH on four different days. All of the signals from the different telescopes are then processed using a technique called very-long-baseline interferometry (VLBI), and the numbers converted to an image.

VLBI is a type of ‘interferometry’, where multiple telescopes capture the same image, have their different images superimposed, and then information extracted from the interference pattern produced. The EHT team used two different algorithms and ended up with matching results.

“One of the main reasons we’re observing at radio is that radio VLBI is the only way we can get this angular resolution,” explains Borkar. VLBI allowed EHT to achieve an angular resolution high enough to read a newspaper in New York from a café in Paris, according to the press release.

Radio interferometry allows astronomers to see through the dust and gas cloud surrounding a black hole, and peek at the innermost stable orbit of the accretion disk, up to the point just before the abyss of the event horizon.

The telescopes contributing to this result were ALMA, APEX, the IRAM 30-metre telescope, the James Clerk Maxwell Telescope, the Large Millimeter Telescope Alfonso Serrano, the Submillimeter Array, the Submillimeter Telescope, and the South Pole Telescope. Petabytes of raw data from the telescopes were combined by highly specialised supercomputers hosted by the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory.

The data generated by these telescopes was unprecedented. In one night, the EHT produced as much data as the Large Hadron Collider does in a year, according to some reports. Data collected per session, across all telescopes for one observation, was over 7 petabytes and the rates of data recording at the telescopes were over 16 gbps.

Initial findings show that the black hole spins in a clockwise direction.

### Significance of the findings

This photograph is a measure of the progress in the human understanding of black holes. Just over 100 years ago, black holes were theorised to be a solution for Einstein’s field equations, a set of 10 equations in general relativity that describe gravitation in a curved space time. The definition of a black hole was then very hazy.

Large masses tend to warp space-time but if too much matter or energy was concentrated in too small an area, space-time would collapse, causing a singularity. Einstein couldn’t tell if such singularities could actually exist in reality.

“But in the 1930s, when Subrahmanyan Chandrasekhar put forward his famous paper on the Chandrasekhar Limit in white dwarfs, it became apparent that black holes might be real,” says Borkar.

Physicist Chandrasekhar showed in his paper that stars above a specific mass, 1.44 times that of the sun, would be too unstable to continue forever. In the last stages of their lives, they eventually collapse into themselves forming dense stellar remnants such as neutron stars or black holes, depending on their mass.

Further observations and future telescopes should help us slowly understand the questions posed by general relativity and the processes that occur at these scales of gravitation.

“We are seeking answers to several questions that would form the basis of our understanding of black holes,” elaborates Borkar.

“What are the effects predicted by general relativity near the black hole? Will we need something beyond general relativity to explain what we see? Do all black holes spin? What causes these powerful relativistic jets and how are they launched? We are just beginning to understand the physics of such powerful objects.”

This article has been updated with new information from the six papers released on 10 April.

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## Contents

Supermassive black holes are classically defined as black holes with a mass above 0.1 million to 1 million M . [9] Some astronomers have begun labeling black holes of at least 10 billion M as ultramassive black holes. [10] [11] Most of these (such as TON 618) are associated with exceptionally energetic quasars. Even larger ones have been dubbed stupendously large black holes (SLAB) with masses greater than 100 billion M . [12] Although they noted there is currently no evidence that stupendously large black holes are real, they noted that supermassive black holes almost that size do exist. [13] Some studies have suggested that the maximum mass that a black hole can reach, while being luminous accretors, is of the order of

Supermassive black holes have physical properties that clearly distinguish them from lower-mass classifications. First, the tidal forces in the vicinity of the event horizon are significantly weaker for supermassive black holes. The tidal force on a body at the event horizon is inversely proportional to the square of the mass: [16] a person on the surface of the Earth and one at the event horizon of a 10 million M black hole experience about the same tidal force between their head and feet. Unlike with stellar mass black holes, one would not experience significant tidal force until very deep into the black hole. [17] In addition, it is somewhat counterintuitive to note that the average density of a SMBH within its event horizon (defined as the mass of the black hole divided by the volume of space within its Schwarzschild radius) can be less than the density of water. [18] This is because the Schwarzschild radius is directly proportional to its mass. Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, the density of a black hole is inversely proportional to the square of the mass, and thus higher mass black holes have lower average density. [19]

The radius of the event horizon of a supermassive black hole of

1 billion M is comparable to the semi-major axis of the orbit of planet Uranus. [20] [21]

The story of how supermassive black holes were found began with the investigation by Maarten Schmidt of the radio source 3C 273 in 1963. Initially this was thought to be a star, but the spectrum proved puzzling. It was determined to be hydrogen emission lines that had been red shifted, indicating the object was moving away from the Earth. [22] Hubble's law showed that the object was located several billion light-years away, and thus must be emitting the energy equivalent of hundreds of galaxies. The rate of light variations of the source dubbed a quasi-stellar object, or quasar, suggested the emitting region had a diameter of one parsec or less. Four such sources had been identified by 1964. [23]

In 1963, Fred Hoyle and W. A. Fowler proposed the existence of hydrogen burning supermassive stars (SMS) as an explanation for the compact dimensions and high energy output of quasars. These would have a mass of about 10 5 – 10 9 M . However, Richard Feynman noted stars above a certain critical mass are dynamically unstable and would collapse into a black hole, at least if they were non-rotating. [24] Fowler then proposed that these supermassive stars would undergo a series of collapse and explosion oscillations, thereby explaining the energy output pattern. Appenzeller and Fricke (1972) built models of this behavior, but found that the resulting star would still undergo collapse, concluding that a non-rotating 0.75 × 10 6 M SMS "cannot escape collapse to a black hole by burning its hydrogen through the CNO cycle". [25]

Edwin E. Salpeter and Yakov Zeldovich made the proposal in 1964 that matter falling onto a massive compact object would explain the properties of quasars. It would require a mass of around 10 8 M to match the output of these objects. Donald Lynden-Bell noted in 1969 that the infalling gas would form a flat disk that spirals into the central "Schwarzschild throat". He noted that the relatively low output of nearby galactic cores implied these were old, inactive quasars. [26] Meanwhile, in 1967, Martin Ryle and Malcolm Longair suggested that nearly all sources of extra-galactic radio emission could be explained by a model in which particles are ejected from galaxies at relativistic velocities meaning they are moving near the speed of light. [27] Martin Ryle, Malcolm Longair, and Peter Scheuer then proposed in 1973 that the compact central nucleus could be the original energy source for these relativistic jets. [26]

Arthur M. Wolfe and Geoffrey Burbidge noted in 1970 that the large velocity dispersion of the stars in the nuclear region of elliptical galaxies could only be explained by a large mass concentration at the nucleus larger than could be explained by ordinary stars. They showed that the behavior could be explained by a massive black hole with up to 10 10 M , or a large number of smaller black holes with masses below 10 3 M . [28] Dynamical evidence for a massive dark object was found at the core of the active elliptical galaxy Messier 87 in 1978, initially estimated at 5 × 10 9 M . [29] Discovery of similar behavior in other galaxies soon followed, including the Andromeda Galaxy in 1984 and the Sombrero Galaxy in 1988. [4]

Donald Lynden-Bell and Martin Rees hypothesized in 1971 that the center of the Milky Way galaxy would contain a massive black hole. [30] Sagittarius A* was discovered and named on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown using the Green Bank Interferometer of the National Radio Astronomy Observatory. [31] They discovered a radio source that emits synchrotron radiation it was found to be dense and immobile because of its gravitation. This was, therefore, the first indication that a supermassive black hole exists in the center of the Milky Way.

The Hubble Space Telescope, launched in 1990, provided the resolution needed to perform more refined observations of galactic nuclei. In 1994 the Faint Object Spectrograph on the Hubble was used to observe Messier 87, finding that ionized gas was orbiting the central part of the nucleus at a velocity of ±500 km/s. The data indicated a concentrated mass of (2.4 ± 0.7) × 10 9 M lay within a 0.25″ span, providing strong evidence of a supermassive black hole. [32] Using the Very Long Baseline Array to observe Messier 106, Miyoshi et al. (1995) were able to demonstrate that the emission from an H2O maser in this galaxy came from a gaseous disk in the nucleus that orbited a concentrated mass of 3.6 × 10 7 M , which was constrained to a radius of 0.13 parsecs. Their ground-breaking research noted that a swarm of solar mass black holes within a radius this small would not survive for long without undergoing collisions, making a supermassive black hole the sole viable candidate. [33] Accompanying this observation which provided the first confirmation of supermassive black holes was the discovery [34] of the highly broadened, ionised iron Kα emission line (6.4 keV) from the galaxy MCG-6-30-15. The broadening was due to the gravitational redshift of the light as it escaped from just 3 to 10 Schwarzschild radii from the black hole.

On April 10, 2019, the Event Horizon Telescope collaboration released the first horizon-scale image of a black hole, in the center of the galaxy Messier 87. [2]

In February 2020, astronomers reported that a cavity in the Ophiuchus Supercluster, originating from a supermassive black hole, is a result of the largest known explosion in the Universe since the Big Bang. [35] [36] [37]

In March 2020, astronomers suggested that additional subrings should form the photon ring, proposing a way of better detecting these signatures in the first black hole image. [38] [39] [40]

The origin of supermassive black holes remains an open field of research. Astrophysicists agree that black holes can grow by accretion of matter and by merging with other black holes. [41] [42] There are several hypotheses for the formation mechanisms and initial masses of the progenitors, or "seeds", of supermassive black holes.

One hypothesis is that the seeds are black holes of tens or perhaps hundreds of solar masses that are left behind by the explosions of massive stars and grow by accretion of matter. Another model hypothesizes that before the first stars, large gas clouds could collapse into a "quasi-star", which would in turn collapse into a black hole of around 20 M . [43] These stars may have also been formed by dark matter halos drawing in enormous amounts of gas by gravity, which would then produce supermassive stars with tens of thousands of solar masses. [44] [45] The "quasi-star" becomes unstable to radial perturbations because of electron-positron pair production in its core and could collapse directly into a black hole without a supernova explosion (which would eject most of its mass, preventing the black hole from growing as fast). An alternative scenario predicts that large high-redshift clouds of metal-free gas, [46] when irradiated by a sufficiently intense flux of Lyman–Werner photons, [47] can avoid cooling and fragmenting, thus collapsing as a single object due to self-gravitation. [48] [49] The core of the collapsing object reaches extremely large values of the matter density, of the order of ∼ 10 7 g / c m 3 g/cm^<3>> , and triggers a general relativistic instability. [50] Thus, the object collapses directly into a black hole, without passing from the intermediate phase of a star, or of a quasi-star. These objects have a typical mass of

Another model involves a dense stellar cluster undergoing core-collapse as the negative heat capacity of the system drives the velocity dispersion in the core to relativistic speeds. [54] [55] Finally, primordial black holes could have been produced directly from external pressure in the first moments after the Big Bang. These primordial black holes would then have more time than any of the above models to accrete, allowing them sufficient time to reach supermassive sizes. Formation of black holes from the deaths of the first stars has been extensively studied and corroborated by observations. The other models for black hole formation listed above are theoretical.

Independently of the specific formation channel for the black hole seed, given sufficient mass nearby, it could accrete to become an intermediate-mass black hole and possibly a SMBH if the accretion rate persists. [43]

The formation of a supermassive black hole requires a relatively small volume of highly dense matter having small angular momentum. Normally, the process of accretion involves transporting a large initial endowment of angular momentum outwards, and this appears to be the limiting factor in black hole growth. This is a major component of the theory of accretion disks. Gas accretion is the most efficient and also the most conspicuous way in which black holes grow. The majority of the mass growth of supermassive black holes is thought to occur through episodes of rapid gas accretion, which are observable as active galactic nuclei or quasars. Observations reveal that quasars were much more frequent when the Universe was younger, indicating that supermassive black holes formed and grew early. A major constraining factor for theories of supermassive black hole formation is the observation of distant luminous quasars, which indicate that 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.

A vacancy exists in the observed mass distribution of black holes. Black holes that spawn from dying stars have masses 5–80 M . The minimal supermassive black hole is approximately a hundred thousand solar masses. Mass scales between these ranges are dubbed intermediate-mass black holes. Such a gap suggests a different formation process. However, some models [57] suggest that ultraluminous X-ray sources (ULXs) may be black holes from this missing group.

There is an upper limit to how large supermassive black holes can grow. So-called ultramassive black holes (UMBHs), which are at least ten times the size of most supermassive black holes, at 10 billion solar masses or more, appear to have a theoretical upper limit of around 50 billion solar masses, as anything above this slows growth down to a crawl (the slowdown tends to start around 10 billion solar masses) and causes the unstable accretion disk surrounding the black hole to coalesce into stars that orbit it. [58] [59] [60] [61]

Distant supermassive black holes, such as J0313–1806, [62] and ULAS J1342+0928, [63] are hard to explain so soon after the Big Bang. Some postulate they might come from direct collapse of dark matter with self-interaction. [64] [65] [66] A small minority of sources argue that they may be evidence that our universe is the result of a Big Bounce, instead of a Big Bang, with these supermassive black holes being formed before the Big Bounce. [67] [68]

Gravitation from supermassive black holes in the center of many galaxies is thought to power active objects such as Seyfert galaxies and quasars, and the relationship between the mass of the central black hole and the mass of the host galaxy depends upon the galaxy type. [69] [70]

An active galactic nucleus (AGN) is now considered to be a galactic core hosting a massive black hole that is accreting matter and displays a sufficiently strong luminosity. The nuclear region of the Milky Way, for example, lacks sufficient luminosity to satisfy this condition. The unified model of AGN is the concept that the large range of observed properties of the AGN taxonomy can be explained using just a small number of physical parameters. For the initial model, these values consisted of the angle of the accretion disk's torus to the line of sight and the luminosity of the source. AGN can be divided into two main groups: a radiative mode AGN in which most of the output is in the form of electromagnetic radiation through an optically thick accretion disk, and a jet mode in which relativistic jets emerge perpendicular to the disk. [71]

An empirical correlation between the size of supermassive black holes and the stellar velocity dispersion σ of a galaxy bulge [72] is called the M–sigma relation.

### Doppler measurements Edit

Some of the best evidence for the presence of black holes is provided by the Doppler effect whereby light from nearby orbiting matter is red-shifted when receding and blue-shifted when advancing. For matter very close to a black hole the orbital speed must be comparable with the speed of light, so receding matter will appear very faint compared with advancing matter, which means that systems with intrinsically symmetric discs and rings will acquire a highly asymmetric visual appearance. This effect has been allowed for in modern computer-generated images such as the example presented here, based on a plausible model [73] for the supermassive black hole in Sgr A* at the centre of our own galaxy. However, the resolution provided by presently available telescope technology is still insufficient to confirm such predictions directly.

What already has been observed directly in many systems are the lower non-relativistic velocities of matter orbiting further out from what are presumed to be black holes. Direct Doppler measures of water masers surrounding the nuclei of nearby galaxies have revealed a very fast Keplerian motion, only possible with a high concentration of matter in the center. Currently, the only known objects that can pack enough matter in such a small space are black holes, or things that will evolve into black holes within astrophysically short timescales. For active galaxies farther away, the width of broad spectral lines can be used to probe the gas orbiting near the event horizon. The technique of reverberation mapping uses variability of these lines to measure the mass and perhaps the spin of the black hole that powers active galaxies.

### In the Milky Way Edit

Astronomers are confident that the Milky Way galaxy has a supermassive black hole at its center, 26,000 light-years from the Solar System, in a region called Sagittarius A* [75] because:

• 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. [76]
• From the motion of star S2, the object's mass can be estimated as 4.1 million M , [77][78] or about 8.2 × 10 36 kg .
• The radius of the central object must be less than 17 light-hours, because otherwise S2 would collide with it. Observations of the star S14 [79] indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit.
• No known astronomical object other than a black hole can contain 4.1 million M in this volume of space.

Infrared observations of bright flare activity near Sagittarius A* show orbital motion of plasma with a period of 45 ± 15 min at a separation of six to ten times the gravitational radius of the candidate SMBH. This emission is consistent with a circularized orbit of a polarized "hot spot" on an accretion disk in a strong magnetic field. The radiating matter is orbiting at 30% of the speed of light just outside the innermost stable circular orbit. [80]

On January 5, 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sagittarius A*. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sagittarius A*, according to astronomers. [81]

### Outside the Milky Way Edit

Unambiguous dynamical evidence for supermassive black holes exists only in a handful of galaxies [83] these include the Milky Way, the Local Group galaxies M31 and M32, and a few galaxies beyond the Local Group, e.g. NGC 4395. In these galaxies, the mean square (or rms) velocities of the stars or gas rises proportionally to 1/ r near the center, indicating a central point mass. In all other galaxies observed to date, the rms velocities are flat, or even falling, toward the center, making it impossible to state with certainty that a supermassive black hole is present. [83] Nevertheless, it is commonly accepted that the center of nearly every galaxy contains a supermassive black hole. [84] The reason for this assumption is the M–sigma relation, a tight (low scatter) relation between the mass of the hole in the 10 or so galaxies with secure detections, and the velocity dispersion of the stars in the bulges of those galaxies. [85] This correlation, although based on just a handful of galaxies, suggests to many astronomers a strong connection between the formation of the black hole and the galaxy itself. [84]

The nearby Andromeda Galaxy, 2.5 million light-years away, contains a (1.1– 2.3) × 10 8 (110–230 million) M central black hole, significantly larger than the Milky Way's. [86] The largest supermassive black hole in the Milky Way's vicinity appears to be that of Messier 87 (i.e. M87*), at a mass of (6.4 ± 0.5) × 10 9 (c. 6.4 billion) M at a distance of 53.5 million light-years. [87] [88] The supergiant elliptical galaxy NGC 4889, at a distance of 336 million light-years away in the Coma Berenices constellation, contains a black hole measured to be 2.1 × 10 10 (21 billion) M . [89]

Masses of black holes in quasars can be estimated via indirect methods that are subject to substantial uncertainty. The quasar TON 618 is an example of an object with an extremely large black hole, estimated at 6.6 × 10 10 (66 billion) M . [90] Its redshift is 2.219. Other examples of quasars with large estimated black hole masses are the hyperluminous quasar APM 08279+5255, with an estimated mass of 2.3 × 10 10 (23 billion) M , and the quasar S5 0014+81, with a mass of 4.0 × 10 10 (40 billion) M , or 10,000 times the mass of the black hole at the Milky Way Galactic Center.

Some galaxies, such as the galaxy 4C +37.11, appear to have two supermassive black holes at their centers, forming a binary system. If they collided, the event would create strong gravitational waves. [91] Binary supermassive black holes are believed to be a common consequence of galactic mergers. [92] The binary pair in OJ 287, 3.5 billion light-years away, contains the most massive black hole in a pair, with a mass estimated at 18 billion M . [93] In 2011, a super-massive black hole was discovered in the dwarf galaxy Henize 2-10, which has no bulge. The precise implications for this discovery on black hole formation are unknown, but may indicate that black holes formed before bulges. [94]

On March 28, 2011, a supermassive black hole was seen tearing a mid-size star apart. [95] That is the only likely explanation of the observations that day of sudden X-ray radiation and the follow-up broad-band observations. [96] [97] The source was previously an inactive galactic nucleus, and from study of the outburst the galactic nucleus is estimated to be a SMBH with mass of the order of a million solar masses. This rare event is assumed to be a relativistic outflow (material being emitted in a jet at a significant fraction of the speed of light) from a star tidally disrupted by the SMBH. A significant fraction of a solar mass of material is expected to have accreted onto the SMBH. Subsequent long-term observation will allow this assumption to be confirmed if the emission from the jet decays at the expected rate for mass accretion onto a SMBH.

In 2012, astronomers reported an unusually large mass of approximately 17 billion M for the black hole in the compact, lenticular galaxy NGC 1277, which lies 220 million light-years away in the constellation Perseus. The putative black hole has approximately 59 percent of the mass of the bulge of this lenticular galaxy (14 percent of the total stellar mass of the galaxy). [98] Another study reached a very different conclusion: this black hole is not particularly overmassive, estimated at between 2 and 5 billion M with 5 billion M being the most likely value. [99] On February 28, 2013 astronomers reported on the use of the NuSTAR satellite to accurately measure the spin of a supermassive black hole for the first time, in NGC 1365, reporting that the event horizon was spinning at almost the speed of light. [100] [101]

In September 2014, data from different X-ray telescopes have shown that the extremely small, dense, ultracompact dwarf galaxy M60-UCD1 hosts a 20 million solar mass black hole at its center, accounting for more than 10% of the total mass of the galaxy. The discovery is quite surprising, since the black hole is five times more massive than the Milky Way's black hole despite the galaxy being less than five-thousandths the mass of the Milky Way.

Some galaxies lack any supermassive black holes in their centers. Although most galaxies with no supermassive black holes are very small, dwarf galaxies, one discovery remains mysterious: The supergiant elliptical cD galaxy A2261-BCG has not been found to contain an active supermassive black hole, despite the galaxy being one of the largest galaxies known ten times the size and one thousand times the mass of the Milky Way. Since a supermassive black hole will only be visible while it is accreting, a supermassive black hole can be nearly invisible, except in its effects on stellar orbits.

In December 2017, astronomers reported the detection of the most distant quasar currently known, ULAS J1342+0928, containing the most distant supermassive black hole, at a reported redshift of z = 7.54, surpassing the redshift of 7 for the previously known most distant quasar ULAS J1120+0641. [103] [104] [105]

In February 2021, astronomers released, for the first time, a very high-resolution image of 25,000 active supermassive black holes, covering four percent of the Northern celestial hemisphere, based on ultra-low radio wavelengths, as detected by the Low-Frequency Array (LOFAR) in Europe. [106]

Hawking radiation is black-body radiation that is predicted to be released by black holes, due to quantum effects near the event horizon. This radiation reduces the mass and energy of black holes, causing them to shrink and ultimately vanish. If black holes evaporate via Hawking radiation, a supermassive black hole with a mass of 10 11 (100 billion) M will evaporate in around 2×10 100 years. [107] Some monster black holes in the universe are predicted to continue to grow up to perhaps 10 14 M during the collapse of superclusters of galaxies. Even these would evaporate over a timescale of up to 10 106 years. [108]

## How scientists saw the ‘invisible’—and captured the first image of a black hole

The Event Horizon Telescope peered into the Messier 87 galaxy.

This black hole resides 55 million light-years from Earth and has a mass 6.5-billion times that of the Sun. EHT

Scientists with the Event Horizon Telescope (EHT) announced Wednesday that they’ve successfully imaged the event horizon of a supermassive black hole at the heart of the Messier 87 galaxy, nearly 55 million light-years away from Earth. A fiery maelstrom, the new image comes two years after the team initially captured their data, and ends a long wait for one of the most exciting astrophysical endeavors in modern memory.

“Black holes are the most mysterious objects in the universe,” Sheperd Doeleman, the director of EHT and a scientist at the Harvard–Smithsonian Center for Astrophysics, told the audience at a National Science Foundation press conference in Washington, D.C. “Because they are so small, we’ve never seen one. We are delighted to be able to report to you today that we have seen, and taken an image, of a black hole.”

The gravity exerted by a black hole is so powerful that light cannot even escape it, which obviously makes it nigh impossible to actually take a picture of one. But black holes possess what’s called an event horizon: a boundary designating the point of no return. Light and matter that cross this threshold will not escape the black hole, but spacetime is warped at the event horizon such that it creates a glowing circle of accreting matter. It creates a sort of silhouette of the object—that’s what the EHT captured.

Despite the name, EHT is actually a project comprised of eight different telescopes at different observatories around the world operating in synchronicity to image the black holes in the center of M87 as well as the supermassive black hole at the center of our own Milky Way galaxy, Sagittarius A*. EHT made its first data capture in 2006, and has since added more and more observatories to its network, which now includes submillimeter telescopes in Hawaii, Arizona, Chile, Antarctica, Mexico, and Spain. Doeleman explained that M87’s supermassive black hole was the first source they imaged, but they are currently working to image Sagittarius A*.

The new picture comes from data captured over a span of nine days in April 2017. It’s taken two years to actually unpack and analyze all of the observatories’ data, in part because the files are too massive to transfer digitally. Hard drives had to be physically ferried from the observatories in order for scientists to process the data. The Antarctic dataset, in particular, remained inaccessible for months because of extreme weather.

Roger Blandford, a theoretical astrophysicist at Stanford University who was not involved with EHT, told Popular Science the image is a “tribute to the hard work by the team and 50 years of ingenuity by radio astronomers before them honing the craft of interferometry.”

The different observatories that make up the EHT are all can make different radiofrequency observations of different objects in space. In this instance, they were all aligned to look at the radiation emitted by each black hole’s event horizon, working in concert to provide the sort of extreme optical resolution necessary to image something so small and so far away. Daniel Marrone, an astronomer at the University of Arizona and a member of the EHT team, told the audience at Wednesday’s press conference that while the black hole is 6.5 billion times the mass of the sun, the event horizon is basically just one-and-a-half light-days across. For reference, M87 itself, already an impressive body to image at 55 million light-years away, is 120 light-years in diameter. Doeleman calls the feat the “equivalent of being able to read the date on a quarter in LA when we’re standing here in Washington, D.C.”

Before the announcement, it wasn’t quite clear exactly what EHT was going to reveal to the world. Andrea Isella, a Rice University astronomer who was not involved with the project, told Popular Science beforehand that while we’ve obviously never had direct observations of Sagittarius A*, we’ve known about its existence for decades. We can observe its gravitational effects on objects in the vicinity. “We see stars orbiting around something that doesn’t meet any optical light,” he says. “From this motion, we can measure the mass of the black hole—estimations on the order of millions of solar masses.”

Blandford previously highlighted the image’s potential for affirming whether Einstein’s theory of general relativity—the model for how we characterize the relationship of gravity and spacetime—could correctly describe how gravity works in relation to these ultra-massive behemoths, perhaps shedding more light on the properties of black holes themselves. While general relativity has already been tested many times through weaker situations like gravitational lensing (how light bends when it crosses massive objects), it’s never been tested in a strong gravitational field like a black hole.

The EHT team on Wednesday affirmed that the new data is consistent with previous models used to characterize both black holes and general relativity. Avery Broderick from University of Waterloo explains that were Einstein wrong, the silhouette of the black hole could have looked very different—misshapen, or even missing entirely. Instead, it was circular and conformed to structural expectations.

“Today, general relativity has passed another crucial test,” says Broderick.

“To some extent, black holes are actually very simple objects,” says Isella. They are defined by what he explains are two major parameters: mass (which is already estimated through the orbit of the stars around it), and rotational spin. An image of a black hole can give you a direct line into figuring out these parameters. Any significant deviations from what we expect mean that there is some critical missing piece we haven’t yet considered. But the new image is encouraging news that everything we’ve learned about black holes, without even having seen one, has been on point.

The new findings will influence myriad astrophysical and cosmological investigations. In the immediate future, Blandford hopes “they will help us understand what happens to gas and magnetic fields outside the event horizon, how the disks of gas swirling around the black hole behave, and how relativistic jets [ionized matter expelled at the speed of light] are made.” Broderick explained the data has already been used to determined M87’s black hole spins clockwise, and possesses a bright crescent-like feature with a dark interior.

Down the road, Blandford thinks astronomers could use the data to get a better glimpse of the behavior of individual stars orbiting the galactic center, and the role of hot gas just outside black holes in influencing the spin of the objects themselves. EHT team member Sera Markoff from the University of Amsterdam discussed how this type of work can be used to better understand how jets of radiation and particles expelled by black holes affect galactic growth and evolution.

But besides the scientific relevance of the image, there’s also a technological milestone here worth highlighting. EHT is, in many ways, a kind of proof-of-concept for acquiring high-resolution images of a celestial object that’s very small and very far away. Accomplishing this type of feat basically opens up a whole methodology for conducting more audacious astronomical investigations.

“A big chunk of investigations in astronomy deal with trying to image very small objects,” says Isella. “The implication is, we should be able to add more telescopes and achieve better quality images moving forward, as well as image other black holes,” said Isella.

That may not be greatly apparent on first glance of the image, which is certainly blurrier than most of the public might have hoped. The image is compressed a million times over from 5,000 terabytes’ worth of data, and the sharpness unfortunately still seems to fall off. But it could be made better through different approaches in follow-up observations, like in employing new algorithms, and the addition of more telescopes with higher frequency.

In fact, astronomy is already well-used to that sort of step-by-step process. Take a planet like Pluto, for example. Our very first view of the dwarf planet was an absolute mess by today’s standards, but with time, we managed to find something that much more closely resembled an actual planet with surface features. And it wouldn’t be until the New Horizons flyby, 85 years after we snagged the first image of Pluto, that we could finally see its hazy atmosphere, rock formations, and true surface colors.

The team will have 11 telescopes under the project by 2020, and Doeleman and his colleagues expressed a desire to eventually put a telescope in space to further their efforts. While the new image of M87’s supermassive black hole has not radically changed our understanding of the universe, it helps open the door to a whole new view of space.

“We’ve exposed part of the universe that we thought were invisible to us before,” said Doeleman. “Nature has conspired to let us see something that we thought was invisible.”

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