The History of the Discovery of Sgr A*
At the center of the Milky Way hides a four-million solar mass black hole. The confirmation of its existence, which took place thanks to the study of star orbits conditioned by its immense gravity, earned the Nobel Prize 2020 for physics to Andrea Ghez and Reinhard Genzel. But the story of the observations that led to the discovery of this supermassive black hole begins in the 1950s and deserves to be told
A powerful radio source towards the Galaxy’s darkened center
In 1968, Eric E. Becklin and Gerald Neugebauer, two Caltech astronomers, managed to scan the central parsecs of the Milky Way in four different infrared wavelengths, obtaining the best results at 2.2 µm. Overcoming 25 magnitudes of obscuration due to the dust in the interposed spiral arms, they discovered swarms of stars huddled together with an unlikely density, compared to the enormous distances that, in the galactic periphery, separate the Sun from its neighbors. An article published in Scientific American in April 1974 (R.H. Sanders and G.T. Wrixon, “The Center of the Galaxy”) evocatively summarized what Becklin and Neugebauer had observed.
The two authors wrote that infrared observations showed that the galactic center contains about one million stars per cubic parsec, a stellar density about one million times that of the Sun’s surroundings. This implies that a living being on a planet orbiting a star at the Milky Way’s center would see a million bright stars like Sirius, the brightest star in our sky. The integrated luminosity of all the stars in the night sky of such a planet would be equal to about 200 full moons. Under these conditions, optical astronomy should limit itself to studying only the brightest nearby objects. Even the light from the closest galaxies would be dimmed. However, they also pointed out that it is quite unlikely that any form of life could exist on planets in the galactic nucleus, given that at such high stellar densities, the stars would graze each other so frequently that they would tear the planets from their orbits every a few hundred million years.
But in those central parsecs of the Milky Way, there weren’t just myriads of stars. Since the 1950s, radio telescopes had revealed that an extremely powerful radio source existed in the direction of the galactic center. The source was first clearly detected by two Australian scientists, Jack Hobart Piddington and Harry Clive Minnett, who used two small antennas, a 3 m diameter mobile dish and a 4.8 by 5.5 m paraboloid, located at Potts Hill, a small town a few kilometers from Sidney. Piddington and Minnett published the results of their sky survey, performed in radio waves at 1210 and 3000 MHz, in an article that appeared in 1951 in an Australian scientific journal almost unknown in the West. In the article’s abstract, you can read:
A new “discrete source” of peculiar spectrum was discovered very close to the centre of the Galaxy. Evidence suggests that the power output of this and some other sources in the radio spectrum may exceed the total power output of the Sun.
The word ‘total’ appeared in italics in the article to emphasize the magnitude of the phenomenon the two researchers had come across. However, the limited resolution of their instruments allowed them to establish only a raw limit to the source’s size, whose diameter could not be greater than 1.5 degrees. Even the location was affected by a certain inaccuracy. The recorded signal’s peak came from a place on the border between the constellations of Sagittarius and Scorpius, with an uncertainty of about 2 arc minutes for right ascension and about 1 degree for declination.
The source coincides exactly with the Milky Way’s center
Most of the articles that appeared in the early 1950s referred to that powerful radio emission as “the Galactic center source.” It seems that the first to indicate it with the name Sagittarius A and the abbreviation Sgr A were John D. Kraus, Hsien-Ching Ko, and Sean Matt in 1954, in a paper listing the radio sources they identified in a sky survey at 250 MHz. From 1958–59, the name and abbreviation became commonplace, over time supplanting any alternative denomination.
However, in those years, radio telescopes’ resolution was not yet sufficient to distinguish the internal structure of the source nor to understand exactly what it was. The research work was mainly aimed at determining as accurately as possible the extent of Sgr A and its position concerning the galactic center. In a study published in 1954 in the Australian Journal of Physics, the four authors — Bolton, Westfold, Stanley, and Slee — listed eleven radio sources, all with a size greater than 1 degree, indicated by capital letters from A to L. Among them, the brightest was the source L, corresponding to Sagittarius A:
The observations indicate that this source provides the greatest flux density and has the most peaked brightness distribution in both longitude and latitude of all the extended sources. The position of its centre is close to the accepted position of the galactic centre.
It is difficult to believe that its high flux density is due to the fortuitous superposition of radiation from a number of objects in the line of sight. We are left with the inference that there is an extended physical object at the centre of the Galaxy, which is an unusually intense source of radio noise.
The circumstantial confirmation that Sagittarius A was right in the Milky Way’s center comes with the 1960s. In an article dated March 16, 1960, published in Monthly Notices of the Royal Astronomical Society (“The position of the galactic center”), authors Jan Hendrik Oort, one of the pioneers of radio astronomy, and his student Gerrit Willem Rougoor wrote:
The direction to the radio source Sagittarius A is found to be within 0.03° of the direction to the galactic centre as determined from a number of precise optical and radio observations […]. This, together with the fact that the source is unique among the known sources, makes it highly probable that Sgr A is situated at the centre.
However, if the direction was exactly that of the galactic center, what evidence was there that the powerful radio source was really located in the galactic center and not, for example, near the Sun? The main evidence indicated by Oort and Rougoor was a strong absorption line, corresponding to the position of Sgr A, which appeared associated with the so-called 3-kiloparsec arm, a vast expanding cloud of hydrogen, located 5–6 kiloparsecs from the Sun, between the galactic center and us. The presence of that line proved that Sgr A was beyond the 3-kiloparsecs arm with respect to the solar system, that is, on the side of the galactic center.
“In view of the evidence given,” concluded Oort and Rougoor, “it seems fairly safe to assume that Sagittarius A can be identified with the galactic centre.”
Finer details about the structure of Sagittarius A are discovered
During the 1960s, the resolution of observations in radio waves and the infrared progressively improved. In 1966, Barry G. Clark and David E. Hogg, using the newly completed two-element Green Bank interferometer, observed 146 radio sources at the frequency of 2695 MHz (11 cm), reaching a resolution of around 10 arc seconds. Not yet sufficient to observe the fine structure of Sagittarius A, but enough to detect the presence of a compact flux source.
In 1968, Eric E. Becklin and Gerry Neugebauer observed Sagittarius A in the infrared, at 1.65, 2.2, and 3.4 µm, with an angular resolution of 0.08 to 1.8 arc minutes. They found that the structure consisted of four distinct elements:
- a principal source with a diameter of 5 arc minutes;
- a point source centered on the latter;
- an extended background;
- some other extended additional sources.
An important aspect emphasized by Becklin and Neugebauer was that the size and position of the infrared source coincided with those of the radio source, defined by previous studies:
We consider the further agreement between the position and extent of the infrared source and the radio source Sagittarius A as conclusive evidence that Sagittarius A also lies at the dynamical center of the Galaxy.
As for the origin of the infrared source, the two authors did not have certain elements to decide whether it was, in whole or in part, a thermal source, due to the heat emitted by a cluster of bright stars, absorbed and re-radiated to longer wavelengths by interposed dust, or a non-thermal source, such as synchrotron radiation, generated by an object capable of accelerating charged particles to relativistic speeds within magnetic fields.
Their calculations indicated that, if the source at the center of the galaxy were a cluster of bright stars, it would have a luminosity in the order of three hundred thousand solar luminosities, a mass equal to 700,000 solar masses, and a density greater than 10⁸ solar masses per cubic parsec.
Although they favored the stellar origin of the observed infrared radiation, Becklin and Neugebauer remarked an interesting clue. The energy distribution of the infrared source at the center of the Milky Way, once corrected by taking into account the extinction caused by the interposed dust, showed striking similarities with that coming from NGC 1068, a Seyfert galaxy, and from 3C 273, the first quasar identified and the brightest known, that is, two non-thermal sources.
What if it was a “Schwarzschild throat”?
Seyfert galaxies, quasars: Astronomers were beginning to wonder if the infrared and radio source at the Milky Way center had anything to do with a superdense object, powered by gravitationally attracted gas.
The English astrophysicist Donald Lynden-Bell, in an article published in 1969 in Nature, proposed the idea that quasars, far and powerful radio sources of the early universe, were a so to speak juvenile manifestation of supermassive objects, still existing at the center of galaxies, even those that populate the contemporary universe. He proposed that after several million years, the phase of paroxysmal activity, in which the quasar, powered by enormous quantities of hydrogen, produces energy in the order of 10⁶¹ ergs, ends. Then it remains the dead object, made of superdense matter, containing between a million and a billion solar masses, enclosed in a volume no greater than that of the solar system.
In 1969, Lynden-Bell was not yet calling them ‘black holes.’ (John Wheeler had coined the term only a few years earlier.) Instead, he used the expression “Schwarzschild throat” to name the event horizon, i.e., the place from which nothing, neither matter nor light, can escape the gravitational pull of the former quasar, and proposed his idea of where to find these superdense “corpses”:
Nothing can ever pass outwards through the Schwarzschild sphere of radius r=2GM/c², which we shall call the Schwarzschild throat. We would be wrong to conclude that such massive objects in space-time should be unobservable, however. It is my thesis that we have been observing them indirectly for many years.
As Schwarzschild throats are considerable centres of gravitation, we expect to find matter concentrated toward them. We therefore expect that the throats are to be found at the centres of massive aggregates of stars, and the centres of the nuclei of galaxies are the obvious choice.
In 1971, in a study published with Martin Rees, Lynden-Bell, taking up the theory of galactic nuclei as the places to search for finding supermassive objects that were once quasars, now explicitly spoke of a black hole at the center of the Milky Way, as what best could explain the concentration of ionized material in the central parsecs of the galaxy. As for its mass,
It seems that any value less than about 10⁸ M⊙ [one hundred million solar masses] is compatible with present knowledge.
But the black hole hypothesis remained just a hypothesis in the absence of conclusive evidence. Lynden-Bell knew this well and knew that higher resolutions had to be achieved in scanning the galactic center if one hoped to find that compact, small-diameter radio source associated with the ionized region found in the central parsecs of the Milky Way, which would confirm the black hole conjecture observationally.
Therefore, Lynden-Bell performed with Ronald D. Ekers interferometric observations at 5 GHz, in which the resolution of 6 arc seconds in an east-west direction and 18 arc seconds in a north-south direction was achieved. But it still wasn’t enough. The research revealed two main components in the Sagittarius A region, both of which could be explained without resorting to a black hole. In an article also published in 1971, Lynden-Bell and Ekers objectively admitted that their attempt was unsuccessful:
Although stimulated by the black hole idea our observations are thus most simply explained in terms of young stars and giant H II regions. Apart from the doubleness of the central source there is nothing in these observations to suggest violent events of black holes.
In 1973, George H. Rieke and Frank J. Low mapped the galactic nucleus in the infrared, at 3.5, 5, 10.5, and 21 µm, achieving an even greater resolution of 5.5 arc seconds in each direction, corresponding to a linear diameter of 0.3 parsecs (based on the distance of 10 kiloparsecs at which the galactic center was then thought to be). At the different wavelengths investigated, at least five discrete sources appeared, resolved with respect to the background, which in turn appeared to be divided into three components. However, the spectra and luminosities favored a thermal explanation for all sources. In other words, it seemed to be simple stellar radiation, still no evidence of black holes.
Interferometry allows astronomers to understand that Sgr A has a diameter smaller than one light-day
That there was something more in the Milky Way’s central parsec than what had hitherto emerged was now more than a suspicion. Bruce Balick and Robert Brown eventually won the race to discover the invisible signal first. In an article published in December 1974 in The Astrophysical Journal (“Intense sub-arcsecond structure in the galactic center”), the two announced to the scientific world:
The detection of strong radio emission in the direction of the inner 1-pc core of the galactic nucleus is reported.
The difference between this research and the previous ones was in the resolution, much higher, and the signal’s clarity. Balick and Brown observed Sagittarius A on February 13 and 15, 1974, using the new interferometer at the National Radio Astronomy Observatory in Green Bank, Virginia, with a 35 km baseline. The interferometer consisted of three 26 m antennas, which could be spaced up to 2.7 km from each other, and a fourth 14 m antenna, located 35 km south-east on the top of a mountain. The system operated at 2.7 and 8.1 GHz, reaching a resolution of about 0.7 and 0.3 arc seconds, respectively. Given the right weather conditions, it was possible to pick up the exact center of the galactic radio source with such equipment much more distinctly than had ever been possible in previous observations. The signal appeared immediately clear and powerful:
For Sgr A West, the [interference] fringes were so strong as to be detectable above the noise level in only a few seconds.
Balick and Brown had observed a powerful and compact radio source, contained in a region of 1×3 arc seconds, whose internal structure consisted, depending on the data interpretation model, of one or two elements, whose angular diameter did not exceed 0.1 arc seconds. At the galactic center’s estimated distance, this meant that the emitting object’s diameter was less than or equal to one light-day and was within a region no larger than a thousand astronomical units (less than 1/63 of a light-year). The source was slightly offset to the west of the point indicated with the number 1 in the Rieke and Low’s map of the Galactic Center, reproduced in the previous image.
The radio source’s structure and location reinforced the hypothesis that the Milky Way’s core was a currently quiescent version of an active galactic nucleus. According to Balick and Brown, the galactic center may once have been the site of highly violent phenomena, similar to those observed in the BL Lacertae objects.
Yeah, it’s a black hole!
In 1975, Ukrainian astrophysicist Iosef Samuilovich Shklovskii published in a Soviet newspaper (Pis’ ma v Astronomicheskii Zhurnal) an article translated in the West with the title “Is the galactic nucleus a black hole?” Building on earlier works by Downes, Rieke, Low, and others, but not quoting the previous year’s Balick and Brown article, Shklovskii interpreted the differences observed in the radiation flux from Sgr A West in the infrared and radio waves as evidence of the non-thermal nature of the emitting object.
If the non-thermal source emitted synchrotron radiation, then the calculations indicated that it could be an object with a size not larger than one-thousandth of an arc second, equal to a linear size of about 6 astronomical units. Shklovskii hypothesized that that non-thermal source was produced by the synchrotron radiation emanating from particles falling on the accretion disk of a black hole of about 30,000 solar masses, placed just in the Milky Way center.
On the other side of the Iron Curtain, a study published in 1977 in The Astrophysical Journal informed the scientific community on the results of observations of the galactic center, carried out on March 6, 1976, with the VLBI interferometer, using three radio telescopes with dishes of 64, 40, and 36.5 m, respectively. The four authors — Lo, Cohen, Schilizzi, and Ross — reported that observations at 3.7 cm indicated that the compact source of radio waves at the center of the Milky Way had a linear size of about 140 astronomical units but a much smaller core of size not far from those foreseen by Shklovskii:
While the source structure is far from being well determined observationally, it appears that the compact source in the galactic center is at most ~140 AU in size, has a ~10 AU core that may be varying with time, a brightness temperature greater than ~10⁸ K, and a radio luminosity greater than ~10³³ ergs s⁻¹. In any case, the internal source must supply energy at a rate appreciably higher than the observed radio luminosity from an extremely small volume of space at the galactic center. If the source itself is a pulsar […] the observed radio luminosity and the radio source spectrum […] would make this pulsar unique.
But no, it wasn’t a pulsar nor a 30,000-solar-mass black hole, as Shklovskii speculated. Today we know that that powerful and compact radio source is a 4-million solar mass supermassive black hole. We know this for the unequivocal trace left on the nearest stars’ orbits by the object’s gravitational attraction, whose position corresponds to the radio source Sgr A*. And we know that the event horizon of the black hole lurking in the galactic center has a radius of just 10 microarcseconds, that is, millionths of an arc second. Not so little, however, to be out of the reach of modern interferometers.
The name issue
We conclude this account with the story of how that radio source — which became for all Sagittarius (or Sgr) A*, with the asterisk, since the 1980s, to distinguish it from the complex and large surrounding radio source, which is Sagittarius A without an asterisk — acquired that strange hallmark.
The inventor of the asterisk was Bob (Robert) Brown, one of the two discoverers of the compact radio source observed in 1974. This was not actually the first name suggested for that radio source. In 1980, Reynolds and McKee had proposed to call it GCCRS, a kind of tax code that stood for “Galactic Center Compact Radio Source.” Fortunately, the attempt had no imitators. In 1982, Backer and Sramek proposed Sgr A(cn), where ‘cn’ stood for “Compact Non-thermal” source. Not even this proposal was followed. Instead, the formula with the asterisk, introduced by Brown in 1982, soon spread.
How the idea was born was explained by its own inventor in a 2003 article:
Scratching on a yellow pad one morning I tried a lot of possible names. When I began thinking of the radio source as the “exciting source” for the cluster of H II regions seen in the VLA maps, the name Sgr A* occurred to me by analogy brought to mind by my Phd dissertation, which is in atomic physics and where the nomenclature for excited state atoms is He*, or Fe* etc.