A Powerful Radio Source Towards the Galaxy’s Darkened Center

In 1968, Caltech astronomers Eric E. Becklin and Gerald Neugebauer successfully surveyed the central parsecs of the Milky Way across four different infrared wavelengths, achieving optimal results at 2.2 µm. They managed to penetrate 25 magnitudes of obscuration caused by the dust in the spiral arms and uncovered clusters of stars tightly packed together, contrasting with the vast distances found in the outer regions of the galaxy. An article published in Scientific American in April 1974 by R.H. Sanders and G.T. Wrixon highlighted their findings.

The two scientists noted that infrared data indicated the galactic center harbors around one million stars per cubic parsec, a stellar density approximately one million times greater than that found around the Sun. This density suggests that a being on a planet orbiting a star in the Milky Way’s core would witness a million bright stars akin to Sirius, the brightest star visible from Earth. The combined luminosity of all visible stars in that scenario would equate to about 200 full moons. Given such conditions, optical astronomy would be restricted to observing only the most luminous nearby objects, and even the light from the closest galaxies would appear faint. Nevertheless, they also posited that life could hardly exist on planets within the galactic nucleus due to the proximity of stars, which would disrupt planetary orbits every few hundred million years.

Yet, the central parsecs of the Milky Way hosted more than just a multitude of stars. From the 1950s onward, radio telescopes had detected an extremely powerful radio source directed towards the galactic center. This source was first distinctly identified by Australian scientists Jack Hobart Piddington and Harry Clive Minnett, who employed two small antennas, a 3 m diameter mobile dish and a 4.8 by 5.5 m paraboloid, located near Sydney. Their findings, reported in a 1951 article in an obscure Australian scientific journal, stated:

> A new “discrete source” of peculiar spectrum was discovered very close to the center 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 emphasis on the word ‘total’ in italics underscored the extraordinary nature of the phenomenon they encountered. However, the limited resolution of their instruments only allowed them to establish a rough limit on the source’s size, which could not exceed 1.5 degrees in diameter. The location of the signal was also somewhat imprecise, with the peak recorded at the boundary between the constellations of Sagittarius and Scorpius, with uncertainties 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 publications from the early 1950s referred to that potent radio emission as “the Galactic center source.” It appears that the first to designate it as Sagittarius A (abbreviated as Sgr A) were John D. Kraus, Hsien-Ching Ko, and Sean Matt in 1954, in a paper listing radio sources identified during a sky survey at 250 MHz. By 1958–59, the name and abbreviation became widely accepted, eventually overshadowing other potential names.

However, during this time, the resolution of radio telescopes was inadequate to discern the internal structure of the source or fully comprehend its nature. Research primarily focused on accurately determining the extent of Sgr A and its position relative to the galactic center. A 1954 study published in the Australian Journal of Physics by Bolton, Westfold, Stanley, and Slee listed eleven radio sources, all larger than 1 degree, labeled from A to L. The brightest among these was source L, identified with 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 center is close to the accepted position of the galactic center.

> 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 center of the Galaxy, which is an unusually intense source of radio noise.

The circumstantial evidence supporting Sagittarius A's position in the Milky Way’s center emerged in the 1960s. In a March 16, 1960 article published in Monthly Notices of the Royal Astronomical Society (“The position of the galactic center”), authors Jan Hendrik Oort, a pioneer in radio astronomy, and his student Gerrit Willem Rougoor stated:

> The direction to the radio source Sagittarius A is found to be within 0.03° of the direction to the galactic center 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 center.

However, if the direction aligned with the galactic center, what evidence substantiated that the powerful radio source was indeed located there rather than, for instance, near the Sun? The primary evidence highlighted by Oort and Rougoor was a strong absorption line linked to Sgr A, associated with the 3-kiloparsec arm, a vast expanding cloud of hydrogen situated 5–6 kiloparsecs from the Sun, positioned between us and the galactic center. The existence of this line confirmed that Sgr A was beyond the 3-kiloparsec arm relative to the solar system, placing it on the galactic center's side.

> “In view of the evidence given,” Oort and Rougoor concluded, “it seems fairly safe to assume that Sagittarius A can be identified with the galactic center.”

Finer Details About the Structure of Sagittarius A Are Discovered

Throughout the 1960s, advancements in radio wave and infrared observation resolution continued. In 1966, Barry G. Clark and David E. Hogg used the newly constructed two-element Green Bank interferometer to observe 146 radio sources at a frequency of 2695 MHz (11 cm), achieving a resolution of about 10 arc seconds. Although this was insufficient to capture the fine structure of Sagittarius A, it was adequate to detect a compact flux source.

In 1968, Eric E. Becklin and Gerry Neugebauer conducted infrared observations of Sagittarius A at 1.65, 2.2, and 3.4 µm, achieving an angular resolution ranging from 0.08 to 1.8 arc minutes. They identified four distinct elements within the structure:

1. A principal source with a diameter of 5 arc minutes;
2. A point source centered on the principal source;
3. An extended background;
4. Additional extended sources.

Becklin and Neugebauer emphasized a critical point: the size and position of the infrared source coincided with those of the previously defined radio source:

> 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.

Regarding the origin of the infrared source, the two authors lacked definitive elements to determine whether it was entirely or partially a thermal source (from heat emitted by a cluster of bright stars, absorbed and re-radiated by intervening dust) or a non-thermal source (like synchrotron radiation produced by an object accelerating charged particles to relativistic speeds within magnetic fields).

Their calculations suggested that if the source at the galaxy's core were a cluster of bright stars, it would exhibit a luminosity approximately three hundred thousand times that of the Sun, with a mass around 700,000 solar masses and a density exceeding 10 solar masses per cubic parsec.

While they leaned towards a stellar origin for the observed infrared radiation, Becklin and Neugebauer noted a fascinating clue: the energy distribution of the infrared source in the Milky Way’s center, adjusted for the extinction caused by intervening dust, bore striking similarities to that from NGC 1068, a Seyfert galaxy, and 3C 273, the first identified quasar and the brightest known, both of which are non-thermal sources.

What If It Was a “Schwarzschild Throat”?

Seyfert galaxies and quasars led astronomers to ponder if the infrared and radio source at the Milky Way center was related to a superdense object fueled by gravitationally attracted gas.

Astrophysicist Donald Lynden-Bell, in a 1969 article published in Nature, theorized that quasars, distant and powerful radio sources from the early universe, were juvenile forms of supermassive objects still present at the centers of galaxies, including those in the contemporary universe. He suggested that after millions of years, the energetic phase of a quasar, sustained by vast amounts of hydrogen, would cease, leaving behind a dead object composed of superdense matter, containing between one million and one billion solar masses within a volume no larger than that of the solar system.

In 1969, Lynden-Bell did not yet refer to these as ‘black holes’ (the term was coined by John Wheeler a few years prior). Instead, he used the term “Schwarzschild throat” to describe the event horizon, the boundary beyond which nothing, not even light, can escape the gravitational pull of a former quasar. He speculated on their location:

> 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 centers of gravitation, we expect to find matter concentrated toward them. We therefore expect that the throats are to be found at the centers of massive aggregates of stars, and the centers of the nuclei of galaxies are the obvious choice.

In 1971, in a study published with Martin Rees, Lynden-Bell further explored the idea that galactic nuclei were the ideal locations to find supermassive objects that were once quasars, now explicitly referring to a black hole at the Milky Way's center as the best explanation for the concentration of ionized material in the central parsecs of the galaxy. Regarding its mass, he stated:

> It seems that any value less than about 10? M* [one hundred million solar masses] is compatible with present knowledge.

However, the black hole hypothesis remained speculative due to the lack of conclusive evidence. Lynden-Bell acknowledged that improved resolution was necessary to investigate the galactic center for a compact, small-diameter radio source linked to the ionized region in the central parsecs of the Milky Way, which would validate the black hole theory.

Consequently, Lynden-Bell collaborated with Ronald D. Ekers to perform interferometric observations at 5 GHz, achieving resolutions of 6 arc seconds in the east-west direction and 18 arc seconds in the north-south direction. Despite these efforts, the findings revealed two primary components in the Sagittarius A region, both of which could be explained without invoking a black hole. In a 1971 article, Lynden-Bell and Ekers candidly noted their limitations:

> 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 higher resolution of 5.5 arc seconds in each direction, corresponding to a linear diameter of 0.3 parsecs (based on the then-accepted distance of the galactic center). Multiple distinct sources emerged from the various wavelengths studied, resolved from the background, which appeared to consist of three components. Nevertheless, the spectra and luminosities suggested a thermal explanation for all sources, indicating they were likely simple stellar radiation with no evidence of black holes.

Interferometry Allows Astronomers to Understand That Sgr A Has a Diameter Smaller Than One Light-Day

It became increasingly evident that something more significant was present in the Milky Way’s central parsec than previously recognized. Bruce Balick and Robert Brown ultimately made the breakthrough in detecting the invisible signal first. In a December 1974 article in The Astrophysical Journal (“Intense sub-arcsecond structure in the galactic center”), they announced their findings:

> The detection of strong radio emission in the direction of the inner 1-pc core of the galactic nucleus is reported.

The distinguishing factor in this research was the significantly higher resolution and clarity of the signal. Balick and Brown observed Sagittarius A on February 13 and 15, 1974, employing a new interferometer at the National Radio Astronomy Observatory in Green Bank, Virginia, with a 35 km baseline. The interferometer was comprised of three 26 m antennas spaced up to 2.7 km apart, and a fourth 14 m antenna situated 35 km southeast on a mountaintop. Operating at 2.7 and 8.1 GHz, they achieved resolutions of approximately 0.7 and 0.3 arc seconds respectively, enabling them to pinpoint the center of the galactic radio source with unprecedented precision. The signal was clear and robust:

> 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 detected a powerful and compact radio source confined within a region of 1×3 arc seconds, with an internal structure comprised of one or two elements, each with an angular diameter not exceeding 0.1 arc seconds. Based on the galactic center's estimated distance, this indicated the emitting object's diameter was no greater than one light-day, confined within a volume smaller than a thousand astronomical units (less than 1/63 of a light-year). The radio source was slightly offset westward from the previously indicated point by Rieke and Low.

The structure and location of the radio source reinforced the idea that the Milky Way’s core represented a presently dormant version of an active galactic nucleus. According to Balick and Brown, the galactic center might have once experienced highly energetic phenomena, akin to those observed in BL Lacertae objects.

Yes, It’s a Black Hole!

In 1975, Ukrainian astrophysicist Iosef Samuilovich Shklovskii published an article in a Soviet newspaper (Pis’ ma v Astronomicheskii Zhurnal) later 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 referencing Balick and Brown's findings, Shklovskii interpreted the variations in radiation flux from Sgr A West in both infrared and radio waves as evidence of the non-thermal nature of the emitting object.

If the non-thermal source emitted synchrotron radiation, calculations indicated it could be an object no larger than one-thousandth of an arc second, equivalent to a linear size of about 6 astronomical units. Shklovskii speculated this non-thermal source originated from synchrotron radiation generated by particles falling onto the accretion disk of a black hole with a mass of approximately 30,000 solar masses, located at the center of the Milky Way.

Conversely, a study published in 1977 in The Astrophysical Journal reported observations of the galactic center conducted on March 6, 1976, using a VLBI interferometer with three radio telescopes of varying sizes: 64, 40, and 36.5 m. The authors—Lo, Cohen, Schilizzi, and Ross—found that observations at 3.7 cm indicated the compact radio source at the Milky Way's center had a linear size of about 140 astronomical units, with a much smaller core size consistent with Shklovskii’s predictions:

> 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 ~10? 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.

However, it was neither a pulsar nor a 30,000-solar-mass black hole, as speculated by Shklovskii. Today, we recognize that this powerful and compact radio source is a 4-million solar mass supermassive black hole. This conclusion is based on the unmistakable evidence of the gravitational influence of the object on the orbits of nearby stars, which aligns with the position of the radio source Sgr A*. The event horizon of the black hole concealed in the galactic center has a radius of just 10 microarcseconds, or millionths of an arc second, a size still detectable by modern interferometers.

The Name Issue

We conclude this narrative by recounting how this radio source, now known as Sagittarius (or Sgr) A*, acquired its unique designation to distinguish it from the complex and extensive surrounding radio source, referred to as Sagittarius A without the asterisk.

The term with the asterisk was coined by Bob (Robert) Brown, one of the discoverers of the compact radio source in 1974. Interestingly, this was not the first name proposed for that radio source. In 1980, Reynolds and McKee suggested calling it GCCRS, an acronym for “Galactic Center Compact Radio Source,” but fortunately, this did not gain traction. In 1982, Backer and Sramek proposed Sgr A(cn), where ‘cn’ stood for “Compact Non-thermal” source, yet this suggestion also failed to gain popularity. Instead, the asterisk introduced by Brown in 1982 quickly became the standard.

Brown explained the origin of the name 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.