Sagittarius A*

Lore & Background

Brown using the baseline interferometer of the National Radio Astronomy Observatory. The name Sgr A* was coined because the radio source was 'exciting', analogous to excited states of atoms denoted with asterisks. Observations of stars orbiting Sgr A*, particularly star S2, have been used to determine its mass and radius limits. In 2002, an international team led by Reinhard Genzel reported the motion of S2 over ten years, ruling out a cluster of dark stellar objects and strengthening evidence for a massive black hole. The image confirms the object contains a black hole. The radio emission from Sgr A* varies on the order of minutes, complicating analysis. The telescope's measurement tested Einstein's theory of relativity more rigorously than before, and the results matched perfectly.

Reader's Guide

Sagittarius A* is significant as the supermassive black hole at the center of the Milky Way, providing the best empirical evidence that such objects exist. Its discovery and subsequent observations have confirmed that a black hole is the only explanation for the compact, massive object at the Galactic Center. This work tested general relativity under extreme conditions and matched predictions perfectly. The mass and orbital dynamics of stars like S2 have allowed precise measurements of Sgr A*, establishing it as a key laboratory for studying black hole physics. Sgr A* remains a focus for understanding how supermassive black holes grow and interact with their environment, including the potential absorption of smaller black holes and stars.

Did You Know?

• The radio emission from Sgr A* varies on the order of minutes, complicating analysis.

From Radio Whispers to a Named Object

His measurements, however, fell short of reaching the true Galactic Center as we understand it today. The identification came later through the work of Jack Piddington and Harry Minnett, who used the CSIRO radio telescope at Potts Hill Reservoir in Sydney to isolate a discrete, bright source they labeled "Sagittarius-Scorpius." Follow-up observations with the larger 80-foot instrument at Dover Heights convinced them enough to publish a letter in Nature proposing this as the probable location of the Galactic Center. Following the standard convention of the era, sources were assigned capital letters within each constellation in order of brightness, with "A" reserved for the brightest emitter.

A Layered Radio Landscape

Sagittarius A is not a single object but a layered radio complex sitting at the heart of the Milky Way, nestled between the constellations Scorpius and Sagittarius. From our vantage point near the Sun, the region is completely invisible to optical telescopes because vast clouds of interstellar dust threaded through the galaxy's spiral arms absorb and scatter visible light. That same dust lane is responsible for the Great Rift, the dark gap that slices through the galaxy's bright central bulge as seen from Earth. Within the radio band, three distinct components overlap in a nested arrangement. The largest is Sagittarius A East, a broad supernova remnant. Inside it, offset from the center, sits Sagittarius A West, a structure with a spiral-like appearance. At the very heart of West lies Sagittarius A*, an extraordinarily bright and compact radio source. This concentric geometry — East encompassing West, West cradling A* — gives the region its distinctive multi-scale character and has made it one of the most studied radio sources in the sky.

The Black Hole and Its Neighborhood

The compact radio source Sagittarius A* is widely regarded as the most credible candidate for the supermassive black hole anchoring the Milky Way. Evidence comes largely from stellar dynamics: the Very Large Telescope in Chile and the Keck Observatory in Hawaii have tracked stars racing around Sgr A* at velocities unmatched elsewhere in the galaxy. Rather than being swallowed past the event horizon as some models predicted, G2 broke apart, a behavior that led researchers to propose it — along with an earlier cloud, G1 — was actually the remnant of a star possessing a stronger self-gravity than a typical diffuse cloud. Scientists speculated this surge might signal the black hole transitioning into a new activity phase, or that it had simply stripped away the outer envelope of G2 during its close passage.

Sgr A East: A Remnant with Unusual Chemistry

Sagittarius A East, the broadest component of the Sgr A complex, is a supernova remnant roughly 27 light-years across and situated about 7 light-years from the central black hole. Although its classification as a supernova remnant was initially questioned, the detection of elevated ratios of manganese to iron and nickel to iron in its emission lines aligns closely with theoretical predictions for such objects. X-ray spectroscopy further suggests the progenitor event may have been a Type Iax supernova, which would make this the first confirmed example of that rare class within our own galaxy. The remnant's appearance shifts dramatically between observing bands: in radio, the brightest emission traces the outer rim, while in X-rays the center dominates, a pattern likely produced by interactions with the dense interstellar medium of the Galactic Center. One study also found overionized plasma within the region, an atypical condition that could result from ionization by charged particles streaming from Sgr A*, rapid post-explosion cooling, or thermal exchange with surrounding material. A leading hypothesis holds that the exploding star was gravitationally compressed during a close flyby of the central black hole.