Radio waves

Historical image of the location of the discovery of the first cosmic radio waves

Radio astronomy is taken back to its celestial origins with the first image of the supermassive black hole at the center of the Milky Way galaxy. The Event Horizon Telescope (EHT), a global array of millimeter wave radio telescopes, has captured a groundbreaking new image of where the first cosmic radio waves were identified.

A Very Large Array image of the central region of the Milky Way. The bright spot marked by the circle is Sagittarius A*, where our galaxy’s central black hole is located. Image Credit: NRAO/AUI/NSF

In 1932, Bell Telephone Laboratories engineer Karl Jansky made the first detection marking the beginning of radio astronomy.

The new EHT image represents the culmination of a long history of studies of the Milky Way, which began in 1610 when Galileo Galilei used his telescope to discover that our galaxy, appearing to the naked eye as clouds, is actually made up of stars. William Herschel, a British astronomer, created a rudimentary map of the Milky Way in 1785.

Harlow Shapley, an American astronomer, discovered the Milky Way’s core in 1918 using the recently discovered distance-measuring technique afforded by Cepheid variable stars to conclude that the Milky Way’s globular star cluster halo is centered on a location in the constellation Sagittarius.

Thick clouds of gas and dust hide this region from visible light telescopes.

In 1928, Bell Laboratories hired Jansky to determine the cause of noise interfering with shortwave radiotelephone communications. By 1932 he had developed a highly directional antenna and identified a variety of noise sources. However, “a very continuous static-like whistle of unknown origin” remained a mystery.

The time of day when this whistle appeared changed with the seasons. Jansky checked various astronomy textbooks on the advice of an astronomical acquaintance and decided in December 1932 that the strange whistle came from “beyond the solar system”.

In April 1933, he revealed this in an article he gave at a rally in Washington, DC. May 5and1933, the New York Times released his statement on the front page.

Jansky was featured on a national radio network ten days later, saying he had pinpointed the location of the noise he saw in the sky, “this seems to confirm Dr. Shapley’s calculation that radio waves seem come from the center of gravity of our galaxy.

Sagittarius A, the brightest source of radio emission in this constellation, was later named in his honor. Australian radio astronomers traced the source of the emission to the center of the galaxy in 1951.

In 1974, Bruce Balick and Robert Brown used the Green Bank Interferometer at the National Radio Astronomy Observatory to identify a very bright, compact object, which Brown eventually named Sagittarius A*.

The powerful radio emission from the object, abbreviated Sgr A*, has become the main theory of what powers it. The mass of the object was estimated to be 3 million times that of the sun by infrared and submillimeter investigations in 1994.

A 10-year investigation of the orbital motion of a star called S2 near Sgr A* was published in 2002 by a team led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics. According to the findings, the central object is more than 4 million times the mass of the sun.

Another team published a paper in 2009 based on additional measurements of stellar orbits in the region, concluding that the primary object is most likely a black hole because no other phenomenon can compress so much matter into such a small space.

This investigation and others by Sgr A* won UCLA’s Genzel and Andrea Ghez the 2020 Nobel Prize in Physics for “the most compelling evidence yet of a supermassive black hole at the center of the Milky Way. “.

The EHT collaboration’s generation of an image that matches theoretical predictions of what should be observed around a black hole strengthens the argument.