Black hole of the Milky Way became the “birth cry” of radio astronomy

(ORDO NEWS) — The first image of the supermassive black hole at the center of our Milky Way galaxy brings radio astronomy back to the sky.

The Event Horizon Telescope (EHT), a worldwide network of millimeter-wave radio telescopes, has taken a new, iconic image of the same region where the first cosmic radio waves ever detected came from.

This discovery, made by Bell Telephone Laboratories engineer Karl Jansky in 1932, marked the beginning of radio astronomy.

The new EHT image is the culmination of a long history of exploration of the Milky Way, beginning with Galileo Galilei, who discovered with his telescope in 1610 that our galaxy, which appears to be a cloud to the naked eye, is actually composed of stars. In 1785, British astronomer William Herschel made a simple map of the Milky Way.

In 1918, American astronomer Harlow Shapley determined the center of the Milky Way using a newly discovered distance measuring instrument provided by the Cepheid variable stars and determined that the halo of globular star clusters surrounding the Milky Way was concentrated in a region in the constellation Sagittarius. This region is hidden from visible light telescopes by dense clouds of gas and dust.

Jansky was hired by Bell Laboratories in 1928 and given the task of identifying sources of noise interfering with shortwave radiotelephone communications.

He developed a highly directional antenna and by 1932 identified a number of noise sources. However, one mystery remained – “a very persistent static-type hiss, the origin of which is unknown.”

The time of day when this hissing appeared varied with the seasons. At the suggestion of an astronomer friend, Jansky turned to astronomy textbooks and in December 1932 concluded that the strange hiss was coming from “outside the solar system.”

He announced this in a paper he presented at a meeting in Washington, D.C., in April 1933. His statement was published on the front page of The New York Times on May 5, 1933.

Ten days later, Jansky was interviewed by a nationwide radio station and said that he had determined the position in the sky of the noise he had detected, and “this seems to confirm Dr. Shapley’s calculation that radio waves seem to come from the center of gravity of our galaxy.”

Later, this area was named Sagittarius A, as the brightest source of radio emission in this constellation. In 1951, Australian radio astronomers further narrowed the range of radiation sources to the center of the galaxy.

In 1974, Bruce Balik and Robert Brown, using the National Radio Astronomy Observatory’s Green Bank Interferometer, discovered a very bright and compact object, which Brown later named Sagittarius A* (adding an asterisk).

The black hole has become the leading explanation for what is causing the object’s bright radio emission, abbreviated as Sgr A*. In 1994, infrared and submillimeter studies estimated the object’s mass to be 3 million times that of the Sun.

In 2002, a team led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics reported a 10-year study of the orbital motion of the star S2 near Sgr A*. As a result of the study, it was concluded that the central object is more than 4 million times more massive than the Sun.

In 2009, another group of researchers reported further observations of stellar orbits in the region and concluded that the central object is likely a black hole, since no other phenomenon is known to be able to accommodate such a mass in such a small space.

This work and other research on Sgr A* earned Hansel and Andrea Ghez of the University of California the 2020 Nobel Prize in Physics for “the most convincing evidence for the existence of a supermassive black hole at the center of the Milky Way.”

The EHT Collaboration’s acquisition of an image consistent with theoretical predictions of what should be seen around a black hole today makes this evidence even more compelling.


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