Astronomer’s Notebook: What is at the center of our galaxy, the Milky Way?

An invisible monster lurks beyond the stars of the constellation Sagittarius near the border with Scorpius: a supermassive black hole, designated as Sagittarius A* (pronounced “Sagittarius A-star”), and abbreviated Sgr A*.

Sgr A* emits enormous amounts of energy in the form of x-rays and radio waves, far greater than would be expected from an ordinary star-sized body.

After mapping the motions of stars near Sagittarius A* for decades, astronomers concluded that Sgr A* has a mass of 4.3 million solar masses, but its size is in the same ballpark as a red supergiant star like Antares.

Most extraordinarily, it is invisible!

The only possible single object that could fit this description is a supermassive black hole.

In 2000, Reinhard Genzel and Andrea Ghez shared half the Nobel Prize in P