Something Frightening in the Core

Proof for the existence of members of the other broad category of larger-than-stellar black holes the supermassive ones has also begun to emerge in recent years. These giants always appear to inhabit the centers, or cores, of galaxies, so it has become common to refer to them as “galactic black holes.” The reasons that it took so long to verify their existence are fairly simple. First, the cores of galaxies are extremely far away; even the center of our own Milky Way lies at the considerable distance of about twenty-six thousand light-years. Second, the galactic cores are also generally blocked from easy viewing by dense layers of gases, dust, and other cosmic debris.

In spite of these obstacles, astronomers persevered. Over the years, new and larger telescopes, along with more sophisticated detection equipment, revealed more and more information about the Milky Way’s core. There, it became clear, many huge stars lie very close together. Some of them are as large as 120 or more times the size of the Sun, and many of them float among the expanding and often overlapping gaseous remnants of many prior supernovas. “Like silken drapes blown in the wind,” writes noted science writer Robert Zimmerman,

the erupting waves of gas from scores of supernovas sweep through an inner region approximately 350 light-years across, filling space like froth and geysers. Here supergiant stars many times more massive than the sun and rare elsewhere in the galaxy number in the hundreds.

A Hubble Space Telescope photo of the galaxy NGC 4414. Both it and the Milky Way, which it resembles, likely contain giant black holes.

And within those 350 light-years are three of the galaxy’s densest and most massive star clusters, surrounded by millions of additional stars. So packed is this core that if the solar system were located there, a handful of stars [in addition to the sun] would float among the planets.

More ominously, astronomers also discovered something dark, monstrous, and frightening in the crowded galactic core. Almost all stars and other matter there are sweeping very rapidly around an extremely massive object. The first hints that something unusual lay in the center of our galaxy came in the 1950s. Radio telescopes, huge bowl-shaped antennas that gather and record radio waves from outer space, showed that a powerful source of these waves lies in the galactic core. These early images were crude and inconclusive. And thanks to the masses of gases and dust obscuring the core, visual images showed nothing.