The beast at the heart of our galaxy lurks so stealthily that for a long time many scientists were not certain it existed at all.
The first hint of that monster came in February 1974, when astronomers Bruce Balick and Robert Brown discovered an unusually compact source of radio waves at the exact center of the Milky Way, some 26,000 light-years away. Brown subsequently named this enigmatic object Sagittarius A* (pronounced “A star”), after the constellation where it appears in the sky. Then came more hints of something strange in the neighborhood: a trickle of X-rays from the same spot, tendrils of agitated gas surrounding it and, most telling, a small group of stars racing around madly for no obvious reason.
Tracking the motions of those stars enabled astronomers to estimate the mass of the unseen object directing the action. From there they built a convincing case that Sagittarius A* was in fact a black hole — the biggest one in the galaxy, with a mass 4.3 million times that of the sun and a diameter of about 25 million kilometers. At the black hole’s outer boundary, known as the event horizon, the fabric of space pours inward like a waterfall at the speed of light. Anything caught up in that flow takes a one-way journey to oblivion, winking out of our reality as it crosses the event horizon and then entering an inner realm cut off from the rest of the universe.
