High winds are the norm at the center of the Milky Way. Astronomers have now clocked suns orbiting the galactic core at a staggering 3,000 miles (4,800 kilometers) per second. At this rate, Earth would complete its orbit around the sun in a mere three days. What lurks at the galaxy’s core that can accelerate stars to such speeds?
Astronomers have considered various possibilities. Does the center of the galaxy harbor a tight cluster of superdense stellar remnants (neutron stars)? Or perhaps a huge ball of subatomic neutrino particles?
But these and other more exotic possibilities were eliminated in the spring of 2002 when a star called S2 swept down in its highly eccentric orbit and passed within 17 light-hours of the Milky Way’s center — a minuscule distance in galactic terms. In 17 hours, light travels three times the distance between Pluto and the sun.
Only one object is compact enough and has sufficient mass to accelerate stars to such a high speed: a supermassive black hole. Astronomers had suspected that a black hole must lie at the Milky Way’s core, but plotting the orbit of S2 and other stars dramatically strengthened the evidence.
Our central black hole is small by the standard of what lurks in the hearts of other galaxies. Observations of the giant elliptical galaxy M87 suggest the presence of a black hole 6 billion times more massive than the sun. The interaction of two supermassive black holes probably produces the intense X-rays streaming from the galaxy NGC 6240. The Andromeda Galaxy may harbor a black hole of 140 million solar masses.
