The action is likely to unfold in two acts, said Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Germany. His team discovered the gas cloud, G2, in 2011.

The first stage could come just weeks after the cloud, already stretched into a spaghetti-like strand by the black hole’s gravity, makes its closest approach to Sagittarius A*. Models suggest that will occur around March 31, give or take three weeks, Dr. Gillessen said.

At closest approach, the cloud’s distance from the black hole will still be about 200 times Earth’s distance from the sun. Nonetheless, the passage might be near enough for G2 to plow into the outer edge of the swirling disk of matter believed to surround and feed the black hole.

The shock wave generated by that encounter may create X-rays and radio waves that could be detected by telescopes. If the cloud contains a star at its center, as some astronomers have proposed, it may produce a stronger shock and generate more light, said Sera Markoff of the University of Amsterdam.

The encounter could have another effect: Disruptions in the cloud might alter the direction in which radio waves vibrate as they travel through. Simultaneous observations with telescopes tuned to different radio wavelengths have a chance of discerning the waves’ twists, giving astronomers new details about the cloud’s properties, said Geoffrey Bower, a radio astronomer with Asiaa, the Taiwanese astronomy organization, in Hilo, Hawaii.

But most of the potential fireworks are at least a year away, perhaps decades. That is how long it may take material torn from G2 to spiral inward through the black hole’s feeding disk, coming 100 times closer to the hole than it is now. At that location, gas becomes hot enough to radiate at high intensity before it is swallowed.

Moreover, astronomers think that if the gas cloud is to be swallowed, it will have to lose much of its angular momentum, or orbital rotation, around the black hole.