The Moon creeps up on the twin stars of Gemini this evening. As night falls, Pollux and Castor are above the Moon. Pollux is on the left, and is a bit brighter than its “twin.”

There’s a lot more to Gemini than just the twins — or even the other stars that are visible to the unaided eye. In fact, one of its most intriguing objects produces most of its energy in the form of X-rays and gamma rays, which are far beyond the range of the human eye.

The object is known as Geminga — short for “Gemini gamma-ray source.” It was discovered in the 1970s, but it took decades for astronomers to figure it out.

Geminga is a neutron star — the dead core of a star that exploded as a supernova more than 300,000 years ago. It’s more massive than the Sun, but only about a dozen miles in diameter. That means it’s compressed to billions of times the density of ordinary matter.

Geminga rotates four times per second. As it spins, it sends out beams of energy — mostly gamma rays, the most powerful form of energy.

The dead star is racing through the galaxy at about 270,000 miles per hour. As it rams into gas in its path, material in a disk around the star is ripped away, forming a tail. At the same time, jets of particles that beam out from its poles form a double tail about half a light-year long. So Geminga is like a fast, heavy ship plowing through the interstellar sea, leaving a long, bright wake in its path.

Script by Damond Benningfield