The color of the laser is precisely tuned to energize a layer of sodium atoms found in one of the upper layers of the atmosphere — one can recognize the familiar color of sodium street lamps in the color of the laser.
This layer of sodium atoms is thought to be a leftover from meteorites entering the Earth’s atmosphere. When excited by the light from the laser, the atoms start glowing, forming a small bright spot that can be used as an artificial reference star for the adaptive optics. Using this technique, astronomers can obtain sharper observations. For example, when looking towards the center of our Milky Way, researchers can better monitor the galactic core, where a central super massive black hole, surrounded by closely orbiting stars, is swallowing gas and dust.
Taken with a wide angle lens, this photo covers about 180° of the sky.