Sentry Beam

Sentry Beams are metallic robots that make their only appearance in Super Mario Galaxy and its sequel Super Mario Galaxy 2. They are enemies that will try to defeat Mario by firing circular emanating lasers at him.

Super Mario Galaxy
Sentry Beams make their first appearance in the Flipswitch Galaxy (where one guards the Power Star) and then reappear later in the Battlerock Galaxy, Bowser Jr.'s Airship Armada, the Toy Time Galaxy, the Deep Dark Galaxy, the Buoy Base Galaxy, and the Dreadnought Galaxy.

Super Mario Galaxy 2
In Super Mario Galaxy 2, they only appear in a few galaxies, namely the Space Storm Galaxy, the Mario Squared Galaxy, the Shiverburn Galaxy, and the Grandmaster Galaxy, among others.

Traits and Abilities
Sentry Beams constantly remain stationary and attack by firing lasers in a circular shock wave formation. If the shock waves touch Mario, he will be electrocuted and lose one section of his Health Meter. Sentry Beams will only fire these lasers when Mario gets near enough to them, and there is a limit as to how far they can go before they disappear. On the Landing Platform in the Dreadnought Galaxy, these boundaries are clearly defined by large white circles on the ground around the Sentry Beams, which the shock waves cannot go past.

Sentry Beams cannot be destroyed by any means. However, there is a way to stun them, temporarily stopping them from attacking and giving Mario refuge from their barrage of shock waves. Mario can either jump on top of the robot or fire a Star Bit at it, causing it to become stunned for several seconds. If Mario jumps on the top of the robot twice, the spring beneath its head becomes exposed and Mario can then use them like a trampoline. Some Sentry Beams do not fire lasers at all, but still have a springy neck nevertheless.

There are also two very rare variants of the normal Sentry Beam; one has a nut on top of it, while the other has a bolt. They are both yellow in color instead of purple. They can both be found in the Buoy Base Galaxy on the Poké Ball Planet in Super Mario Galaxy, with the nut-like robot on the bottom of the planet and the bolt-like robot on the top. The Sentry Beam with the nut is very much like the original Sentry Beam robot, though with no springy neck. Once Mario or Luigi spins the bolt of the bolt-like Sentry Beam, it will disappear and the planet will open up to reveal the Power Star. Nut-like Sentry Beams also appear on the Watermelon Planet in the Deep Dark Galaxy.

Sentry Beams also have a spiked yellow variant which attacks in the same way as its purple counterpart, though Mario cannot stun it due to its spikes. They also lack eyes.

The final Sentry Beam variant is a gray, mobile unit with sinister-looking eyes, which hovers around with the aid of a propeller on its bottom. This variant is able to shoot a straight laser beam at Mario from its mouth. The laser beam is very fast and very hard to avoid. Though it cannot be defeated, if Mario stands on its head, it will cease all action. However, once Mario starts moving again it will continue to attack.

All Sentry Beams are invincible, and they cannot be defeated by anything less than Rainbow Mario himself. Mario can throw Bob-ombs at the robots to delay their attacks, but this still will not destroy them.

Trivia

 * Note that the Sentry Beams on the Starting Planet of the Shiverburn Galaxy fire yellow circular laser beams instead of the normal red, most likely implying that the beams are in fact electric, rather than regular laser beams.