How do water masers produce their radio signals that can be picked up on Earth by radio telescopes?

Water masers are produced when water vapour (H₂O) molecules in space act as a natural microwave laser. The word maser stands for Microwave Amplification by Stimulated Emission of Radiation.

Here’s how it works:

1. Warm, dense gas is needed. Water masers occur where temperatures are typically 300–1000 K and the gas density is much higher than in ordinary interstellar space.
2. Water molecules are pumped into an excited state. This pumping is usually caused by:
   • collisions in shock waves,
   • infrared radiation from warm dust,
   • or a combination of both.
3. A population inversion develops, meaning more water molecules are in an excited energy level than in a lower one. This is the essential condition for maser action.
4. When a microwave photon at the correct frequency passes through, it stimulates an excited water molecule to emit an identical photon. The new photon has the same frequency, phase, and direction, so the signal is amplified.
5. If the radiation travels through a long path of water-rich gas, the amplification becomes enormous, producing the very bright maser signals detected by radio telescopes.

The strongest astronomical water maser transition is at 22.23508 GHz (the one you’re observing with your Norsat LNB). Despite the enormous apparent brightness, the emission comes from very small regions—often only a few astronomical units across.

Water masers are commonly found in:

• Star-forming regions, where powerful outflows and shocks surround young stars (e.g. W49, W3(OH), Orion KL).
• The envelopes of old giant and supergiant stars, where expanding gas contains abundant water vapour.
• The centres of some galaxies, where gas orbiting a supermassive black hole forms extremely luminous “megamasers.”

For amateur radio astronomers, these masers appear as very narrow spectral lines, often only a fraction of a MHz wide, with their exact observed frequency shifted slightly by the Doppler effect due to the motion of the gas. This is why your SDR spectrum around 22.235 GHz (after downconversion by the LNB) shows narrow peaks rather than broad emissions.