What are masers? How can they be used to measure the distance to a galaxy?

Imagine being able to boost a signal as naturally as plugging an amplifier into your guitar, but with microwaves in space. This is what occurs with Microwave Amplification by Stimulated Emission of Radiation, commonly known as MASER. These natural cosmic phenomena are analogous to lasers, which work with light, but MASERs operate in the microwave range of the electromagnetic spectrum.

In astrophysical contexts, MASERs occur when molecules in space, such as water or methanol, get excited—usually through heat or radiation. When these molecules return to their lower energy state, they emit microwaves in a coherent and amplified form. As you might have already guessed, 'coherent' here means that all the waves are in phase, which translates into a strong and focused beam of microwaves. Since the direction is well-defined, scientists can use MASERs along with the precise art of radio astronomy to unlock cosmic secrets, including measurements that can lead us to calculate vast interstellar distances.

To measure the distance of far-off cosmic objects such as galaxies, astronomers utilize a high-resolution technique named Very Long Baseline Interferometry (VLBI). This technique involves a network of radio telescopes scattered across the Earth, which work collectively to function like a giant antenna. Through VLBI, astronomers can partake in a sort of cosmic triangulation.

When these telescopes simultaneously observe the same object, like an astronomical maser, they collect data that can be correlated to produce an image of tremendous precision. This method allows the measurement of angles with extraordinary accuracy, and by tracking the apparent motion of MASERs over time, scientists can derive information on the object's speed and trajectory. By further employing these observations over prolonged durations, it is possible to create a remarkably detailed picture of the space object's motion—akin to gauging the stride of a sprinter in a cosmic race. This information, fortified by the laws of physics, unlocks the capability to calculate the maser's distance from Earth with surprising precision.

You may have experienced the Doppler shift without realizing it—imagine an ambulance siren that changes pitch as it zooms past you. In astronomy, the Doppler shift is an equally critical and fascinating phenomenon. It refers to the change in frequency—or color—of light (or other electromagnetic radiation) from a source due to its motion relative to the observer.

A source moving toward the observer will have its waves squished together, causing the frequency to increase—this is known as blue-shifting. Conversely, if the source is retreating, then the waves stretch apart, and the frequency decreases, leading to red-shifting. Hence, by analyzing the spectral lines of light from celestial bodies, such as the MASERs from galaxies, and noting how they are shifted from their expected positions, astronomers can deduce whether these bodies are moving towards or away from us. Even more impressively, by calculating the degree of this shift, they can determine the speed at which these objects travel through space. This technique becomes a powerful tool when combined with VLBI, as it allows for the complex motions and distances of galaxies to be teased out, merely by studying the language of light they emit.