First, as you navigate this site, there will be links marked with two stars ‘**’ such as this **one. Visiting these links will give a more advanced overview of the topic. Be warned, however, these advanced links would require physics knowledge at least at a college Freshman/Sophomore level!
Although telescopes are blind to the third direction, it can discern different frequencies of light (exactly how the human eye detect color). Now, the Doppler effect is a phenomenon where if a light source is moving towards an observer, the observer will observe an increase in the light’s frequency (vice versa for a light source moving away from the observer).
This effect can be understood by noting the wave property of light. Take a look at the animated gif below:
The source (red dot) is emitting a wave at a constant frequency (the expanding circles). Since the source is moving, each new circle is emitted at a point further to the right than the previous circle. The wavefronts at the direction of the movement (right) get bunched up together, producing an increase in frequency (vice versa for waves emitted to the left).
What does this look like? If I drive a red car towards you at very large speeds, you will see my car look blue! We can literally change the color of an object by putting it in motion! Of course, the shift in frequency is very small for the typical speeds we experience on Earth. As such, this effect is not very perceptible on our daily lives. In astronomical context, however, objects can be moving extremely fast (think thousands of kilometers per second!). At these velocities, the change in color can be easily observed by telescopes.
How does this help an astronomer?
The amount of frequency shifted by the Doppler effect is related mathematically to the velocity of the emitting source. Therefore, a telescope can discern the velocity of an astronomical object. A caveat however, is that the classical Dopper effect formula is only valid for movement along the line between the observer and the source. We can use the Doppler effect to figure out the velocity along our telescope’s line of sight, but not perpendicular to it.
Ok, how does that help the astronomer?
Next we need to learn about spectral lines. The idea is that particles (atoms or molecules) can either produce or absorb light at very specific frequencies. Different species of atoms or molecules have spectral lines at different frequencies. This resulted in different species of atoms and molecules having different color. For example, a copper solution is blue, while an iron oxide solution is red.
Now, we combine our knowledge that different atoms and molecules have different color with the fact that we can literally change the color of objects by putting them in motion. Since we know what the color of a specific atom is when it is at rest (we did a lot of laboratory experiments to figure this out), we can figure out how fast they are moving by relating the change in color with the velocity. Remember that different atoms start at different frequencies. So, not only can astronomers measure how fast an object is moving, we can also figure out what that object is made of!
We can distinguish between a cloud of hydrogen moving towards us at 200 km/s with a cloud of carbon monoxide moving away from us at 50 km/s!
That’s nifty, but how does that help you with depth perception?
Time to move on! On the subsequent pages, I will show you how we can translate velocities to the elusive third dimension using a case study: the exploding star Cassiopeia A!