X is for X-Ray Astronomy

X-rays have a shorter wavelength and higher frequency than visible light.  Most people encounter x-rays for the first time in a medical setting, but x-rays are extremely useful for astronomical purposes as well.  That's because plenty of objects emit x-rays.

Here's a video that discusses the history, present, and possible future of x-ray astronomy.

W is for Wavelength

Electromagnetic waves bombard us from every direction each second of every day.  One of the important properties of EM waves is the wavelength. We can only visually observe waves of a certain wavelength, which is often referred to as the visible spectrum.  

To learn more about the higher and lower wavelength EM waves you cannot see, watch the video below.

The following video gives a brief explanation of the Doppler Effect, which deals with the shift in wavelength observed when the object emitting the waves is in motion relative to an observer.