Here we can't see in an image that two stars are present, but we see periodic dimming in the amount of light we receive because one star is eclipsing the other.  So we can infer that we are viewing a binary.

Spectroscopic Binaries

Here, again, we can't see both stars in an image, but the spectrum shows periodic Doppler shifts indicating a binary.  In this case, we have a Double-Line Spectroscopic Binary.  Note how all three of Kepler's Laws are reflected in the motions of the stars.

Get used to these demonstrations by playing with them for a few minutes. You will be asked to use them later in order to answer questions. Just read the instructions and make sure you understand them before you move on.