All of the stars we see in our night sky belong to our own Milky Way galaxy. Most galaxies external to our own require a telescope or photography to see clearly. When a planet is the furthest from the sun in the sky, usually making it the easiest to observe. Both the light and the loss of contrast make it difficult to find fainter stars and nebulae. This is becoming a bigger problem for both amateur and professional astronomers.
Astronomers use magnitudes to describe the brightness of an object in the sky.
It is a non-linear, numerical scale where the smaller numbers represent brighter objects. Generally, the unaided eye can see stars down to magnitude 6 in good skies. Pluto is magnitude 14, and thus, is invisible without optical aid or photography. Objects are highest in the sky, and thus least affected by atmospheric distortions, when on the meridian. More recently, it is used to describe clouds of gas in space.