VII. BEYOND THE SOLAR SYSTEM: THE MILKY WAY 
Stars
When we look into the night sky, it is populated with star systems of all shapes, sizes and colors. Some are supergiants hundreds of times bigger than the sun, some are puny, no bigger than the Earth. Some are blue some are red, they come in many colors. Some are solitary, the majority (maybe as many as 75%) come in multiple star systems. Some have planets, others do not. Some stars are relatively stable, others vary in size and brightness. 

The brightness that we see here on earth called the apparent magnitude is a function of two things: the actual amount of light that the star puts out and its distance from the earth. The apparent magnitude scale of stars was first established by Hipparchus, a hellenistic astronomer.  His original magnitude scale was qualitative. Today we have extended and refined the scale into a quantitative scale and a first magnitude star is 2.5 times brighter than a second magnitude star, a sixth magnitude star is 100 times dimmer than a first magnitude star, etc..  The brighter the star, the lower the number.  In order to be able to compare stars without the influence of distance, astronomers also talk about how bright a star would be at a standard distance (taken to be 10 parsecs or 32.6 LY). We call this absolute magnitude. For example the sun has an apparent magnitude of -26.7, but an absolute magnitude of +4.8. Because stars do not put out all of their energy in the visible spectrum, we use a third measure, luminosity, or the total amount of energy a star puts out.  There are six luminosity classes.  It turns out that two factors affect luminosity, surface area and surface temperature. 
 
LUMINOSITY CLASSES
It further turns out that 
Ia
Bright Supergiant
Ib
Supergiant
II
Bright Giant
III
Giant
IV
Subgiant
V
Main Sequence Star
 
Open cluster Globular cluster
Stars are not scattered evenly in space; they tend to come in clusters. Some clusters are open, often rich in bright, bluish younger stars. These are more commonly found within the star and dust lanes of the Milky Way. Other clusters exist in, or surround the center of the galaxy in a halo, much as comets surround the plane of the solar system. These globular clusters with up to a million stars or more are rich in older, more yellow stars. 
   
   The Eagle Nebula: a bright nebula                                          The Horsehead Nebula: a dark nebula
Between the star systems lies interstellar space. This is not a vacuum devoid of matter, but space that contains dust and gases. In many places this dust and these gases are tenuous; in other places they are concentrated into nebulae (clouds). Some of these nebulae are close to stars and will fluoresce (glow) from the energy of nearby stars. Others will simply reflect the light of these stars. Both of these nebulae are called bright nebulae because they give off light. Other, often dense, nebulae, far away from bright stars will actually block starlight. These are called dark nebulae
Since Hubble, we have come to understand that these stars, nebulae and clusters are not evenly and randomly distributed across the universe, but that they clump up (as does all matter) into larger aggregates called galaxies.