. The word Galaxy comes from the Greek word for milk. Galileo looked up at the milky streak that divides the night sky and proposed that it might be the aggregate light from millions of stars, very far away. It wasn't until the early 20th century when people like Vesto Slipher and Edwin Hubble looked closely at "Sky Survey" photographic plates and Spectroscopic data and realized that the "spiral nebulae" were actually galaxies. There must be at least 100 million galaxies in the universe.
Large (Barred) Spiral galaxy – 3 major arms typical of spirals, extending from the nucleus
Diameter: around 100,000 ly When seen from the side, it looks like a disk
Thickness: may be up to 30,000 ly at nucleus (near Sagittarius), and as little as 1000 ly at edges
Number of stars - 200 to 400 million stars
Age - some stars may be 13.6 byo
The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that the solar system lies close to the galactic plane.
Milky Way may be a Barred Spiral. Why can't we be sure?
The Solar System's position in the Milky Way
Recent estimates place us around 26000 ly from the central bulge
Older estimates has us as far as 35000 ly out
We are on the Orion arm/spur
240my to make circle around milky way on Orion arm section.
220km/s sun's velocity around galactic center
Milky Way Nucleus
Sagittarius A* is a bright and very compact source of radio emission at the center of the Milky Way Galaxy.
We now have evidence that it is a highly dense compact object, a supermassive Black Hole.
It may have more than 3 million solar masses, in a volume less than 6.25 light-hours (45 AU).
Democritus and Galileo both thought that the Milky Way might be the light from many distant stars. Later, Galileo could see them with his telescopes.
Origin of name – The Milk of Hera
Notice the high inclination of the ecliptic from the galactic plane. That is why all cultures knew it.
What keeps it all together?
When you get a few friends together, you call it a party.
When you get enough stars together we call it a galaxy.
Does this analogy work? Nope, there's more to a party than just people. You can't put your finger on all aspects of what keeps the party going, just like you can't account for all of the mass of a galaxy. Some of it simply doesn't give off light. We call it Dark Matter and Dark Energy, simply because we can't "put our finger on it" yet.