Definition and Discovery

A Galaxy is a system of stars and Planetary systems, dust, and gas held together by gravity. We live in the Milky Way Galaxy, a Spiral galaxy.


. 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.

Shapes of the galaxies

  1. Spiral
    1. Spiral galaxies have a nucleus and have arms that spiral away from it
    2. Largish - their diameters range in size from 20,000 light years to 100,000 light years
    3. Examples - M31 and Milky Way
    4. Most consist of at least one billion stars
    5. How might this happen?
  2. Barred Spiral
    1. A barred galaxy is a type of galaxy where the spiraling arms start at the ends of a bar that goes through the center of the galaxy.
    2. How can this be?
  3. Elliptical
    1. Round galaxies without spiral arms
    2. Resemble the Nucleus of spiral galaxies
    3. Can be very dim...
    4. Sometimes very small and densely packed... sometimes exceptionally huge
    5. Can be seen from great distances... in other words, some are very old
    6. What happened to their spiral arms?
  4. Irregular
    1. Not spiral or elliptical
    2. Irregular in shape (also asymmetrical)
    3. Various reasons for irregular shape:
      1. Collisions with other galaxies
      2. Gravitational effects of close galaxies
      3. Recently merged with smaller globular cluster
  5. Lenticular
    1. Seen edge-on from our point of view, it is like looking at a cross-section of a lens.
    2. They may be Spiral, Barred Spiral, or a Hoag Object (Elliptical with a surrounding ring). We generally cannot tell. Sombrero-IR.jpg

Formation Theories

  1. Bottom-up theory - smaller objects (groups of stars) formed first, then merged to form galaxies.
  2. Top-down theory – large clouds formed, then star formation began.
  • Perhaps both are correct...
  • Nevertheless, we think it all started with a big bang ~13.7bya, and galaxies began forming fairly soon afterward.
  • It's more important to understand the relationships between the shapes than to just be able to identify them.

Our Milky Way Galaxy

  1. Description
    1. Large (Barred) Spiral galaxy – 3 major arms typical of spirals, extending from the nucleus
    2. Diameter: around 100,000 ly When seen from the side, it looks like a disk
    3. Thickness: may be up to 30,000 ly at nucleus (near Sagittarius), and as little as 1000 ly at edges
    4. Number of stars - 200 to 400 million stars
    5. Age - some stars may be 13.6 byo
    6. 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.
    7. Milky Way may be a Barred Spiral. Why can't we be sure?
  2. The Solar System's position in the Milky Way
    1. Recent estimates place us around 26000 ly from the central bulge
    2. Older estimates has us as far as 35000 ly out
    3. We are on the Orion arm/spur
    4. 240my to make circle around milky way on Orion arm section.
    5. 220km/s sun's velocity around galactic center
  3. Milky Way Nucleus
    1. Sagittarius A* is a bright and very compact source of radio emission at the center of the Milky Way Galaxy.
    2. We now have evidence that it is a highly dense compact object, a supermassive Black Hole.
    3. It may have more than 3 million solar masses, in a volume less than 6.25 light-hours (45 AU).
  4. Discovery
    1. 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.
    2. Origin of name – The Milk of Hera
    3. Notice the high inclination of the ecliptic from the galactic plane. That is why all cultures knew it.
  5. 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.