Milky Way and Galaxies

• Galaxy is a collection of stars, planets, hydrogen gas, and interstellar matter and debris. It is also bound together by mutual gravitational attraction.
• The farthest distance traveled by man-made objects from Earth is 152.2 AU by Voyager 1.
• Milky Way galaxy has been obtained by observing from the inside. "Observe trees in the middle of a dense forest."
• Milky Way Galaxy is a spiral galaxy.
• The view of the Milky Way Galaxy from the top or bottom looks like a spinning pinwheel.
• The Sun is located on one of the spiral arms, about 25,000 light years away from the center of the galaxy.
• Immanuel Kant said that our galaxy is flattened with the Sun near the center.
• Immanuel Kant also said that if the galaxy is spherical, we should be seeing the same number of stars in all directions. This led to conclude that the galaxy is disk-like rather than a ball of stars.
• William Herschel supported Kant's idea by counting the number of stars in different directions.
• Herschel stated that most of the stars lay in a flattened structure encircling the sky.
• William Herschel found out that the disk of the galaxy is 5 times longer than its width.
• HII Regions characterize the spiral nature of a galaxy; made up of free electrons and protons ionized by young stars.
• Jacobus Kapteyn and Harlow Shapely used global clusters to estimate the size of the galaxy, and it pegged at around 25,000 parsecs or 80,000 light-years.
• Kapteyn and Shapely said that based on the galaxy's mass, there could be 200 billions stars within it.
• The structure of our galaxy is a disc that extends to about 30 kiloparsecs, and is made of stars, open clusters, and nearly all our galaxy's dust and gas. It also has a subtle swoop and spiral arms.
• The halo of the Milky Way galaxy is a globular clusters surrounding the disk. Old stars with little to no gases make up the halo.
• Majority of the galaxy's mass is found in the halo.
• Bulge is the dense cloud of stars surrounding the center.
• Radius of bulge is around 2 kpc
• The bulge contains little gas and dust.
• The formation of stars is scarce in the bulge region.
• The sun is located somewhere along the disk between the center and its edge.
• The sun swirls around the center along with the other stars and other interstellar materials.
• Elliptical galaxies are mostly reddish and lacks hot and bright stars. It is also indexed with numbers 0-7 based on their roundness with E0 being spherical and E7 being highly elliptical.
• All spiral galaxies have halos.
• Spiral galaxies have disks with 2 or more arms swirling out of their centers.
• Spiral galaxies contain young stars and sites of star formation.
• Spiral galaxies are easier to notice since they are luminous.
• Spiral galaxies are classified into several categories - S for spiral and SB for barred spiral.

  a for large nuclei and few hot stars

  b for intermediate values

  c for small nuclei and tons of hot and bright stars
• Irregular galaxies lack uniform shapes and visible patterns, and are often found in clusters.
• Irregular galaxies can be seen as splotches and patches of chaotic mixture of stars, gas, and dust
• The shape of an irregular galaxy could be a result of gravitational interaction or collisions with other galaxies.