What is a “star cluster”?
- stars formed together at same time
- stars may be gravitationally bound together
- two types: open (galactic) and globular
What is a “galaxy”?
A large group of stars outside of our own Milky Way (Vastly different sizes and luminosities)
- Made of billions to trillions of stars
- Also may have gas and dust
- Spiral - rich in gas and dust
- Elliptical - lack large amounts of gas and dust
- Irregular - result from galaxy collisions/mergers; very active star formation
What is a Light Year?
- A light year is the distance light travels in a year.
- Light moves at a velocity of about 300,000 kilometers (km) each second; how far would it move in a year? About 10 trillion km (or about 6 trillion miles).
It would take 11,780 Earths lined up side to side to bridge the 1 AU between Earth and Sun.
The Local Group
Composed of three (3) Galaxies:
- Milky Way Galaxy
- The Milky Way Galaxy is a giant disk of stars 100,000 light-years across and 1,000 light-years thick.
- It takes 200 Million years for the Sun to complete one orbit.
- There are over 100 Billion stars in the Milky Way
- Andromeda Galaxy
- Triangulum Galaxy
Gravitationally bound together—orbiting about a common center of mass
The region of the Galaxy within about 32.6 light-years of the Sun (65 light-years diameter) is considered its neighborhood.
The Local Supercluster
- A cluster of many groups and clusters of galaxies
- Largest cluster is the Virgo cluster containing over a thousand galaxies.
The observable Universe is 27 Billion light-years in diameter.
Probably 100 Billion galaxies in the Universe