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Define terms of Galaxy Clusters, Basic Astronomical Objects, Astronomical Distance Units, Terms relating to motion

Our solar system "The Milky Way" consists of:-

-          Sun (which is a star)

-          9 planets & their moons

-          Asteroids; chunks of rock

-          Comets; balls of ice

-          Countless tiny particles of interplanetary dust

- Galaxy: is a great island of stars, containing a few million to trillion or more stars.

The Milky Way
- Disk- shaped collection of stars
- contains more than 100 billion stars

- one of the two largest galaxies among the 40 galaxies in the local group

Galaxy Clusters: Groups of galaxies with more than a few dozen members.

Star: Large and glowing balls of gas that generate heat and light through nuclear fusion in their cores.
                   - Protostars: objects in the process of  becoming stars
                   - Neutrons: stars that have died.

Basic Astronomical Objects

Planet: A moderately large object that orbits a star. Planets may be rocky, icy or gaseous in composition, and they shine primarily by reflecting light from their star.

Moon/ Satellite: An object that orbits a planet

Asteroids: chunks of rock

Comets: balls of ice 

Collections of Astronomical Objects:

Super Cluster: A gigantic region of space that where many individual galaxies and clusters are packed closer together than anywhere else in the universe.
Universe: The sum total of all and energy , that is, everything within and between all galaxies.
Observable Universe: The portion of the entire universe that can be seen from Earth.

Astronomical Distance Units 

Astronomical Unit (AU): The average distance between the Earth and the Sun , which is about 150 million k.m
Light- Year: The distance that which light can travel in 1 year, which is about 9.46 trillion kilometers.

Terms relating to motion: 

Rotation: The spinning of an object around its axis. Earth rotates once each day around its axis, which is an imaginary line connecting the North and South poles.

Revolution: The orbital motion of an object around another. Ex. The Earth revolves around the sun once each year.

Expansion (of the universe):  We say that the universe is expanding because the average distance between galaxies is increasing in time. Note that while the universe as a while is expanding, individual galaxies and their contents are NOT expanding because of gravity forces.A planet is defined as an object that:-

1) orbits a star (but is itself neither a star nor a moon)
2) is massive enough for its own gravity to give it a nearly round shape.
3) Has cleared the neighborhood around its orbit

-The nearest star system to us is a 3-star system called; Alpha Centauri is about 4.4 light years away.

- The earth orbits the sun at a speed in excess of 100,000 km/h in a path called ecliptic plane.

-The axis tilt happens to be oriented so that it points directly at a star called polaris/North Star. (tilted by 23.5 degrees.

-The universe began in the big bang.