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What is a galaxy?

A MASSIVE SPINNING cluster of stars, celestial objects, solar systems, gas and dust clouds (like nebulae); all held together by a strong force called GRAVITY.

Theorized to have formed from huge clouds of gas billions of years ago.

Areas began to collapse where there was more gravity and the gas clouds began to spin.

Over time creating a shape that is either… SPIRAL , ELLIPTICAL, or IRREGULAR

In 1764, Charles Messier labeled it as the Andromeda Nebula-M31.

However the GREAT DEBATE happened between two astronomers in 1920.

Harlow Shapley and Heber Curtis debated over whether Andromeda was a spiral nebula or a totally separate galaxy.

But then in 1923 Edwin P. Hubble measured flickering stars known as cepheid variables (stars that pulsate)

and found that this flickers was caused by both nearby dust and gas.

By using the Hooker telescope with its 100-inch mirror, on top of Mount Wilson in California.

By measuring the change in brightness of these stars Hubble was able to calculate just how far away these stars were.

This then found that these stars were MUCH further away than what we measured the size of our Milky Way galaxy to be.

This showed that these stars were then part of an entirely SEPARATE galaxy.

Hubble published this discovery in 1929.

We then knew that our universe wasn’t just our Milky Way Galaxy.

Courtesy Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF