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Credit & Copyright: Courtesy
Carnegie Institution for Science
Explanation:
How big is our universe?
This question,
among
others,
was debated by two leading astronomers in 1920 in what has since
become known as
astronomy's
Great Debate.
Many astronomers then believed that our
Milky
Way Galaxy was the entire universe.
Many others, though, believed that our galaxy was just
one of many.
In the
Great Debate,
each argument was detailed, but no consensus was reached.
The answer came over three years later with the detected variation
of single spot in the
Andromeda Nebula, as shown on the
original glass discovery plate
digitally reproduced here.
When Edwin
Hubble compared images, he noticed that this
spot varied, and on October 6, 1923
wrote "VAR!" on the plate.
The best explanation, Hubble knew, was that this spot was the
image of a variable star that was very far away.
So M31 was really the
Andromeda Galaxy --
a galaxy possibly similar to our own.
Annotated 100 years ago, the
featured image
may not be pretty, but the variable spot on it opened a window
through which humanity gazed knowingly, for the first time, into a
surprisingly vast cosmos.
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NASA Official: Jay Norris. Specific rights apply.
A service of: LHEA at NASA / GSFC
& Michigan Tech. U.
Based on Astronomy Picture
Of the Day