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From: Henry Spencer <>
Subject: Re: Who first proved that galaxies were collection of stars?
Date: Sun, 8 Mar 1998 23:49:43 GMT

In article <01bd4ac3$7b22d380$d1cea8c1@default>,
rabuelow <> wrote:
>...In the mid-1920s Edwin Hubble found Cepheides in the
>Andromeda nebula and thus was able to prove that is doesn't belong to our
>home galaxy. (At that time he thought that the nebula was around 1 million
>light years away but Walter Baade showed in the 1940s that the distance is
>twice as much.)

And it was also Baade who finished settling the matter, by resolving
separate stars in the galactic core of Andromeda, establishing it as a
galaxy much like our own.  (His two major accomplishments were related:
he resolved the stars while gathering evidence for the theory that there
were two separate star populations in local galaxies, one older than the
other... and it was the recognition of the two populations which caused
reassessment of the distance scale, because each population has its own
Cepheids, and you have to know which flavor of Cepheid you're looking at
to estimate its distance.)

Interestingly enough, it was World War II that enabled Baade to do this.
Most US astronomers switched temporarily to doing war work of some kind,
but he couldn't -- he was German and wasn't allowed to.  So he had Mount
Wilson and the 100-inch telescope almost to himself, and could patiently
experiment and tune his procedures for the best possible results.  And to
cap it off, blackout procedures in Los Angeles gave him much darker night
skies than usual.
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         |

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