Posted
on December 30, 2014
We
Earthlings are not alone in our endless circuits around the Sun –
there are seven other planets and vast numbers of other celestial
bodies circling our star, making up our Solar System.
We
Solar System-ites are not alone in our rush through space – our sun
is but one of 100 BILLION stars in a vast, turning pinwheel in space,
our Milky Way Galaxy.
Well,
on this date in 1924, astronomer Edwin Hubble announced the discovery
that a small smudge in our skies – what had been considered a
spiral-shaped nebula (cloud of dust and gas in space) inside the
Milky Way – was in fact another entirely-separate galaxy!
In other words, the Andromeda Nebula had to be renamed the Andromeda Galaxy.
In other words, the Andromeda Nebula had to be renamed the Andromeda Galaxy.
Because
the newly discovered independent galaxy was 2-and-a-half billion
light years away from our galaxy, it seemed obvious that it was like
another island in an ocean of empty space. Hubble referred to it as
another “island universe.” He and other scientists immediately
wondered if all the other known spiral nebulae are also separate
galaxies, and of course it turns out that the ocean of space is
dotted with many island universes.
(We
now use the word universe
to mean everything we can observe in the cosmos. And we use the word
galaxy
to mean a group of stars that, along with gas and dust and black
holes and such, are held together by gravitational attraction.)
It
turns out, indeed, that there are at least 100 billion galaxies in
the universe! Here are a few of my favorites:
I
guess you can say that, on this date in 1924, our known universe got
a whole lot bigger!
How
did he do that?
I
hope you noticed that Hubble was not the first to actually see the
Andromeda galaxy; instead, he was just the first to understand what
it was: not a cloud of dust and a few stars within the Milky Way
Galaxy, but its own separate collection of billions of stars and dust
well outside the Milky Way.
The
key to Hubble's discovery was figuring out how far away Andromeda
really was. An astronomer named Henrietta Leavitt had figured out
that a certain kind of variable star – a Cepheid variable – has a
set ratio between its maximum brightness and the period of its
brightness changes. A Cepheid that is quite close to us looks
brighter than a Cepheid that is far (just as a nearby candle seems
brighter than a distant candle), so we can use the dependable ratio
to compute how far away the Cepheid is.
In
other words, Leavitt's careful observations and analysis meant that
we now had a measuring stick to use in space! And Hubble had to do
some very careful observation and analysis to detect Cepheids as far
away as those in Andromeda. But he succeeded in detecting some –
hence his discovery we honor today!
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| This is what happens when galaxies collide. |
Also
on this date:
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ahead:
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