December 30 – Another “Island Universe” Is Announced

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.

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!

This is what happens when
galaxies collide.

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