December 30 – Another “Island Universe” Is Announced

nPostednon December 30, 2014

nWenEarthlings are not alone in our endless circuits around the Sun –nthere are seven other planets and vast numbers of other celestialnbodies circling our star, making up our Solar System.

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WenSolar System-ites are not alone in our rush through space – our sunnis but one of 100 BILLION stars in a vast, turning pinwheel in space,nour Milky Way Galaxy.

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nWell,non this date in 1924, astronomer Edwin Hubble announced the discoverynthat a small smudge in our skies – what had been considered anspiral-shaped nebula (cloud of dust and gas in space) inside thenMilky Way – was in fact another entirely-separate galaxy!

Innother words, the Andromeda Nebula had to be renamed the AndromedanGalaxy.

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Becausenthe newly discovered independent galaxy was 2-and-a-half billionnlight years away from our galaxy, it seemed obvious that it was likenanother island in an ocean of empty space. Hubble referred to it asnanother “island universe.” He and other scientists immediatelynwondered if all the other known spiral nebulae are also separatengalaxies, and of course it turns out that the ocean of space isndotted with many island universes.

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n(Wennow use the word universento mean everything we can observe in the cosmos. And we use the wordngalaxynto mean a group of stars that, along with gas and dust and blacknholes and such, are held together by gravitational attraction.)

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nItnturns out, indeed, that there are at least 100 billion galaxies innthe universe! Here are a few of my favorites:

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nInguess you can say that, on this date in 1924, our known universe gotna whole lot bigger!

nHowndid he do that?

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nInhope you noticed that Hubble was not the first to actually see thenAndromeda galaxy; instead, he was just the first to understand whatnit was: not a cloud of dust and a few stars within the Milky WaynGalaxy, but its own separate collection of billions of stars and dustnwell outside the Milky Way.

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nThenkey to Hubble’s discovery was figuring out how far away Andromedanreally was. An astronomer named Henrietta Leavitt had figured outnthat a certain kind of variable star – a Cepheid variable – has anset ratio between its maximum brightness and the period of itsnbrightness changes. A Cepheid that is quite close to us looksnbrighter than a Cepheid that is far (just as a nearby candle seemsnbrighter than a distant candle), so we can use the dependable rationto compute how far away the Cepheid is.

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nInnother words, Leavitt’s careful observations and analysis meant thatnwe now had a measuring stick to use in space! And Hubble had to donsome very careful observation and analysis to detect Cepheids as farnaway as those in Andromeda. But he succeeded in detecting some –nhence his discovery we honor today!