## Saturday, February 11, 2012

### Music on My Mind

Yakko's Universe Song! And he's right, you know. Suppose that this period is the size of the earth:

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Then this is the size of the sun, more or less, in earth-size-units:

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If we were to take a period to indicate an earth-diameter, then the distance from the earth to the sun would be somewhat more than eleven thousand six hundred periods. Suppose that this is the entire distance between the earth and the sun (an AU):

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Then this is the radius of the solar system in AUs, more or less, if we take it to include everything within the orbit of Pluto:

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That's the radius, mind you; the diameter would be twice as long. But it's bigger than that, really; if we include all the comets orbiting the sun, then it would end up being somewhat more than a thousand times longer. I will spare you that. Likewise, the distance to the Proxima Centauri, the nearest start to our sun, would be about six or seven thousand times longer than the above sequence.

Suppose that this is the diameter of the whole solar system, including the Kuiper Belt and the Oort Cloud (which is the measure that's more-or-less a thousand times the above sequence of periods):

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Then the distance to Proxima Centauri, the star nearest to our sun, doesn't look so bad, being a bit shorter than this in whole-solar-system-units:

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But if we wanted to indicate the radius of the Milky Way, we would need a fair number more than ten thousand periods.

Suppose that this is the diameter of the entire Milky Way galaxy:

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Maybe you want to get to the nearest galaxy, not counting the Canis Major Dwarf galaxy, which is technically within the Milky Way itself. That would probably be the Sagittarius Dwarf Elliptical galaxy, which is (if you find the diameter of the Milky Way to be a conveniently traversable distance) quite close in diameter-of-Milky-Way units:

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A little closer than that, actually.

But what if the real action is in the nearest spiral galaxy? That's Andromeda. Taking the same measure, diameter-of-Milky-Way units, the distance is, more or less:

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The local neighborhood is (conveniently, if a bit self-centeredly) called the Local Group, and in diameter it is about a hundred times our diameter-of-Milky-Way unit. Suppose this is one galaxy, like the Milky Way:

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There are at least this many galaxies in the Local Group:

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They are not all the same size. The Milky Way is the second largest; Andromeda is larger. (Although if we measure by mass, the Milky Way might be the largest.) Most of the others are small as galaxies go, which is still very, very big.

The Local Group is one of the two major groups in the Virgo Supercluster, the other being the Virgo Cluster. If this is diameter of the Local Group:

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Then this is more or less the diameter of the Virgo Supercluster:

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The Virgo Supercluster is a small supercluster among a large population of superclusters. There are about 10 million superclusters in the observable universe. Truly, truly, it's a great big universe.