It’s a Big Galaxy Out There

Last month, I was trying to remember how far away an open star cluster was. I was showcasing NGC 457 at a state park star party. This cluster is a cute appearing cluster in Cassiopeia. It is called the ET cluster or the Owl cluster. For kids these days, I call it the WALL-E cluster.

Since I could not remember its distance, I came up with something that seemed like it might make sense. I said that it was probably a couple hundred light years away. Boy was I wrong. I should have just said that I could not remember. Now there are maybe a dozen kids and parents roaming the world thinking that ET is a lot closer to home than he really is…

Of course, hundreds of light years away is still a long, long way.

Here is one way of thinking of these distances. It is about 3000 miles from the east coast of the US to the west coast. The circumference of the earth is about 24000 miles. Light travels at about 186000 miles in one second.

That means it would take about 1/7 of a second for light to circle the earth. 1/50th of a second is all it would take for light to make it from the left coast to the right coast of the US.

The moon is about 240,000 miles from the earth on average. So it takes about 1-1/3 seconds for light to travel from the moon to the earth. The sun is about 93,000,000 miles from earth. So it takes about 500 seconds or 8-1/3 minutes for the light from the sun to reach the earth.

Jupiter is hanging high in the evening sky right now. It is about 484 million miles from the sun. That is five times as far as the earth is from the sun. One way to talk about distances in the solar system is to measure in Astronomical Units (AU). One AU is the average distance of the earth from the sun. Jupiter’s distance from earth ranges from 4 AU to 6AU. In light minutes, Jupiter’s distance from the sun is about 41 minutes. But when I look at Jupiter right now, the time it takes for light to leave the sun, travel to Jupiter, reflect off the clouds and come back to my eye is closer to 80 minutes – almost an hour and a half.

I was pointing out Neptune to one of our club members from the light polluted city streets of San Antonio last week. Neptune is 4.5 billion miles from the sun. That is about 6.7 light hours from the sun. When I was pointing out Neptune, its light had been traveling for more than 13 hours since it left the surface of the sun. Another way to think of Neptune’s distance is that it is about a million times as far away as the distance across our country.

But that’s just peanuts.

The second nearest star is about 3 light years away. That’s 2000 times as far away as Neptune. Sirius, the brightest star in our sky is about 8-1/2 light years away. Star clusters are much further away. The Pleiades are a close cluster and are about 440 light years away. Maybe that is what I was thinking when I told my listeners that the ET cluster was several hundred light years away. As it is, the light from the Pleiades left those young suns about the time that Galileo was born.

But the ET cluster is a lot further away. When I finally looked it up while hiking yesterday, I was taken back when I read that this cute little open cluster is about 9000 light years away. The light I looked at the other night started heading this way about the time that people first began writing in Mesopotamia and the Chinese began making wine.

That’s a long time ago, and the galaxy we live in is a big place.