Friday, July 11, 2008

How Far?

When I show things off at a star party, we usually look at things that cover a long range of distances, from near to far.

Very Close: 1-2 Light Seconds Away

During the day, we'll sometimes put a telescope on a nearby landscape feature such as a mountain top or a cell tower. The distance to these objects is familiar to most people. It's usually fractions of a mile to a few hundred miles. Astronomically speaking, these objects aren't at all distant.

Close: 1-6 Light Hours Away

We also show off the Moon both in day and night. It is only a quarter million miles away. It takes light about one and a quarter seconds to go this distance, so we might say that it is just over a light-second away.

The Sun and bright planets range in the millions of miles. The sun is about 93 million miles away, planets range from a bit under half this distance to several times this distance. The distance to the sun is about eight light minutes. The planets range from about three light minutes (a very close pass with Venus) to just about six light hours (Pluto and Eris.)

Nearby: 4-25 Light Years Away

The next step in distance takes us from measurements in light hours to measurements in light years. The nearest star is a component of the triple-sun Alpha Centauri system, called Proxima Centauri. It's just over four light years away. This is about 37,000 light hours away. The nearest stars are a whole lot further away than the solar system's planets, the visible comets, the asteroids, the Moon and Sun. Sometimes in art you'll see a star shown in front of the Moon (or within the horns of a crescent Moon.) This can never happen, they are millions of times further away than the Moon. (Yes, except for our own star, the Sun. But it's still hundreds of times farther away than the Moon.)

Nearby stars include Altair, in the Eagle, Sirius in the Big Dog, and Procyon in the Little Dog. Near range out to about 25 light years away or so.

Middle Distance: 25-100,000 Light Years Away

Stars in our galaxy range from these nearby neighbors to the stars in the globular clusters. These are about as far away as you can get while still being in the Milky Way galaxy. The ones you can see can be about as far away as 80,000 light years. Most of the stars you'll see at night are far closer, however, from the nears stars above out to distances of a few hundred light years.

This range includes the nebulas, like the Ring Nebula, Orion Nebula, and Lagoon Nebula. It also includes the star clusters.

Far: Millions of Light Years Away

Here we get to the galaxies. The nearest galaxy to ours (not counting small companions to the Milky Way, like the Magellenic Clouds) is the Andromeda Galaxy, at about 3 million light years distance. A whole bunch of galaxies in the Virgo Cluster is about 50 million light years away. These objects are all far further from us than any of the individual stars, clusters, or nebula you will see through a telescope that are part of our galaxy. Telescopes with mirrors or main lenses larger than 10 inches or so can show individual items in the closest galaxies, like the globular clusters and nebulas in the Andromeda galaxy. But normally any of these you see are in our galaxy, and normally over 100 times closer than even the nearest galaxy.

If you want an idea of what you can see in different sizes of telescopes, see What Can I See in My Telescope?

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