Posted by: T. Boyd | June 15, 2009

How Far the Stars?

The heavens are telling the glory of God;
and the firmament proclaims his handiwork.
Day to day pours forth speech,
and night to night declares knowledge.
There is no speech, nor are there words;
their voice is not heard;
yet their voice goes out through all the earth,
and their words to the end of the world.
(Ps. 19:1-4 RSV)

Astronomers state that some distant galaxies are 15 billion light years away. How do they know that? They estimate this with a series of extrapolations. Only for nearby stars can the distance be measured directly. The others are indirect.

The first step in the process is to use the parallax method similar to the first kind of rangefinders used on golf courses or for hunting. This technique uses two lenses separated a few inches from each other, like a person’s eyes, giving two slightly different views of an object.

The two views are merged by an adjustment from which the distance to the object is produced. (Our brains do that automatically, so we can reach out and put our hands right on an object, or cast a fishing lure right where we want it to go.)

In the case of astronomy, the two views are taken 1/2 year apart, so that the earth has traveled to the other side of the sun between measurements – a separation of 186 million miles. About 100,000 star distances have been determined this way. The closest star (other than our sun) is about 4 light years away (the distance that light travels in 4 years), and the furthest one determined this way is about 3300 light years away.

An astronomer chooses a far away star and compares its color spectrum to the tabulated data to find a close by star that matches. They compare the brightness of the two stars. Knowing how brightness varies with distance, they can estimate the distance to this distant star. Simple? Not really, but it is logical.

The problem is that it is based on several assumptions. And the uncertainty in the distance measurement is significant. But it is the best that can be done at this time.

So, how far the stars? Even though the stars are silent, as the psalmist says, their “voice speaks” by their light, by their spectra. We can estimate that they are, indeed, very far away. Yet even at their great distance, they bring a grandeur and majesty to the night sky that has amazed every generation.


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