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Candles in the Night

When the distance to a light increases, the brightness of the light drops by the square of the distance. A light moved 10 times farther away will only be 1/100 as bright. We can use a star of known brightness to measure distances across the unbridgeable chasm of space. Such star is called a standard candle. If we located a star just like our Sun but a trillion times dimmer, we would know it was a million times the distance to the Sun or about 15 light years away. Unfortunately, there are two major problems with using sun-like stars. (1) We need an independent method to know a star's brightness before we can use the star as a measuring rod. Without an independent double check, it is impossible to judge how big (and therefore how bright) a star is. Some stars are giants and very bright. Other stars are dwarf and very dim. (2) Main stream sun-like stars really aren't all that bright. They are so dim that we can't see them in our nearest extra-galactic neighbors. At best, sun-like stars are short range measuring tools.

In 1912 Henrietta Levitt was examining photographs of the Magellanic Clouds, two of the nearest extra-galatic star systems. She noticed that some of these stars varied regularly in brightness just like the large bright variable stars we call Cepheids in our own galaxy. Since all the stars in the Clouds are about the same distance, differences in brightness reflect size, not distance. She determined that stars with long periods were brighter than those with short periods. This was the crucial independent double check we needed.

All we needed was to determine the distance of a Cepheid in our own galaxy and we would have our standard candle in the night. Alas, even the nearest Cepheid Variable (the pole star Polaris) is too far away to be measured by surveyor's trigonometric methods. Scientists were forced to use a technique called star streaming to determine the distance. To get an idea what this is like, imagine driving in a huge Bumpem Car ride. All of the cars move in random directions and various speeds. However, the cars nearest you will generally seem to move faster. The cars farther away seem to move more slowly on average. If you measure enough cars and factor in your own car's speed, eventually you can determine by statistical methods the approximate distance to the other cars. Using a similar reasoning, measuring the apparent motions and speed of stars in our galaxy allows us to estimate the distance to the nearest Cepheids.

The importance of Miss Levitt's candles in the night can hardly be underestimated. They allowed Edwin Hubble to determine the distance to the Great Galaxy in Andromeda (M31). This in turn established the expanding universe, the birth, maturation and death of stars, black holes, the Big Bang and just about all of modern cosmology. In spite of the tremendous advances this allowed us to make, we need to exercise some judgment. This type of measurement is probably no more accurate than ±10%.

Even Miss Levitt's candles are not bright enough for really vast distances. From them we calibrate brighter beacons such as supernova, whole galaxies and ultimately the red shift of light. So when we read that the Hubble Space Telescope has found a galaxy twelve billion light years way that was formed within a billion years of the Big Bang, remember how shaky our measurements are. A 10% error is longer than the time since the first multi-celled creatures evolved until now.

As the we enter the New Year, some old friends are making their last appearances of the season as some new friends come into view. Look for Venus in the western sky these evenings just after sunset. During January ,Venus will be passing by two very hard to find planets, Uranus and Neptune. On the 8th Venus it will be located about equidistant and slightly below between these two planets. With binoculars, see if you can find a greenish "star" [Uranus] and a bluish "star" [Neptune] about five degrees from Venus. [Five degrees is 10 Moon diameters in the sky.]

After a fine Fall and early Winter, Jupiter and Saturn in the western skies are getting ready to leave us until late next Summer. Don't miss a chance to see these two spectacular planets before they disappear behind the Sun. Mars rises just after midnight. As the season passes it will rises progressively earlier. Look for it in the eastern sky just about 1 AM. By sunrise it will be almost overhead. Mercury makes a brief appearance in the eastern sky in the morning twilight. Look for a pinkish "star" where the Sun is rising.

Throughout the night, the great Winter constellations shine. The constellation Gemini (the Twins), stand with their feet just above Orion's Club. The two brothers are Castor (Alpha Geminorum) and Pollux (Beta Geminorum). Stars prefixed "alpha" are supposed to be the brightest stars in their constellation, but Pollux is brighter than Castor. In fact, even Alhena (Gamma Geminorum), at the tip of Castor's foot, is brighter than Castor. I've often wondered but do not know whether these stars were mislabeled or if somehow Castor has dimmed. Various civilizations see the constellations differently. We see the Big Dipper where the Romans saw the Great Bear. Where we see a Teapot, the Romans saw an Archer. However, just about every civilization sees Gemini as Twins.

Leslie Coleman
Author:
Leslie Coleman
Entry Date:
Dec 1, 1998
Published Under:
Leslie Coleman's Columns
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