Your Handle on the Night Sky for Aug. 28, 2008

by Dan Pope

This week our Moon is not in our evening sky and that will make it easier to locate the dim constellation Cepheus (SEE-fee-us), the King. When we locate the Delta star in Cepheus, we are looking at one of the most famous “standard candles” in the history of astronomy.

We think our most ancient ancestors assumed the stars that appeared to be bigger and brighter were just that - bigger and brighter, but at the same distance from us as the dimmer ones. Undoubtedly there were some who conjectured otherwise but no one had scientific proof to support their conjectures.

By the early 1600’s, Johannes Kepler, using Tycho Brahe’s data, had worked out a model of our Solar System that showed the relative distances among the planets and our Sun. For instance he had determined that Saturn was about ten times as far away from the Sun as Earth but he didn’t know the actual distances from the Sun. Once any one of the distances between two objects in our Solar System was determined then all the other distances could be calculated by using Kepler’s model.

The invention of the telescope in the early 1600’s made it possible to make increasingly more accurate observations. Giovanni Cassini used a method known as “parallax” to calculate the distance from Earth to Mars. The parallax method depends on the apparent shift of an object when compared with its background. To illustrate this method, hold your index finger at arm’s length and notice how your finger appears to jump as you look at it with just your right eye then just your left eye. By measuring the distance between your eyes and the angle your finger appears to jump you could use trigonometry to calculate the distance from your eyes to your finger. In 1672 Cassini was in Paris, France and he sent Jean Richer to Cayenne, French Guiana in South America, 4,500 miles away. They both observed Mars at the same time and recorded the positions of the background stars. Using parallax he was able to calculate the distance to Mars. Kepler’s model of relative distances was then used to calculate all the other distances in our Solar System. No two places on Earth are far enough apart to use the parallax method to calculate the distance to the nearest stars, beyond our Sun. But the distance from the Earth to the Sun was now known (93 million miles) and that distance could be used with the parallax method to calculate the distances to stars several hundred light years away.

In 1784 John Goodricke (1764-1786) was the first person to discover that ‘Delta’ was a variable star. Delta varies in its apparent brightness from magnitude 3.48 to 4.37 in 5.366 days. The research on variable stars grew slowly over the next 120 years.

In 1912, Henrietta Leavitt (1868-1921) published her research into variable stars of the same type as ‘Delta’ but in the nearby Small Magellanic Cloud. She found a correlation between the period (length of time from bright to dim and back to bright again) and luminosity (brightness) of the ‘Delta’ type of variable stars. She concluded that these stars were all of the same absolute brightness, which meant that those that appeared dimmer were farther away. In 1913 Ejnar Hertzsprung used Leavitt’s results to calculate the distances to some ‘Delta’ type stars in our Milky Way and the Small Magellanic Cloud. Delta became known as ‘the standard candle’ for measuring distances to the stars. Up until the 1920s, the prevailing thought was that our Milky Way was our entire Universe. In 1924 Edwin Hubble found a ‘Delta’ type star in what was then named the Andromeda Cloud. He used Leavitt and Hertzsprung’s distance calculating formulas to show that the Andromeda Cloud was far outside of our Milky Way Galaxy. In fact it was not a ‘cloud’ in our Milky Way Galaxy but a galaxy in its own right - far, far away – the Andromeda Galaxy. Suddenly it was realized that our Universe was much, much larger than anyone had previously conjectured. Hubble extended his distance measuring work and is credited with showing that our Universe is expanding. It is an understatement to say it made a huge and dramatic change in our outlook and the study of astronomy. The Andromeda Galaxy is the most distant object that can be seen by the naked eye, 2.5 million light years!

Estimating distances within the Cosmos is no trivial matter. The techniques now used are many and complex. Among the latest and most sophisticated is the European Space Agency’s satellite named Hipparcos. One of the results is a catalogue that contains greatly improved accuracy for positions, motions, brightness and colors for 2,539,913 stars.

Next week we will explore Cassiopeia, Cepheus’ Queen.

Clear skies.