Miss Leavitt's Discovery


The Perimeter Series lecture that took place at the Bruce County Museum and Cultural Centre on Wednesday April 30, 2008 was well attended. It was part of the PI series that will continue in two weeks.

The subject of the lecture by George Johnson, a science writer,  was Miss Henrietta Leavitt (shown), a woman who worked as a 'computer' at Harvard University in the early 20th century.  The Astronomy Department was populated by people such as Harlow Shapely.

They had been using men for a long time to do calculations, but they proved unreliable.  Women without regard to higher education were enlisted in this tedious process of examination and calculation.

So women became human 'computers'.  They looked at glass plates taken from telescopic pictures.  They were asked to compute many things and worked in teams.  One of these women, Henrietta Leavitt, was a college graduate, who had partially lost her hearing and was not easily employed as a teacher or in other areas.  The women worked for 25 cents per hour.

In the early 1900's the canopy of the sky consisted of the wandering planets and the fixed stars which rotated uniformly across the scene of the observer.  Every object seen by eye or telescope was seen to be part of the same thing that we today call the Milky Way.

One problem that loomed large was the distances between our earth and the objects seen in the sky.  In particular, the modern telescopes could see concentrations of light that took on odd shapes like swirling masses and spirals.  They were all thought to be part of the known universe, that is, what we now know as the Milky Way.



The mathematics for objects relatively close to us like the moon, planets and some nearby stars (less than 100 light years) could be worked out by observations and trigonometric means as shown in the picture above for a near star.  But, some objects appeared to be so far away that this method just did not work because no observation point in the earth's orbit around the sun had a broad enough base to make the calculations accurate.  Astronomers knew that some object were at great distance, but how great, they just did not know?

Miss Leavitt and others reasoned that if they could determine how bright an object was at these distances, then they could at least compare one object to another, if the same brightness or relative brightness could be known in two or more different places.  Miss Leavitt was a simple 'computer' and not part of the faculty, one must remember. 

How could these measurements be possible, then?

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Inverse Square Law

Light Luminosity is diminished as it moves away from the Source

Everyone knew that light dimmed according to the inverse square law.  If we have a light bulb of 100 Watts, it will have a certain luminosity (energy emitting) at 1 foot, but at 2 feet the brightness will diminish by not twice, but by 4.   You can get a 'feel' for this with a 100 Watt bulb by placing your hand very near the bulb and them moving it away in increments. You'll see that the heat (energy) drops off like the brightness. This is called the inverse square law.  So if you measure brightness at distances it will have a luminosity of 1/4, 1/9, 1/16, ... as you increase the distance by 2, 3, 4, .... units.

The City Lights Analogy

There are many ways to explain how Astronomers know how far away things are, but perhaps the clearest analogy is that of the City Lights. Imagine being on the roof of a tall building with no way to leave. At night you can see lights in all directions. How can you map your "universe"? For nearby lights, you can use triangulation by just stepping off the base of the triangle and doing some sums.

By observing from different locations on the roof, and measuring changes in the relative positions of the lights, you can determine how far away the lights are. Once you know how far away the lights are, you can determine their true brightness. In nearby towns, the lights are all too far away to measure their distances by triangulation, but you can recognize lights of the same types as those you see nearby.

Since you know how bright these nearby lights are, you can calculate how far off the lights in the town are. For distant towns, you cannot even see individual lights, but you know how far away the nearby towns are, and how much total light the towns emit, so you can use this information to estimate how far the distant towns are.

At even greater distances, only clusters of towns are visible, and finally only great urban complexes, but at each stage you can use what you already know to discover facts about the more distant universe.

This was the reasoning of Miss Leavitt, but what could she use as the base standard?  She did not have a 1000 Watt bulb to obtain a start.  You might say that every star has the brightness of the Sun, but you would be wrong because some of the stars are 10,000 times as bright and more.  If you made that wrong turn, then everything would seem closer to in the view of your guess about the world.


There are a class of star called a Cepheid. Cepheids are named for a star in the constellation Cepheus, the first star of this type discovered. These stars pulsate in a regular way,

Cepheid variables are yellow-white giant stars that pulsate and vary in brightness, but in a regular way. The brighter the absolute magnitude of a Cepheid, the faster it pulsates. It is easy to measure the period of a Cepheid variable, and armed with this information, Miss Leavitt and her superiors were able to determine the absolute brightness of the star. How did this work?  We'll follow  the City Lights example, taking into account the distances to nearby Cepheids that were known by triangulating them. 

Miss Leavitt noticed the key fact of the relationship of brightness and period, which allowed astronomers to create a standard scale that can be used for vast distances across space.  The inverse square law played its part.

For the first time it was recognized that the Milky Way was just one of an unknown number of galaxies.  One Cepheid is the North Star, Henrietta was a star too!

For an introduction see Miss Leavitt, the Computer