Miss Leavitt's Stars; George Johnson; 2005; Atlas Books; 162 pgs, Acknowledgements, notes, index,

​I have often thought about how much further  mankind might be if we (men) had not banished  half of the  worlds brightest  minds (women) to a life of drudgery and gestation.  But every now and then, one or two women manage  to beat the odds. Henrietta Swan Leavitt was one.

Miss Leavitt  was an astronomer, although her official title was "computer".  She was a diligent  researcher who spent much of her time measuring the apparent brightness of stars recorded on photographic plates.

In her day (circa 1915), the words "galaxy" and "universe' were synonyms.  Examining the brightness of star can give clues to how far away it is. If you know how far away it is, you can know its absolute brightness and vice versa. Ms Leavitt studied stars in the Magellanic Clouds. Some stars are called "variable" stars, Their brightness varies over time.  Some of those stars are Cepheid  variables. What Miss Levitt discovered was that all Cepheid variables that share the same period also have the same absolute brightness. In other words, Cepheids can be used as a "standard candle". Once the distance to one Cepheid star is known, it is trivial to  calculate how far away other  Cepheids are. The story of determining the size of the universe is some what more complex. For example, inter stellar dust mucks up the calculations. But Henrietta's discovery was the first step. Before her discovery, Andromeda was though to be a cloud inside the Milky Way, which in turn was thought to be about 100,000 light-years across. Her work showed that Andromeda was another galaxy even larger than the Milky Way, and was something like two million light years away.

Another well known modern standard candle today is the type 1A supernova.

This book is a quick read, and an interesting recounting the squabbles between stubborn  astronomers as the moved slowly towards a much grander description of the universe in which we live.

Miss Leavitt was born in 1868 and died of cancer in 1921.              

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