she ruled the known universe

By the late 19th century astronomy had been revolutionized by photography.  Long exposures accurately produced much more detail than the best human eyes could hope to record and spectrography lead to the creation of astrophysics.  A real revolution was underway and large sums of money were going into new telescopes.

 

There was a fly in the ointment.  A night's worth of human observations were equivalent to a few kilobytes of information.  A good photographic plate contained thousands of times more.  Astronomy was entering its first age of big data. Finding items of interest and making accurate measurements required a lot of computation.  That meant human computers.

 

To create a catalog of the location, brightness and color of every star in the observable universe Edward Pickering created of the first computer centers at the Harvard College Observatory  The first computers were Harvard students, but Pickering found the young men unreliable.  He switched to female computers .. largely educated women who were unmarriageable for one reason or another, allowing them to have a full time job.  Pay was a bit more than a female school teacher made -  twenty cents an hour.  This group became one of most remarkable computer centers of all time.

 

Henrietta Leavitt  studied music at Oberlin and went on to the Harvard Annex (now Radcliffe) to get a college degree.  She enjoyed astronomy so much that her well-to-do family supported her volunteer work at the Harvard Observatory for a few years.  Her speciality was variable stars - the oddballs that changed in brightness over relatively short amounts of time.

 

She went back to her family taking a position as an arts assistant at a local college, but continued to work on a paper based on her observations.  She became ill and became deaf making marriage and work in music unlikely.   Henrietta got in touch with Pickering.  She could come back, but he couldn't pay her.  She went back to work without a salary - after all, her family was expected to support her.  Years later he started to pay her. 

 

She set out to characterize about 1800 variable stars in the Large and Small Magellaniac  Clouds.  As she worked something started to stand out in her mind.  If she assumed these stars were a similar distance away, after all, they were in the same cluster,  she deduced the period over which their brightness changed was related to their brightness. 

 

The apparent brightness of a light changes by the inverse square of its distance.  Double the distance to a light and it appears a quarter as bright.    Henrietta had discovered if two of these variables - known as Cepheid variables - had the same period, but one was one hundred times dimmer, you could deduce it was ten times farther away than the brighter star.  

 

Measuring the distance to objects in the Universe is core to astronomy and cosmology.  Distances to nearby objects - out to about fifty light years away - could be measured using parallax.  Hold a finger in front of your face and look at it with your left eye.  Now your right.  You'll notice a shift in position.  In astronomy you note a position at one part of the Earth's orbit and measure again six months later. Knowing the diameter of the Earth's orbit and the angular difference between the two measurements allows you to calculate the distance with trigonometry.  Unfortunately the technique  only works for nearby objects as the angles become too small to measure with increasing distance. 

 

By the early teens distances to several nearby Cepheids had been measured using parallax. Leavitt's yardstick had been calibrated.  Her result was one of the most important developments in astronomy - certainly worthy of the Nobel Prize.   She was unknown outside of astronomy, but was finally nominated for the Nobel Prize in Physics.  When information was being collected by the Nobel committee it was discovered cancer had taken her a few years earlier.

 

In the mid twenties Edwin Hubble used the Hooker telescope and Leavitt's relation to measure the distance to Cepheid variables in what was then called the Andromeda nebulae. The distances he found were mind blowing - about a million light years.¹  Andromeda was much farther away than anyone had imaging and couldn't be in the Milky Way.  It was a galaxy and the Universe was made of more than just the Milky Way. 

 

The physics of the how Cepheids work took decades to work out, but her relation didn't require a detailed knowledge of the mechanism.²  It became a workhorse and is still regularly used out to distances where you can still see Cepheids.  Over time a series of other yardsticks have been created all using her notion of a knowable absolute  brightness and the inverse square law.  It's how science advances, but someone has to take that first huge leap.  Someone should make a movie about these remarkable computers, particularly the deaf one who figured out how to measure from here to the stars. 

 

__________

¹ The currently measured distance is something over twice as far out - well within his measurement error.  The million light year distance was mind blowing at the time.

A small detail.  Astronomers often use the parsec as a unit of length rather than light-years.  A parsec is a parallax-second - the distance one astronomical unit (the radius of the Earth's orbit) subtends an angle of one arc second..   it's about 3.26 light-years.  Everyone else seems used to light years so I use them here.

² Cepheids are bright  yellow giants - anywhere from 4 to 20 times the mass of the Sun and up to 100,000 times as luminous.  During their period they see their radius change about as much as 25 percent.   They have a lot of helium in an outer layer.  Helium changes in opacity depending on how ionized it is.  Doubly ionized helium - helium missing both electrons - is more opaque than singly ionized helium.  As helium heats up it becomes more ionized.  At the dimmest part of the star's cycle the outer layer of helium is most opaque.  It absorbs heat from the star's radiation and expands.  As it expands it cools and the helium layer becomes more transparent.  At some point it begins to contract due to gravitation getting hotter as it goes.  The transparent mostly singly ionized helium becomes doubly ionized and opaque and the cycle starts over.  It's a huge fusion powered heat engine.