On a clear night in central Idaho, you can see the sweep of our own Milky Way galaxy split the velvet sky. Although we now know the observable Universe spans 13 billion light years, in the 1920s, astronomers didn’t even know how big the Milky Way was. In fact, many astronomers believed our galaxy comprised the entire universe and that what we now know as different galaxies were just strange nebulae within the Milky Way. The story of how astronomers finally took the true measure of the Universe as a result of the the tireless efforts of a human computer.
Astronomical Rule-of-Thumb
For many decades, astronomers have been able to measure distances out to about 100 lightyears using a technique called parallax. This technique is, in fact, the same way your brain measures distances across the room. If you hold your thumb out at arm’s length and blink one eye closed and then switch eyes, you’ll see your thumb leap back and forth against the background. The closer your thumb is to your face, the farther it will leap. Your brain automatically turns this parallactic shift between your eyes into depth perception, and, depending on your visual acuity, it allows you to gauge distances out to maybe 10 meters (30 feet). Beyond that distance, there’s too little parallactic shift for your eyes to see.
Astronomers can use the Earth’s orbital motion to see parallactic shifts for some objects but only out to about 100 lightyears, barely 0.1% of the distance across the Milky Way. However, even the nearest stars exhibit very tiny shifts. The very first parallax measured wasn’t measured until 1838, when the German astronomer Friedrich Bessel detected a shift in the position of the star 61 Cygni of 0.3 arcseconds, the angular equivalent of seeing a dime at a distance of about a mile.
But, just like your brain uses other clues for more distant objects, astronomers also use other techniques. Bizarrely, the foundational technique that finally elucidated the Universe’s vastness involves stars that inflate and deflate like balloons.
Unrequited Love
Henrietta Swann Leavitt could be forgiven for believing she was born in the wrong generation. A Massachusettsan with deep family roots, she attended Oberlin and Harvard college in the late 1800s, an era when women weren’t even allowed to vote. She fell in love with astronomy during her last years at college and attempted to pursue a career in astronomy, only to be stymied by Victorian-era strictures on female ambition. Undeterred, she joined the increasingly renowned group of human computers at the Harvard College Observatory. One of the only opportunities for women to contribute to professional astronomy, members of the Harvard computers helped establish the stellar type system still used today and determine the compositions of the stars.
Leavitt herself studied a group of stars, later named “Cepheid variables”, that waxed and waned in brightness over days to months. Leavitt analyzed thousands of such stars and found that the period over which such a star oscillated in brightness depended on the brightness of the star: the brighter the star, the longer the oscillation period. This result transformed astronomy.
Prior to Leavitt’s work, astronomers couldn’t tell whether a star was dim because it was very far from Earth or because it was just a dim star very close to Earth. But with Leavitt’s discovery (now called “Leavitt’s Law”), astronomers could figure out the brightness from the oscillation period and then use the difference between a star’s apparent and absolute brightness to work out its distance.
In the 1920s and 30s, Edwin Hubble used Leavitt’s Law to measure the size and expansion of the Universe. Famously, Hubble showed that the more distant a galaxy was from us, the faster it was moving away, a relationship now known as “Hubble’s Law”, which itself is now used to estimate the distances to the most distant galaxies. Hubble said that Leavitt deserved the Nobel Prize for her discovery, and in 1925, members of the Swedish Academy of Sciences, tried to nominate her. They contacted Harvard to gather evidence of her work, only to learn that Leavitt had died in 1921, and, unfortunately, Nobel Prizes are not awarded posthumously.
Across the Universe
Leavitt’s Law allowed astronomers to leap across the Universe and determine not just the distances to faraway galaxies but also understand their evolution across time. A vast and bewildering zoo of galaxies emerged, many with surprising and unusual properties. For example, ultra-faint dwarf galaxies (UFDs) are itty-bitty galaxies with less than one hundred thousand stars, making them the faintest galaxies in the Universe. (The Milky Way, a typical medium-to-large galaxy, has 100 billion stars.) Puzzlingly, UFDs contain a significant amount of dark matter. In fact, they are the most dark matter-dominated systems known.
Astronomers believe that UFDs are telling us something important about the early Universe and the origin of dark matter and energy. Recent work has hypothesized that UFDs may have formed very early in the Universe’s history, and untangling this knotty problem will rely on calculating the galaxies’ distances and ages, estimates that, at bottom, will rely on the tireless work of a human computer.
On Friday, Mar 3 at 7:30p MT, join Boise State Physics for our First Friday Astronomy event when we will host Dr. Katy Rodriguez Wimberly, who will discuss her research on the evolution of ultra–faint dwarf galaxies. The event will be presented in-person on campus (room 112 in the Education Building) and via live-stream – http://boi.st/astrobroncoslive.