By Roger Linnett
Henrietta Swan Leavitt’s contribution to astronomy is acknowledged as one of the most significant of the 20th century, but, as is typical of women in science in this country, her name elicits only blank stares and shrugged shoulders.
Born in Lawrence, MA., in 1868, she enrolled in Oberlin College in Ohio in 1885, completed two years, and transferred to the Society for Collegiate Instruction of Women (later Radcliffe College) in Cambridge, MA. She graduated in 1892 at the age of 24. Her degree was a certificate that stated if she were a man she would have been awarded a Bachelor of Arts degree from Harvard.
In the late-1870s Edward C. Pickering, Director of the Harvard College Observatory had embarked on a project to produce a photographic survey; a photo-mosaic of the entire night sky. Pickering recruited as many as 40 women – the work was quite tedious and paid too little to attract any men – to work for him to reduce the stellar data from several thousand photographic plates. Pickering treated them with respect, and tried to make their jobs interesting and stimulating. These human computers were referred to, affectionately, as “Pickering’s Harem.”
When Henrietta first started working at the observatory, she was assigned the job of recording the magnitude of stars, using a microscope and measuring the size of the stars’ disks on the photographic plates, and comparing them against pictures of stars whose magnitudes had been ascertained.
In the mid-1890s, she began the task that would become her life’s work, and profoundly change the science of astronomy. She was asked to search for variable stars. These stars pulsate, as if they were breathing, growing brighter then dimming. Finding them was accomplished by a slick bit of photographic legerdemain.
Two images were taken of the same star field, but at different times, usually days or weeks apart. One of these negative plates was then converted to a positive plate. These two plates were then exactingly aligned, and those stars whose brightness was constant were canceled out while those whose brightness had changed stood out. Henrietta, it seemed, had a genuine talent for ferreting out the rare and elusive stars.
These pulsating stars were called Cepheid (See-fee-id) variables after one of the first stars to be recognized as having this unusual quality, Delta Cepheid (the fourth brightest star, designated Delta – the fourth letter of the Greek alphabet) in the constellation Cepheus, a king from Greek mythology.
In 1896 she spent two years traveling in Europe, and returned to Beloit, Wisconsin, where her father was a minister. During this period she started having health problems and her hearing deteriorated rapidly. She returned to Harvard in August, 1902, but was only able to work four hours a day. She was appointed to the observatory’s permanent staff in 1903.
During the next few years she discovered 1777 variable stars in the Large and Small Magellanic Clouds, which are actually dwarf galaxies gravitationally bound to our Milky Way, but at that time they were believed to be part of it.
In 1908, she published her results in a paper which is now considered an astronomical classic. A subsequent paper published in 1912, “Periods of 25 Variable Stars in the Small Magellanic Cloud” elaborated on her 1908 publication.
It contained the observation that was to become the basis of one of astronomy’s greatest discoveries: “A remarkable relation between the brightness of these variables and the length of their cycle will be noticed.”
When plotted on a graph a Cepheid variable’s changing brightness looks like a playground slide – a quick, steep rise to maximum brightness (the stairs), and a slower undulating decrease of light output (the slide) back to its minimal state, which then repeats. And the graph of every such star produces the exact same figure, although they often differ in size.
This relationship became known as the Period-Luminosity Relationship. The importance of her discovery was soon recognized by others in the astronomical community as a new tool, called a “standard candle,” with which to measure the distances to distant stars by comparing their apparent brightness against one another, that is, if one Cepheid was only half as bright as another, it must be twice as far away, and one only a tenth as bright would be ten times farther, etc.
In 1923-1924 Edwin Hubble – the guy they named the Hubble Space Telescope after – was able to compare photographic plates he had made of assorted “nebulosities,” cloud-like patches of light, and found Cepheid variables embedded in them that proved the “nebula” in Andromeda was in actuality a separate galaxy some two million light-years from the Milky Way.
Using the Cepheid method, astronomers were able to calculate distances out to an estimated three million light years, encompassing about 20 galaxies in what is now known as the Local Group of galaxies. Today, using the HST we can monitor Cepheid variables, as well as other very bright stars in galaxies, out to about a hundred million light years.
In 1925 the Swedish Academy of Sciences wrote to Leavitt concerning their intention to nominate her for the 1926 Nobel Prize in Physics. Sadly, she had died in 1921 due to stomach cancer. The Nobel Prize is not awarded posthumously.
In January, 2009, the American Astronomical Society passed a resolution recognizing Henrietta Leavitt’s contribution to astronomy: “The AAS Council recognizes the 100th anniversary of Henrietta Leavitt’s first presentation of the Cepheid Period-Luminosity relation, a seminal discovery in astronomy that continues to have great significance. The Council was pleased to learn of a resolution adopted by the organizers of the Leavitt Symposium in which it was suggested that this important relation now be referred to as the ‘Leavitt Law.’ Although we recognize that the AAS has no authority to define astronomical nomenclature, we would be very pleased if this designation were used widely.”
One other honor has been accorded Henrietta, again posthumously. A crater on the Moon, crater Leavitt, has been named in her honor.
Find Delta Cepheus in the Night Sky
Finding Delta Cepheus in the night sky is quite easy. Using the two “pointer” stars at the end of The Big Dipper, trace a line to the North Star, Polaris, and continue on that same line almost exactly as far to the other side of Polaris, and voila’ – Delta Cepheus. If you observe over several nights, you will see that it appears to double in brightness. (Compare it against neighboring stars.) Its actual period for one complete cycle is 5 days 8 hours 37.5 minutes.