On the Cusp of Discovery
from his book, How Old is the Universe?
Professor of Astronomy David A. Weintraub is the award-winning author of Is Pluto a Planet? which won great acclaim for its fascinating and approachable style. His new book, How Old is the Universe? answers another compelling astronomy question. In the following excerpt, Weintraub recounts how the work of one astronomer at the turn of the 20th century was critical in determining the age of the universe.
From the Introduction
Ask any astronomer why she believes the universe is 13.7 billion years old, and she will tell you that she does not believe that it is 13.7 billion years old; she knows that it is 13.7 billion years old—give or take a hundred million years…. But why exactly do twenty-first century astronomers think that 13.7 billion years is the right answer? Why not 20 billion years? Why not 6,000 or 50 million or 1,000 trillion years? How do astronomers know that the universe even has an age—that it is not eternal?
From Chapter 15
…[Henrietta] Leavitt graduated from Radcliffe College, known then as the Society for the Collegiate Instruction of Women, in 1892 and went to work in 1893 as a volunteer computer* at the Harvard College Observatory. Soon thereafter, [Observatory director] Edward Pickering assigned to Leavitt the job of identifying variable stars, these being stars for which the output of light varies as a function of time; she quickly became an expert in this task. After three years of unpaid work, she delivered a summary report of her work to Pickering and departed Cambridge, spending the next two years traveling in Europe and then four more years in Wisconsin as an art instructor at Beloit College. Finally, in the summer of 1902, she contacted Pickering and asked permission to return to her work identifying variable stars. She clearly was very good at her work because Pickering immediately offered her a full-time, paid position with a wage of thirty cents per hour, which was significantly above the standard rate of twenty five cents per hour. This decision was one of the wisest Pickering would ever make.
The prototype of the variable stars known as cepheid variable stars, which Henrietta Leavitt would make famous, is Delta Cephei, discovered by John Goodricke in 1784. Cepheids do not simply get brighter and fainter and brighter again with periods of a few days or weeks; as they brighten, they also change color and temperature…becoming cooler and redder when brighter, then warmer and yellower when fainter. In addition, cepheids are distinct as variable stars because of the peculiar ways in which they brighten and fade…from the moment at which they are faintest and begin to brighten, they brighten very steadily, but when they reach maximum brightness and begin to fade, they fade continuously but not steadily. First they fade at an apparently constant rate. But when they are about two-thirds of the way back to minimum brightness, they begin to fade just a little bit less quickly. Then, when they are about seventy-five percent of the way to minimum light they fade much more quickly again. Despite the quirky nature of this pattern, the pattern is dependable and repeatable.
Early in 1904, Leavitt discovered several variable stars in a set of photographic plates of the Small Magellanic Cloud. Later that year, she found dozens more in both the Small and the Large Magellanic Clouds. Her discovery rate rose to hundreds per year and eventually she would identify 2,400 such stars. In 1908, Leavitt published under her own name “1777 Variables in the Magellanic Clouds” in Annals of Harvard College Observatory. …For all but 16 of these stars, she was able to determine “the brightest and faintest magnitudes as yet observed,” but for the remaining 16 of these 1777 stars, which she identified in Table VI of her paper, she also was able to determine their periods of variability…[She wrote that] “it is worthy of notice that in Table VI the brighter stars have the longer periods.” With historical hindsight, this is one of the most understated and important sentences in all of astronomical literature.
Four years later, Leavitt would conclude her work on the variable stars in the Small Magellanic Cloud in a brief, three page paper, “Periods of 25 Variable Stars in the Small Magellanic Cloud,” published as a Harvard College Observatory Circular under the name of Edward Pickering, though Pickering’s first sentence states that “the following statement…has been prepared by Miss Leavitt.” Leavitt focused her attention on the 16 variable stars specifically identified in 1908, along with nine newly identified ones, all of which “resemble the variables found in globular clusters, diminishing slowly in brightness, remaining near minimum for the greater part of the time, and increasing very rapidly to a brief maximum.” These are the cepheids, and those identified by Leavitt had periods that ranged from 1.25 days to 127 days. She then notes, with characteristic understatement, “A remarkable relation between the brightness of these variables and the length of their periods will be noticed…the brighter variables have the longer periods.” That is, brighter stars blinked slowly, fainter stars blinked more quickly…. The keen insight that makes this discovery so important comes next: “since the variable stars are probably at nearly the same distance from the Earth their periods are apparently associated with their actual emission of light.”… Truly, the brighter stars were brighter, the fainter stars were fainter.
The identification of cepheids in spiral nebulae would enable astronomers to measure the distances to these enigmatic objects, proving once and for all that the spirals were distant galaxies.
What does Leavitt’s discovery mean? If we can measure the period of variability for any single cepheid variable star, we instantly know the absolute [brightness] of that star. Since we can directly measure the apparent [brightness] of the cepheid, the combination of the period (which gives the absolute [brightness]) and the apparent [brightness] yields the distance to the star. This is an incredibly powerful discovery…
By using Leavitt’s variable stars, astronomers were on the cusp of a decade of absolutely monumental discoveries: first, the identification of cepheids in spiral nebulae would enable astronomers to measure the distances to these enigmatic objects, proving once and for all that the spirals were distant galaxies; then, the discovery that the distances and velocities of these galaxies are correlated would lead to the discovery of the expanding universe. Ultimately, the expanding universe measurements will give us [a] method for determining the age of the universe.
*A term originally used for a person who makes calculations, especially with a calculating machine.
Excerpted from How Old is the Universe?, published by Princeton University Press. Used by permission, all rights reserved.
photo credit: Jeffrey Newman (Univ. of California at Berkeley) and NASA/ESA