The First Cepheid

By Ken Croswell

Published in Sky and Telescope (October 1997, page 90).

Finding distances is at the heart of astronomy. Most physical characteristics of a star or galaxy, such as its size, mass, age, and energy output, depend critically on how well its distance is known. And distances are often known poorly.

A key tool in building the cosmic distance scale has been the Cepheid variable stars. These pulsating yellow supergiants have been called an astronomer's best friend. They take their name from Delta Cephei.

Because they reveal distances to galaxies, Cepheids have figured prominently in twentieth-century astronomy. During the 1920s Edwin Hubble used them to establish that the spiral "nebulae" lie outside the Milky Way. Cepheids and other yardsticks in these galaxies helped him discover the universe's expansion. Cepheids are just as crucial today, for they are key weapons in the ongoing battle over the size and age of the universe.

Because Delta Cephei is the prototype, many books and articles state that it was the first Cepheid found. But in fact that honor belongs to another bright star, Eta Aquilae. Its variability was spotted more than a month earlier.

The man who discovered Eta Aquilae's changes was Edward Pigott, a wealthy young British amateur in York. Astronomers of Pigott's time knew of only five variable stars: Mira, P Cygni, Chi Cygni, R Hydrae, and Algol. But Pigott rightly suspected there were many more. In 1781 he began a campaign both to monitor the known variable stars and to hunt for new ones.

Collaborating with Pigott was John Goodricke, also in York, who was only seventeen years old. More remarkable than Goodricke's youth was his physical handicap: he was deaf and mute, which is probably why he is better known than Pigott. In eighteenth-century England deaf-mutes were presumed to be idiots and were treated accordingly. But Goodricke's parents refused to accept this view and set him on a vigorous academic course. He developed a passion for science, especially astronomy.

Pigott became Goodricke's friend and mentor and taught him how to study the sky. Sometimes the two observed together from Pigott's well-equipped backyard observatory three blocks from Goodricke's home in York. Other times they observed separately and compared notes.

During the course of this work, on the night of September 10, 1784, Pigott discovered that Eta Aquilae changes brightness. He did so while monitoring another star, Theta Serpentis. Comparing the two, Pigott noticed that the former looked fainter than it did the year before. He soon confirmed Eta Aquilae's variability and discovered what astronomers now recognize as trademark Cepheid behavior: the star rises fast to maximum brightness, then falls slowly to minimum, in a very regular cycle. Pigott established Eta Aquilae's period to within 1 percent of the modern value, 7.18 days.

The same night that Pigott discovered Eta Aquilae's variability, Goodricke noticed an alteration in the light of what would become another prototype variable, Beta Lyrae. And a month later Goodricke discovered the variability of Delta Cephei. On October 20 he noted that one of the stars of Cepheus did not look right. Three nights later he wrote that he was "almost convinced" that the star doing the changing was Delta.

That winter Goodricke continued to observe the star, which unlike Eta Aquilae was circumpolar from England and could be followed all year long. He monitored it night after night and soon found a period close to the modern one of 5.37 days. Sadly, he died in 1786 when only twenty-one and entering what promised to be an important scientific career. As Pigott wrote, "I had the misfortune to lose the best of friends . . . which took away the pleasure I ever had in astronomical pursuits."

Nevertheless Pigott stuck with astronomy, and in 1795 he discovered the variability of R Coronae Borealis, the prototype for another important class of variables.

Neither Pigott nor Goodricke could have guessed how vital Eta Aquilae, Delta Cephei, and their siblings would prove to be. In 1907 Harvard astronomer Henrietta Leavitt discovered the famous Cepheid period-luminosity relation. While examining variable stars in the Small Magellanic Cloud, she noticed that the brighter variables had longer periods. Because all were nearly the same distance from Earth, the long-period Cepheids must be intrinsically brighter than the short-period ones. Five years later in 1912 she confirmed and extended this relation, and astronomers began to use Cepheids to measure distances in the Galaxy and throughout the universe.

Although its historic role has been eclipsed by Delta Cephei and its colorful discoverer, Eta Aquilae is just as bright and its variability just as easy to track. Its visual magnitude ranges from 3.5 to 4.3, so the naked eye is the only instrument you need. The only Cepheid that's noticeably brighter, second-magnitude Polaris, varies so little that to the naked eye it appears constant.

Of these three stars Eta Aquilae is the most luminous, because its pulsation period of 7.18 days exceeds the 5.37-day period of Delta Cephei and the 3.97-day period of Polaris. Eta Aquilae lies too far from Earth to have a reliable parallax, but the standard Cepheid period-luminosity relation says that its mean intrinsic brightness is 2,700 times the Sun's. This luminosity, along with the star's mean apparent magnitude and the known absorption of light by interstellar dust, implies that Eta Aquilae is 900 light-years away.

In recent years astronomers have verified this distance, for Eta Aquilae has a companion. The secondary was discovered in 1979 when a team led by John Mariska (Naval Research Laboratory) used the International Ultraviolet Explorer (IUE) satellite to find that Eta Aquilae radiates more ultraviolet light than a yellow star should. The excess ultraviolet light comes from a small companion star hotter than the Cepheid primary. Although the pair has never been resolved, the IUE spectra suggest that the companion resembles the type A0 main-sequence star Vega. Its ultraviolet flux matches what we would see from Vega if it were 900 light-years away.

Today Cepheids are at the cutting edge of astronomical news as one of the most important intergalactic yardsticks. But the Cepheid story did not start with their namesake. It actually began a month earlier, with the largely overlooked Eta Aquilae.

Ken Croswell is an astronomer in Berkeley, California, and author of The Alchemy of the Heavens and Planet Quest.

"An engaging account of the continuing discovery of our Galaxy."--New York Times Book Review. See all reviews of The Alchemy of the Heavens here.

"An excellent introduction to a branch of astronomy that may eventually help to establish the presence or absence of life elsewhere in the universe...Fascinating and worthwhile."--New York Times Book Review. See all reviews of Planet Quest here.