Sky & Telescope Oct 1997 v94 n4 p90(2) Page 1 The first cepheid. by Ken Crosswell

Knowing the size, mass, energy, and age of any astronomical body depends totally on how well its distance is known, proof enough that distance is important to astronomers. To make calculations of distance, Cepheid variable stars, such as Delta Cephei and Eta Aquilae are used.

© COPYRIGHT 1997 Sky Publishing Corporation presumed to be idiots and were treated accordingly. But Goodricke’s parents refused to accept this view and set The Cepheid variable stars, famous cosmic yardsticks, him on a vigorous academic course. He developed a really ought to be called the Aquilid variables. passion for science, especially astronomy.

Finding distances is at the heart of astronomy. Most Pigott became Goodricke’s friend and mentor and taught physical characteristics of a star or galaxy, such as its him how to study the sky. Sometimes the two observed size, mass, age, and energy output, depend critically on together from Pigott’s well-equipped backyard observatory how well its distance is known. And distances are often three blocks from Goodricke’s home in York. Other times known poorly. they observed separately and compared notes.

A key tool in building the cosmic distance scale has been During the course of this work, on the night of September the Cepheid variable stars. These pulsating yellow 10, 1784, Pigott discovered that Eta Aquilae changes supergiants have been called an astronomer’s best friend brightness. He did so while monitoring another star, Theta (see "Dating the Cosmos: A Progress Report" on page ([Theta]) Serpentis. Comparing the two, Pigott noticed that 42). They take their name from Delta ([Delta]) Cephei, the former looked fainter than it did the year before. He currently high in the northeastern evening sky; it’s plotted soon confirmed Eta’s variability and discovered what on the foldout constellation map on page 83. astronomers now recognize as trademark Cepheid behavior: the star rises fast to maximum brightness, then Because they reveal distances to galaxies, Cepheids have falls slowly to minimum, in a very regular cycle. Pigott figured prominently in 20th-century astronomy. During the established Eta Aquilae’s period to within one percent of 1920s Edwin Hubble used them to establish that the spiral the modern value, 7.18 days. "nebulae" lie outside the Milky Way. Cepheids and other yardsticks in these galaxies helped him discover the The same night that Pigott discovered Eta Aquilae’s universe’s expansion. Cepheids are just as crucial today, variability, Goodricke noticed an alteration in the light of for they are key weapons in the ongoing battle over the what would become another prototype variable, Beta size and age of the universe. Lyrae. And a month later Goodricke discovered the variability of Delta Cephei. On October 20th he noted that Naked-Eye Beginnings one of the stars of Cepheus did not look right. Three nights later he wrote that he was "almost convinced" that the star Because Delta Cephei is the prototype, many books and doing the changing was Delta. articles state that it was the first Cepheid found. But in fact that honor belongs to another bright star currently high in That winter Goodricke continued to observe the star, which the evening sky, Eta ([Eta]) Aquilae. Its variability was unlike Eta Aquilae was circumpolar from England and spotted more than a month earlier. could be followed all year long. He monitored it night after night and soon found a period close to the modern one of The man who discovered Eta Aquilae’s changes was 5.37 days. Sadly, he died in 1786 when only 21 and Edward Pigott, a wealthy young British amateur in York. entering what promised to be an important scientific Astronomers of Pigott’s time knew of. only five variable career. As Pigott wrote, "I had the misfortune to lose the stars: Mira, P Cygni, Chi ([Chi]) Cygni, R Hydrae, and best of friends . . . which took away the pleasure I ever had Algol. But Pigott rightly suspected there were many more. in astronomical pursuits." In 1781 he began a campaign both to monitor the known variable stars and to hunt for new ones. Nevertheless Pigott stuck with astronomy, and in 1795 he discovered the variability of R Coronae Borealis, the Collaborating with Pigott was John Goodricke, also in prototype for another important class of variables. York, who was only 17 years old. More remarkable than Goodricke’s youth was his physical handicap: he was deaf Neither Pigott nor Goodricke could have guessed how vital and mute, which is probably why he is better known than Eta Aquilae, Delta Cephei, and their siblings would prove Pigott. In 18th-century England deaf-mutes were to be. In 1907 Harvard astronomer Henrietta Leavitt - Reprinted with permission. Additional copying is prohibited. - Information Access

C O M P A N Y Sky & Telescope Oct 1997 v94 n4 p90(2) Page 2 The first cepheid. discovered the famous Cepheid period-luminosity relation. KEN CROSWELL is author of the recently published While examining variable stars in the Small Magellanic Planet Quest: The Epic Discovery of Alien Solar Systems. Cloud, she noticed that the brighter variables have 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. Yet even today the calibration of the Cepheid period-luminosity relation remains somewhat unsettled, especially in light of new star distances measured by the Hipparcos astrometry satellite.

A Star to Watch

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. It’s plotted on the fold-out constellation map high in the south-southwest, in Aquila’s eastern wing. Eta’s 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, 2nd-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 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 begin with their namesake. It actually began a month earlier, with the largely overlooked Eta Aquilae.

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