Chapter 1
Historical Overview

1.1 Discovery of the First Pulsating Variable Stars

Although cataclysmic variable stars of the nova or supernova type had been seen since antiquity (Stephenson & Green, 2002), by the sixteenth century stars were generally regarded as fixed and unchanging in both position and brightness (Hoskins, 1982). The outburst of a bright supernova in Cassiopeia in 1572 (Tycho's supernova) startled the astronomical community and reawakened interest in apparently new stars. Almost 14 years later, in 1596, David Fabricius (1564-1617) observed what he thought was yet another new star, this time in the constellation Cetus.

Fabricius's new star was only of the third magnitude, far less brilliant than Tycho's supernova, but nonetheless easily visible to the unaided eye. First seen in August, by October the star had faded below naked-eye visibility. However, Fabricius's star had not forever vanished. A few years later, it was recorded by Johann Bayer, who named the star omicron (o) Ceti and placed it on his 1603 star charts, although Bayer appears to have been unaware that he had rediscovered Fabricius's nova. In 1609, it was Fabricius himself who was surprised to see the new star make a reappearance. While o Ceti certainly seemed unlike Tycho's new star of 1572, Fabricius did not suspect that he had discovered a star that was not a nova but one that instead showed periodic changes in brightness (Hoskins, 1982).

In 1638, Johannes Holwarda (1618-1651) made yet another independent discovery of o Ceti. Like Fabricius, Holwarda watched the star fade from view, only to see it subsequently reappear. Now other astronomers began to pay increased attention to the variable, but nonetheless the periodic nature of its variability still eluded recognition.

Johannes Hevelius (1611-1687) carried out a detailed study of o Ceti, and, in 1662, he published Historiola Mirae, naming the star Mira (meaning the wonderful in Latin), a name which has ever since been applied to it. Ismael Bullialdus (1605-1694) made the next advance. He noticed that the peak brightness of Mira occurred about a month earlier each year, finally discovering the cyclic nature of its brightness changes. In his 1667 Ad astronomos monita duo he determined the period to be 333 days, about 1 day longer than the current determination of the mean period. Mira thus became the first variable star whose period was determined, and the archetype of the class of long-period variable stars now known as Mira variables. It would nonetheless be a long while before it was established that Mira's brightness changes had anything to do with pulsation (Hoskins, 1982).

Mira itself can become as bright as second or third magnitude at maximum light, making it easily visible to the naked eye (Figure 1.1). Other long-period variables, although not as bright as Mira, also have peak magnitudes that place them within the bounds of naked-eye visibility. That begs the question of whether Mira or other long-period variables might have been discovered before the time of Fabricius. Hoffleit (1997) summarized the evidence for pre-1596 observations of Mira. A number of possible pre-1596 observations have been suggested, going back to ancient times. Unfortunately, for none of the proposed pre-discovery observations is the historical evidence ironclad. The case is perhaps strongest for a 1592 "guest star" recorded in Asian records. However, Stephenson & Green (2002) concluded that even that object was unlikely to have actually been Mira.

Figure 1.1 Mira near maximum (a) and minimum (b) light. The image (a) was taken from Römerstein, Germany, on July 17, 2004, whereas the image (b) was obtained on October 30, 2005, from Austria. Several of the nearby stars in Cetus are identified by their Bayer designations, and the Cetus constellation itself is also drawn. The bright object on the upper left corner in (b) is the planet Mars. The July 2004 image presents a view similar to that seen by David Fabricius at his discovery of that long-period variable star in August, 1596. (Images courtesy Till Credner, AlltheSky.com.)

The number of confirmed periodic variable stars increased only slowly following the recognition of the periodicity of Mira. For instance, in 1686 Gottfried Kirch discovered the variability of ? Cygni, a Mira variable with a period of 408 days (Sterken, Broens, & Koen, 1999). R Hydrae, a 384-day-period Mira variable, was found by Maraldi in 1704. A third Mira variable, 312-day-period R Leonis, was found by Koch in 1782. The long-period variability of these stars, and of Mira itself, would eventually be attributed to pulsation. However, little was known of shorter-period variable stars when the British team of Edward Pigott and John Goodricke took up the study of variable stars in earnest late in the eighteenth century.

Pigott and Goodricke soon confirmed the variability of Algol1 (ß Persei), which had been originally established by Geminiano Montanari, and determined its period. Goodricke, in a 1783 report to the Royal Society, suggested that an eclipse might be responsible for the periodic dimming of Algol's light. Pigott discovered the variability of ? Aquilae in 1784, and soon thereafter Goodricke identified the variables ß Lyrae and d Cephei. While ß Lyrae proved to be an eclipsing variable star, ? Aquilae and d Cephei were the first representatives of the important Cepheid class of pulsating stars. The circumstance that Goodricke was deaf (and possibly mute) did not slow the progress of the pair, but Goodricke's untimely death at the age of 21 ended the collaboration in 1786 (Hoskins, 1982; French, 2012).

A list of the earliest discovered variable stars and their discovery dates is shown in Table 1.1, from which cataclysmic variables such as novae or supernovae have been excluded. The first column of this table identifies the variable star by name, while the second indicates into which category the variable star was eventually classified. The discovery date of the variable star is given in the third column, followed by the name of the discoverer. In some cases, there are questions as to what should be called the actual discovery date: the date at which variability was first suspected or the date at which the variability was clearly established. The period of the variable, if it is periodic, is listed in the fifth column, while the brightest visual magnitude and amplitude of the variable are given in columns six and seven. Finally, column eight lists the spectral type of the variable. Note that all of the variable stars in this table reached naked-eye visibility, with the possible exception of S Serpentis, which at its peak brightness is just fainter than the usual magnitude limit for the naked eye.

Table 1.1 Some of the first variable stars discovered.a),b),c)