Variable Stars

Stars with changing magnitudes are called variables (Fig. 13.1). Variations in the brightness of stars were first noted in Europe at the end of the 16th century, when Tycho Brahe's supernova lit up (1572) and the regular light variation of the star o Ceti (Mira) was observed (1596). The number of known variables has grown steadily as observational precision has improved (Fig. 13.2). The most recent catalogues contain about 40,000 stars known or suspected to be variable.

Strictly speaking, all stars are variable. As was seen in Chap. 11, the structure and brightness of a star change as it evolves. Although these changes are usually slow, some evolutionary phases may be extremely rapid. In certain evolutionary stages, there will also be periodic variations, for example pulsations of the outer layers of a star.

Small variations in stellar brightness are also caused by hot and cool spots on a star's surface, appearing and disap pearing as it rotates about its axis. The luminosity of the Sun changes slightly because of the sunspots. Probably there are similar spots on almost all stars.

Initially stellar brightnesses were determined visually by comparing stars near each other. Later on, comparisons were made on photographic plates. At present the most accurate observations are made photoelectrically or using a CCD camera. The magnitude variation as a function of time is called the lightcurve of a star (Fig. 13.3). From it one obtains the amplitude of the magnitude variation and its period, if the variation is periodic.

The basic reference catalogue of variable stars is the General Catalogue of Variable Stars by the Soviet astronomer Boris Vasilyevich Kukarkin. New, supplemented editions appear at times; the fourth edition published in 1985-1987, edited by P.N. Kholopov, contains about 32,000 variables of the Milky Way galaxy.

Fig. 13.1. The variables are stars changing in brightness. Two variables in Scorpius, R and S Sco (Photograph Yerkes Observatory)

Hannu Karttunen et al. (Eds.), Variable Stars.

In: Hannu Karttunen et al. (Eds.), Fundamental Astronomy, 5th Edition. pp. 279-290 (2007) DOI: 11685739_13 © Springer-Verlag Berlin Heidelberg 2007

Fig. 13.1. The variables are stars changing in brightness. Two variables in Scorpius, R and S Sco (Photograph Yerkes Observatory)

Hannu Karttunen et al. (Eds.), Variable Stars.

In: Hannu Karttunen et al. (Eds.), Fundamental Astronomy, 5th Edition. pp. 279-290 (2007) DOI: 11685739_13 © Springer-Verlag Berlin Heidelberg 2007

Fig. 13.2. The location of variables in the HR diagram

ß Cephei stars

Dwarf novae

Classical cepheids

RV Tauri stars

RRLyrae stars

W Virginis stars

Long period variables

Magnetic variables

Dwarf cepheids Sun

T Tauri stars

T Tauri stars

Semi-

regular variables

Flare stars

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