GUIDE TO VARIABLE STARS


INTRODUCTION

On looking up at the night sky it may appear that the stars are invariant, unchanging with time, always in the same place and having the same intensity. This however is not true. Some stars show changes in brightness. These changes may occur within the space of a night, a week, a month, a year, or over many years. The Sun was once thought to be a constant star, but recent satellite observations have shown that it too shows a variation, albeit very small, with time. This has implications for climate change. A study of variable stars is thus important to enable us to understand why and how such variations occur.

THE NAMING OF STARS

Constellation Centaurus
  • We name stars so we can refer to them.
  • The sky is divided into 88 constellations.
  • We start naming with the brightest star.
  • Letters of the Greek alphabet are used starting with alpha.
  • As with any naming scheme we find exceptions.

    The brightest star in the constellation Centaurus is alpha-Centauri (a-Cen for short), the next brightest beta-Centauri, then gamma-Centauri and so on.

NAMING VARIABLE STARS

CLASSIFICATION OF STARS

Star Class
  • Astronomers classify stars according to their temperature.
  • The scheme runs through the letters according to the sequence:
      OBAFGKMRNS
    where O-type stars are hot blue stars and S-type stars are ‘cold’ red stars.
  • You can remember this order through the phrase “Oh Be A Fine Girl, Kiss Me Right Now Sweetheart”.
  • Variable stars are found throughout this range of star types.
  • The Sun is a G-type star.

    CLASSIFICATION OF VARIABLE STARS

    Variable Star Classification Diagram

    EXTRINSIC VARIABLES

    Eclipsing Binaries

    • Two stars with an orbital plane which lies near the line of sight of the observer. One star will periodically block the other’s light, causing the total light received to diminish.
    • A planet orbiting around a star may also cause a small dimming of light when it passes in front of the star.

    • Eclipsing stars
    Rotating Variables

    • A star that rotates may show a variable light output to an observer if the star has dark spots or bright spots on its surface.
    • The Sun shows both these phenomena. The dark spots are called sunspots, and the light areas are called faculae.

    • Spotted star

    INTRINSIC VARIABLES

    Pulsating Stars

    • The size or the shape of the star may change or the star may vibrate.
    • The Sun is an example of a vibrating star, although the resultant light changes are very small. The study of the Sun’s vibrations or oscillations is called helioseismology.

    • Pulsating star
    Eruptive Stars

    • These are also known as cataclysmic stars.
    • The sudden change in light output may be due to changes within the star itself or it may be due to a binary star system in which there is an interaction between the two stars – usually involving the transfer of mass.

    • Eruptive star

    PULSATING VARIABLES

    Pulsating variables are stars that show periodic expansion and contraction of their surface layers. Pulsations may be radial (where the star remains spherical in shape) or non-radial (where the star may become non-spherical). Pulsating variables may be distinguished by their pulsation period, their mass, their age, and the nature of their pulsations. Pulsating variable stars may be subdivided into 5 classes.

    Pulsating Variable Class

    Cepheid Variables

    Cepheids as Distance Markers

    RR Lyrae Stars

    RV Tauri Stars

    Long Period and Irregular Variables

    ERUPTIVE (CATACLYSMIC) VARIABLES

    Eruptive Variables Class

    Supernovae

    Novae

    Other Nova Type Stars

    Flare Stars

    RR Coronae Borealis

    MEASURING VARIABLE STARS

    Astronomer Cartoon
    • The brightness of variable stars was first measured visually by comparing them to known surrounding or comparison stars. Accuracies of 0.1 magnitudes can be achieved by experienced observers.
    • When photography came along, star images on plates were measured with microdensitometers.
    • With the development of electronic photomultipliers, direct photometry at the telescope became possible.
    • Today, with CCD cameras, and computer analysis, many variable star measurements can be made from images obtained with much shorter exposures than are required by film.

    LUMINOSITY AND MAGNITUDE

    VARIABLE STARS TO OBSERVE

    The following table lists some variable stars that are visible to the naked eye.

    NameConstellation TypeMagnitude Period
    AlgolPerseusEclipsing2.2 - 3.52.87 days
    m ScoScorpiusEclipsing3.0 - 3.31.66 days
    MiraCetusLPV/Mira1.0 - 10.2331 days
    d CepCephusCepheid3.8 - 4.65.37 days
    h AqlAquilaCepheid3.7 - 4.47.18 days
    c CygCygnusLPV/Mira2.3 - 14.3407 days
    Note: LPV = Long Period Variable

    THE SUN AS A VARIABLE STAR


    THE SIGNIFICANCE OF VARIABLE STARS

    The study of variable stars is important:

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    Copyright Australian Space Academy © 2007