AAQ Variable Star Section

An Introduction To Variable Stars

The term “Variable Star” comes as a surprise to many people who think of stars as permanent unchanging beacons of our night time sky. Indeed this has been the perception for countless centuries. Variable stars are stars that vary in brightness. Today there are over 30,000 stars classified as variables and many other stars are suspected to be variable.

The first known example of a variable star, apart from novae and supernovae, was a star in the constellation Cetus with the designated name of Omicron (ο) Ceti, more commonly known as Mira (the Wonderful). This third magnitude star was first observed by David Fabricius in 1596 in a part of the sky where no stars appeared on the star charts available at that time. Mira subsequently faded from view and reappeared several months later. The cyclic behaviour of its brightness variations was recognised 42 years later in 1638 and “Variable Star Astronomy” was born.

Variable star astronomy is the cornerstone of understanding how stars behave, what drives this behaviour and how they evolve with time. The now known 30,000 + variable stars are categorised into designated types based on their patterns of behaviour (fluctuations in brightness and spectral class).

There are two main categories of variation:

• intrinsic; and
• extrinsic.

Intrinsic variables are stars whose brightness fluctuations are caused by variations in the energy released by the star. Intrinsic variables can be subdivided into:

• pulsating variables - stars whose outer layers undergo periodic expansion and contraction driven by a temporal imbalance between gravitational and radiative forces;

• eruptive variables - stars undergoing violent processes such as flares in their chromosphere and corona with significant mass loss from the star; and

• cataclysmic variables - stars undergoing rapid brightening due to events either on the star’s surface involving mass transfer from one star to its binary companion followed by a sudden thermonuclear burst within the accumulated material (nova) or an explosive thermonuclear burst in the star’s interior resulting in the destruction of the star (supernova).

Extrinsic variables are stars whose brightness fluctuations are caused by factors other than energy release by the stars. Extrinsic variables can be subdivided into:

• eclipsing binary stars - two stars in a close orbit where one star eclipses (i.e. passes in front of) the other star in our line of sight; or

• rotating stars – stars showing small changes in brightness as they rotate due to non-uniform surface brightness such as large dark patches or star spots.

The above subcategories of intrinsic and extrinsic variables can be further subdivided into classes based on their behaviour. As an example, some of the more notable intrinsic pulsating variable classes are:

• Mira or Long Period Variable stars – red giant or supergiant stars with pronounced periodicity of 80 to 1000 days and a large range in brightness variation (amplitude) of >2.5 magnitudes, they are present throughout the Milky Way;

• Semiregular stars – yellow to red giant and supergiant stars with noticeable periodicity accompanied or interrupted by intervals of irregularity. The period ranges from 20 to >2000 days and the amplitude is usually <2.5 magnitudes;

• Cepheid stars – high luminosity white to yellow supergiant and bright giant stars with periods of 1 to 70 days (NB the period is related to the star’s luminosity with the more luminous stars showing a longer period) and amplitudes of 0.1 to 2.0 magnitudes;

• RR Lyrae stars – rapidly pulsating white stars that are often found in globular clusters, they have pronounced periodicity of 2 hours to 1.2 days and amplitudes of 0.2 to 2.0 magnitudes;

• RV Tauri stars – yellow supergiant stars with alternating deep (fainter) and shallow (brighter) minimum values with periods of 30 to 150 days and amplitudes that may reach up to 4 magnitudes.

The five intrinsic pulsating variable classes described above all consist of giant and supergiant stars with diameters of 5 to 1000 times that of the Sun and luminosities in the order of 100 to 100,000 times that of the Sun.

Observations of variable stars by amateur astronomers are keenly sought by professional astronomers who use the data to determine properties such as mass, temperature and luminosity of the stars and how the stars function and evolve over time (stellar evolution). As such, variable star observing is one of the few fields where amateur astronomers can make a real contribution to the science of astronomy.

The AAQ Variable Star Section was formed in June 1998 to encourage members to undertake and submit observations to databases maintained by the RASNZ and AAVSO. In early 2000 the AAQ decided to select a few of the brighter variable stars that are visible in binoculars for regular monitoring and to develop of our own database. The initially selected stars were L2 Puppis, R Carinae, S Carinae and T Centauri. This list has since been expanded to include 18 variable stars. Refer to the articles by AAQ members for further information on some of the variable stars being monitored by the AAQ. Those wishing to delve deeper into the science of variable stars and stellar evolution are encouraged to visit the web sites listed below.

Anyone wishing to participate in this interesting and rewarding field of astronomy should contact the Variable Star Section Director who will be happy to provide some variable star charts and some initial guidance and training. You will require some basic equipment such as binoculars (7 x 50 or 10 x 50 are ideal), star charts and red light torch to undertake the observations.

American Association Of Variable Star Observers

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Astronomical Association of Queensland 2006. www.aaq.org.au