Variable and Double Stars

Variable and double stars can be very difficult to see even with a good telescope – large observatory telescopes and the Hubble Telescope in Earth’s orbit, tend to be better at distinguishing between stars at extreme proximity compared with their distance from us.

Perhaps more than half the stars in the universe may be double or binary stars. Some are so close together that gas can be pulled from the star with the lower gravity to the other, eventually resulting in the complete absorption of the lesser star. Once this process is complete the dominant star can become unstable, and may explode as a nova or supernova. In fact many binary systems are in fact multiple, with triple systems being the most common.


Very close binaries can make it impossible for astronomers to see the separate stars. These are termed spectroscopic binaries because both stars can be pinpointed through spectral analysis. Eclipsing binaries occur when the orbit of the secondary star moves alternately in front of and behind the primary star, resulting in a fluctuating light pattern. Optical doubles are not binary stars at all, but are in fact and optical illusion where one star is in line with the other but is much closer to the observer.


The brightest (the star with the lowest magnitude) of the binary or multiple system is labeled “A” the next “B” and so on. To observe binaries you need a telescope with a large magnification (at least 50 to 100x), good atmospheric conditions and no artificial light. The larger the magnification the greater the chance of splitting the individual stars.


Binaries visible in spring include Epsilon and Xi in Böotes, Iota I and Zeta in Cancer, Alpha Centauri in Centaurus and Mizar in Ursa Major. Summer binaries include Alpha and Beta in Capricornus, 61, Albireo and Omicron in Cygnus, and Alpha Herculis and Delta in Hercules. Some autumn binaries are Gamma in Andromeda, Zeta in Aquarius, Gamma and Aries in Lambda, Eta and Sigma in Cassiopeia, and Polaris in Ursa Minor. Winter binaries include h3945 in Ursa Minor, Castor in Gemini, Theta in Taurus, and Lambda, Mintaka, Rigel, Sigma and Trapezium in Orion


Variable stars are of three broad types. The first are eclipsing binaries as described above. The second type is pulsating variables. These are older stars that vary their diameters, and consequently their total light output, rather like the appearance of an expanding and contracting heart. This beating may be rhythmic or irregular. The cycle can be from hours (as in the Cepheid variables) to longer than a year.


Eruptive variables are some of the most dramatic stars. They can remain stable for a period of time and then brighten suddenly within days, hours or perhaps minutes. Eruptive stars can go nova, and brighten by more than five magnitudes within a day (such as R Coronae Borealis). Or they can emit their maximum output steadily, and then drop it suddenly as they are obscured by sooty gas they eject.


All variable star types are named the same way. The first variable discovered in a constellation is labeled “R”, the second “S” and continuing through to Z. Following this it is “RR”, “RS” up to “RZ”. Then it’s “AA” to “AZ”, then “BB” to “BZ” and so on, but “JJ” to “JZ” isn’t used. “QZ” is the final designation that can be used in this way. This very strange and bewildering system means that 334 variable stars can be named in any one constellation. But this has not always been adequate. If a number 335 is found, it is labeled V335, the next one V336 and so on.


To find out more about variable stars contact the American Association of Variable Star Observers (AAVSO), which is an international organization that can give you all the important information on how best to observe variables, and how to calculate their magnitudes. You can advise them of the magnitudes of the variables you observe.  The AAVSO provides liaison between people engaged in amateur astronomy and professional astronomers asking for data. You can estimate the magnitude of a variable to a degree of accuracy of 0.1 by comparing it with stars of known magnitude that you can observe. Ask the AAVSO for more information on how to make accurate estimates.

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