Question: I understand that variable stars play a big part of amateur astronomy and offer a way for amateurs to do research. Could you explain the different types and and give some of the basic techniques for observing ? Mitch B.. Pueblo, CO
Answer:
It is true that observing variable stars and recording their light changes remains one of the primary ways amateurs can contribute to astronomy. First some preliminaries. As the name suggests variable stars change their brightness over defined periods of time. One can generally divide variable stars into 1) geometrical variables, and 2) pulsating variables.
The geometrical variables are stars which vary in brightness owing to changes in their geometric configuration, say with respect to another star that is part of the physical system. An example wold be an eclipsing binary with two stars revolving around a common center of mass (barycenter) such that the regular eclipse of one by the other produces light intensity changes. The sketch below shows the basic binary system:
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With larger star of mass m(A) revolving about a barycenter with a star of lesser mass m(B). Here, the two stars are separated by a" seconds of arc.
Now instead of such a plane view imagine the smaller star B regularly passing in front of star A (or vice versa) in the line of sight for an Earth observer. Then the resulting light curve of the system will show fluctuations.
An excellent animation showing what happens - including the corresponding light resulting can be found at this NASA site:
http://www.youtube.com/watch?v=cj0hcA..
The other type of variable star - the pulsating variables - have as their prototype Mira (Omicron Ceti) in the constellation Cetus, the Whale. This pulsating star lies 420 light years from us and varies over a period of 11 months. Mira is peculiar in its variations because they aren't constant, i.e. the peaks aren't always the same brightness. For example, over the centuries Mira has attained a peak magnitude of nearly +2.0 or almost as bright as Polaris, the North star. But in recent years the peak has seldom exceeded +3.5 and note again that the higher the numerical stellar magnitude the dimmer the star, You can learn more about Mira and Mira-type variables at the American Association of Variable Star Observers website, e.g.
https://www.aavso.org/mira-variables-period-changes
In terms of observations of these stars, it is worthwhile to get hold of the July, 2016 issue of Astronomy magazine (p. 62) which shows how CCD cameras can now be incorporated into observations, including of exoplanet .-bearing star systems. (Which exhibit similar photometric characteristics to eclipsing binaries)
In the case of variable stars, it isn't even necessary to have elaborate equipment to do a respectable plot of a light curve. You need only good eyes, good star charts (with magnitudes shown) and the ability to compare the (estimated) brightness of the target star with the known magnitudes of those comparison stars in the vicinity.
One such method is known as the Argelander Photometric Technique or Argelander Step Method. In this technique the target variable (say Mira) has its changing magnitude estimated in steps between different combinations of comparison stars. A step value is then obtained for each evaluation and this is plotted against a magnitude scale which ranges over all the visual magnitudes of the comparison stars used.
An example of the (a-c) and (b-c) runs and estimated steps are given in the following table. for the appropriate Julian date are shown. Again, a designation 'b2v3c' means star b (
In the case of variable stars, it isn't even necessary to have elaborate equipment to do a respectable plot of a light curve. You need only good eyes, good star charts (with magnitudes shown) and the ability to compare the (estimated) brightness of the target star with the known magnitudes of those comparison stars in the vicinity.
One such method is known as the Argelander Photometric Technique or Argelander Step Method. In this technique the target variable (say Mira) has its changing magnitude estimated in steps between different combinations of comparison stars. A step value is then obtained for each evaluation and this is plotted against a magnitude scale which ranges over all the visual magnitudes of the comparison stars used.
For example, in a technical workshop I gave to the Barbados Astronomical Society in 1975 I targeted Mira and used the comparison (reference) stars: Gamma Eridani ( m v = 3.2 (b)) ; Gamma Ceti ( m v = 3.6 (c) ) and Delta Ceti (m v = 4.1 (d) ) Grading of the variable Mira (a, m v = 2.8) was then done in three runs referred to as c-d, a-c and b-c. My chosen instrument for observation was an Asahi Pentax Prism binocular (10 x 50) . Accuracy was enhanced by setting the binocular slightly out of focus so stellar images appeared as slightly extended sources instead of point sources.
The step value is found by first taking the mean value of the steps in the given run, then the mean of the brightness ratios, x: v and v:y where x denotes the brighter comparison star and y the dimmer, say in run x-y. The runs, of course, must be integrated so that the independent step values, i.e. for a-c, b-c, c-d merge together smoothly in the single run, say a-d. (i.e. from the brightest comparison star to the dimmest. This is usually done graphically at the end of all the observations. (Using a plot of integrated step value vs. m v )
An example of the (a-c) and (b-c) runs and estimated steps are given in the following table. for the appropriate Julian date are shown. Again, a designation 'b2v3c' means star b (
The light curve I obtained for Mira over the Julian dates of interest are shown below and the degree of uncertainty was estimated to be + 0.1 magnitude.
This technique was also used to determine the light curve of a Cepheid variable by a Barbadian student who took first place at a Barbados Science Exhibition, e.g.
The best way to get started. of course, is to make contact with the AAVSO and indicate which variables are of greatest interest and what sort of instruments you have. You will then receive an extensive handbook for how to conduct observations tat are worthwhile and how to report them to the group.
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