Wording On Memorial Program

Variable Stars: An Introduction (II) visual estimate

observation methodology
Variable stars are an area of \u200b\u200bparticular interest to the amateur astronomer for two reasons:

1 - is the right person to continue their behavior on a regular basis to be impossible from a professional observatory regular observation of all variables listed.

2-the area of \u200b\u200bee.vv. is open all availability of instruments, since there are variables that can go bright to the naked eye, there are many that can be observed with simple binoculars, and the catalog is extensive and varied enough to be useful telescopes of all kinds.

To this we must add that the observation of e. variables is very demanding on the quality of the optics used and which are not indispensable exceptional conditions of transparency and darkness if we are not studying too faint stars, this means that since the city can keep a lot of variables with very simple telescopes . The task of the amateur in this field will consist of making a measure of brightness or variable star photometry in the study and repeated some time later.

With photometric measurements of different times we can reconstruct the behavior of the selected object. The relationship between date and brightness can be summarized and displayed in a Cartesian diagram, with time on the horizontal axis and the visual magnitude in the vertical axis graph called brightness curve.

photometric To begin our journey will be equipped with specific maps variable stars, which mostly edit AAVSO (American Association of Variable Star Observers) and AFOEV (French Association Observer Variable Star).

maps of type 'a' (on a scale of 5 'per mm) will allow us to locate the variable (probably also with the help of a detailed atlas like Sky Atlas 2000.0 or Uranometria), while the letters 'b' (60 "per mm) will allow us to identify the variable that we seek no error and measure its brightness when it is maximized.

The letters 'c', 'd' and 'e' (on a scale of 40 ", 20" and 10 "per mm, respectively) are available for faint variable stars or weak and will give us maximum field of view restricted and weaker comparison stars, we will use during phases of reduced brightness. Photometry we're going to make it visual, that is, from now on we only need your eye to measure the brightness of the variable.

Once we have located around this we will need to find two stars that we call stars of comparison, a brighter and a weaker one, and we will estimate the difference in brightness of the str. Comp. with e. treating variable assign one of the following degrees of Argelander :

• The grade 1 is set when you enter the variable and the comparison star is a nearly imperceptible difference in brightness, which is only appreciated after careful consideration.
• The grade 2 when the variable is set and e. comparison seem the same brightness at first glance, but soon we see a slight difference in brightness between the two.
• The grade 3 when set between the variable and e. comparison is a moderate brightness difference can be seen from the outset.
• The grade 4 is set when the difference in brightness between the variable and comparison star is remarkable.
• The grade 5 is set when the difference in brightness between the variable and comparison star disproportionate.

Thus we compare the first variable star with a star that has a lower brightness and then with another that is more light.

The brightness value of e. I obtrendremos variable by a simple formula given below:

var m = m to + (m b - m to ) * to / ( to + b )

where var m is the magnitude of the variable star, m to the magnitude of comparison star superior brightness and to the degree of difference between observed brightness; m b is the brightness of the comparison star brightness and lower b the degree of difference observed between this and the variable.

There are cases in which we can discern differences in brightness a little more ambiguous, in which we taking degrees in tenths, so, for example, if a difference of brightness was between 1 and 2 would take a degree 1.5.

This method, although it appears to us fairly intuitive, it is quite reliable. To analyze the degree of precision with which we could discern a difference nominally up to a tenth of a magnitude, which in practice boils down to a confidence interval between 0.2 and 0.4 magnitudes, which is a perfectly acceptable uncertainty in monitoring of stars with an amplitude of variation of 1.5 magnitudes at least, which are the are the program of visual observation.

The time reference we will use is the Julian Day (defined as the average number of solar days elapsed since noon on January 1 4713 BC), which provides a continuous time scale entirely without the changes it is always subject to the civil calendar .

The comments are always referred to the date and time is expressed in Julian day made with an accuracy of several decimal places. Observers have to do Julian Day calendar and tables for fractions of days from the time in universal time, in case you do not have software tools on your computer or on the web to calculate it. ;

not forget, at the time of calculation, the astronomical Julian days are days, that is, beginning at noon, universal time.