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Variable Star - Analysis

The following analysis will be done using the LTImage software. Follow the link if you've never used LTImage.

The purpose of this analysis is to create a light-curve for the target star in question. A light-curve shows how the brightness (or intensity) of a target varies with time. The shape of the curve depends on what type of variable star we are looking at - as expalined in the introduction.

STEP 1: Identify the Target and Comparison Stars

Finding Chart
Finding chart for variable star target

Before taking any measurements, you will need to identify the correct target and comparison stars using an appropriate finding chart, an example of which is shown to the right.

The relative brightness method we are using compares the brightness of our target star (TS) in relation to a nearby star of similar brightness, known as the comparison star (or CS1). To ensure that CS1 is not itself a variable star, we compare it with a second comparison star (CS2). The relative brightness method means that we don't have to worry about the observing conditions changing during the observing run.

STEP 2: Taking the Measurements

In order to create a light-curve we need to make some brightness measurements on observations of the target star in question. Typically you will have a number of observations to analyse. These will have been taken over the course of a night, or even several nights, and will allow you to monitor how the star's brightness changes over time. To remind yourself of how to make brightness measurements with LTImage, click on the following screencast:

 Using LTImage to measure brightness

Our analysis relies on comparing the brightness of stars. In practice this means dividing the number of counts we measure for one star by that for another. For example, if we measured 3000 counts for our target star (TS), and 2800 counts for CS1, then the relative brightness for TS/CS1 is 3000 ÷ 2500 = 1.2. The same goes for the relative brightness of CS1/CS2.

STEP 3: Recording your Results

Your measurements should be recorded on a spreadsheet, such as Microsoft EXCEL as follows:

Date and TimeBrightness (Counts)Relative Brightness
of ObservationTSCS1CS2TS / CS1CS1 / CS2
11/07/10 20:203000250027001.2001.11
12/07/10 20:503050265027481.1501.11
14/07/10 02:152900290026121.0001.11
14/07/20 21:402980280026851.0641.11

We would expect TS/CS1 to change due to the variable nature of TS, however, CS1/CS2 should remain roughly the same. If not, then there is a risk that CS1 is also a variable star, and can not, therefore, be used as a comparison for TS. In this case, we would need to find another comparison star.

STEP 4: Plot results on a graph

The next step is to create a graph of 'relative brightness' against 'time' for all of your data points. This can be done using a computer spreadsheet, such as Microsoft Excel, and is best achieved using a scatter graph that plots individual points without connecting lines. If you are unsure how to achieve this in Excel, then please contact your teacher, or read the following link carefully :

 Scatter Graph Tutorial - Excel 2007

Once your data has been plotted, you should see a graph similar to the following:

Scatter plot of data points

The graph can then be used to identify the type of variable star we are looking at, by comparing the light-curve with those of the different types of variables described in the introduction section of this project.