Target 4:

Target 4 is a dim star, that is showing a wide

range of variance. Its error bars are clearly

visible but not too large. This star is showing

positive signs that it is going to form a very good

wave pattern.

Because of its wave pattern and period of

around 40 days, it appears to be a type of long

period pulsating variable star (for example a

Cepheid) or a rotating variable.

Target 6:

Target 6 is a medium-high magnitude star, that

appears to be varying in a regular pattern.

However, recent data has been going against

the trend of the light curve of target 6.

Like Target 4, this star could possibly be another

pulsating or rotating variable.

Target 3:

Target 3 is another dim star of about 14.3

magnitude, but has one of the largest amounts

of variability out of any of my target stars.

However, because of its very low magnitude,

the error bars for this star are very large. It

shows no pattern in its variance as no two

nights have the same magnitude for this star.

It is hard to determine what type of variable

star this is yet, but because of its variation

across every image, it may be a short period

variable star.

Target 2:

Target 2 is a very bright star, the second

brightest in the field, and also has a high range

of variance (0.25). In the April images target 2

displays a much higher magnitude, whereas in

the June/July/August images, the star has a

much lower magnitude, indicating that it may

be a long period variable star. Recently Target 2

has begun to rise back to its original magnitude.

Comparison Star 5:

Comp 5 is a high magnitude star that has a very low variance. It is a good choice for a comparison star, because it has an ideally bright magnitude, a very low variance and an almost perfect Colour Index (B-V). As you can see, the variance is only 0.06, which indicates that it is much more stable that the target stars.

2. How I chose my field

To choose a field, I set some criteria:

• Near the South Celestial Pole because the Southern skies are less explored and as I live in Australia.

• Near the galactic plane of the Milky Way so there are plenty of stars, but not right in the middle, wherethey are too dense.

• Visible for at least part of most nights of the year from the Siding Spring Observatory in NSW.

From this criteria I chose a constellation, but had to narrow in on part of it that had no known variables.

To check this, I downloaded the data for all the variable stars in my constellation from the AAVSO’s VSXdatabase and used an Excel Pivot Table to break my constellation up into small squares and show a heatmap of how many variable stars are known in each square. A sample pivot table is shown below, and a fieldwith no proven variable stars is annotated onto it in red.

Searching for a Variable StarWill Stamp

Astronomical Society of Victoria, Juniors Section

How do we find them (basic explanation)?

The most common method (and the method that I am using) is called Differential Photometry. Basically,

you take many images over a period of time, and then use photometric software to find stars that are

varying. To do this, you must first choose 3 comparison stars, which have to be bright, white stars that are

very stable. The software compares the brightness of the comparison stars, to the brightness of every

other star in the field, to find any stars that are varying.

3. iTelescopeFor taking our images I used a high-tech telescope operated by the iTelescope

network.

iTelescope is a network of Internet connected telescopes, that allows

astronomers to take images of the night sky. They operate 19 telescopes at 3

observatories located in New Mexico USA, Australia and Spain.

Telescope bookings are made over the Internet and the entire session is fully

automated, including targeting, focusing and capturing the images, which are

then made available for retrieval from the iTelescope.net site.

I used iTelescope T9, located at the Siding Spring Observatory in NSW,

Australia. It has an aperture of 12.5” (317mm) and a focal length of 2171mm,

resulting in field of view of 14.4 x 21.6 arc-mins.

4. CCD Photography for AstronomyAttached to iTelescope’s T9 is a CCD camera, which captures the images.

A CCD (Charged-Coupled Device) is a microchip that constructs an image made up of a grid of thousands of

small rectangles called pixels. Each pixel is an electrically isolated sensor that measures the amount of light

that hits that pixel during the exposure. It relies on the photoelectric effect, whereby a photon hits the CCD

and causes an electron to be released. When the exposure time is complete, the chip counts the number of

electrons that have accumulated in that pixel, effectively acting as a “photon counter” to calculate a digital

representation of the brightness of each pixel making up the image.

Simply put, an image is made up of pixels, and the more light (photons) that hits a pixel, the higher the

reading.

Each pixel is stored as a value called ADU (Analogue-Digital Unit), which is digital number that represents the

number of photons that hit that pixel. The following images show a zoomed in image of a star, and the exact

value of ADUs per pixel.

5. Finding stars and their instrumental magnitudeThe next step is to use photometry software to identify all of the stars in the images and to calculate the

brightness of each one.

To identify the stars in the image, the software is able to identify whether there is a star there or not by

searching for a cluster of high ADUs, as shown in the images.

For each of the stars found, the software calculates the instrumental magnitude, which is an uncalibrated

value that shows how bright the star is compared to the image background (empty space). In basic terms;

the instrumental magnitude is the ADU count of the all pixels in the star, minus the ADU count of empty

space behind it, or how much light the star added to the sky around it.

6. VaST - Variable Star Search SoftwareAfter taking a large number of images, the next step was to search for stars that showed signs of being

variable.

To do this, I used a software package called VaST, which identifies all of the stars in my images and shows

how much each star is varying. The result is a graph that plots the standard deviation of each star (the

amount of variability) over its instrumental magnitude (its relative brightness).

An example plot from VaST is shown below, showing some stars highlighted in green that are prospective

candidates because they have higher magnitude and variance.

7. VPHOT – Annulus and Aperture GapOne of the hardest problems to overcome in photometry, is when one of your targets has another smaller star

very close by, that could possibly be effecting the variability, because the small stars are visible in some images

and not in others. The effect of this can be significantly reduced by using something called the annulus.

8. VPHOT – Calibration using comparison starsOnce the instrumental magnitudes have been calculated for each star in the field, we must now begin to

calibrate the magnitudes of the stars. All magnitudes must be calibrated so that other astronomers can

compare results.

The first step is to identify some stable stars, to be your comparison stars. Comparison stars must have a

constant known magnitude, a specific colour index (B-V must be between 0.35 and 1.0), and be relatively

bright. I used Stellarium to find stars that matched all of these criteria, and found 5. The AAVSO has catalogues

of comparison stars, but I chose a field that had no published comparison stars, so I had to find my own.

By inputting the magnitude value of the comparison stars, the photometry software is able to determine the

calibrated magnitude of every other star in the image by comparing the difference between the comparison

star’s instrumental and calibrated magnitudes, and applying this same translation to the target stars.

Thanks

• Mike Thompson, Backyard Astro Science and Astronomical Society of Victoria

• Chris Rudge, Astronomical Society of Victoria

• Peter Lake, iTelescope.net

• Ms Adolph and Ms Grainger, John Monash Science School

• Duncan Galloway, Monash University

• Peter Stamp, My Dad

9. Results – Light CurvesWhat is a light curve?

A light curve is a graph of the star’s magnitude over time. Light curves are vital to prove and determine what type of

variable star it is. Some light curves show a distinct pattern of variance, while others show no pattern at all and vary

randomly. Discovering the period of the variable is also very important, as it can provide insight into the type of

variable it is. The following graphs are the light curves that I have created from my images:

1. Introduction

Aim: To use differential photometry to discover a variable star.

What is a variable star?

A variable star is a star whose brightness changes over a period of time

that can be random or regular. There are 2 reasons why stars vary. They

are either Extrinsic Variables or Intrinsic Variables.

Extrinsic Variable Stars

An Extrinsic Variable Star varies because of things outside the star,

where an object passes in front of the star and blocks some of its light.

Some examples are:

• An Exoplanet, orbiting its star

• A binary star pair, orbiting each other.

Intrinsic Variable Stars

An Intrinsic Variable Star varies because of internal factors such as

pulsations, eruptions or because of growing and shrinking.

Some examples are:

• Exploding Supernovae• Mira variables that expand and shrink• Cepheids

Exoplanet

Binary Star Pair

Supernova

The annulus is made up of three components that serve

different purposes; the inner annulus, buffer and the

outer annulus. Inside the inner annulus, all of the photons

are measured, and added together to find the

instrumental magnitude of the star. The outer annulus

must not contain any stars, as it is used to find the

brightness of the background (empty sky). The buffer is

used to remove the tiny stars from the image, and all

readings inside this area are ignored.

The three components of the annulus must be

customised for every target, to ensure that the cause of

the variability is not due to tiny stars in the inner annulus.