2.2 Reference stars
Another important consideration in making scientific measurements is to eliminate any external factors that could affect your measurement. In order to look for variation in a star, you need to make measurements over a period of time, perhaps hours or days. The brightness of the star in your image will depend on a number of things that remain constant, such as the distance to the star, the size of the telescope and the sensitivity of the detector. There will also be some factors that vary from one image to the next.
What factors can you think of that may vary from one image to the next?
The transparency of the atmosphere may change, especially if taking images from one evening to the next. Also, for images taken at different times, the height (altitude) of the star above the horizon will be different each time, again resulting in different amounts of absorption and affecting the brightness of the star’s image.
Another source of variation between images would be if we wanted to combine results from different observers, perhaps stationed in different parts of the world and using telescopes of different sizes.
Fortunately, there is a powerful technique that we can use to eliminate all of these factors, leaving only the variation of the star itself. This is centred around the use of a reference star – a second star in the image that is known not to be variable (i.e. whose brightness is constant). Within any single image, factors such as atmospheric absorption will be the same for all stars in the image, so by comparing the brightness of the target star against that of the reference each time, these factors can be eliminated since both target and reference will be affected equally.
Any change seen in the brightness of the target star relative to the reference will then indicate a variation in the star itself.
This technique of using a reference is so powerful that it can even account for different observers using different equipment, provided each observer uses the same reference star. If one observer uses a larger telescope, making the image of the target star brighter, the brightness of the reference star will be increased by the same amount, but the relative brightness would be the same – for instance, if the target is twice as bright as the reference in an image from one telescope, it will still be twice as bright as the same reference star in an image taken at the same time on a different telescope.