Photometry is the measurement of the intensity or brightness of an astronomical object, such as a
star or galaxy by adding up all of the light from the object. For example, a star looks like a point of
light when you look at it just with your eyes but the Earth’s atmosphere smears it out into
something that looks like a round blob when you use a telescope to look at it. In order to measure
the total light coming from the star, we must add up all of the light from the smeared out star.
Photometry is generally used to generate light
curves of objects such as variable stars and
supernovae, where the interest is the variation of
total light energy output by the system over time.
It can also be used to discover exoplanets, by
measuring the intensity of a stars light over a period
of time. Deviations in the light output can indicate
objects in orbit around the star.
These instructions explain how photometry can be
carried out on groups, or clusters of stars, from
images taken with different filters, in order to plot a
color magnitude diagram.
In order to plot a HR diagram, the temperature and luminosity of the stars need to be known. The simplest indication of a star’s temperature is its color. A star’s color is simply a measure of the amount of light from the star in one filter compared to another. The most common color system is B-V, which is simply an object’s magnitude as measured through the B filter, minus its magnitude as measured through the V filter. Other filters may be used however, and it is most common to compare the colors by taking the shorter (more blue) wavelength and subtracting the longer (more red) wavelength.
To find these values, you can use SalsaJ to measure the brightness of as many stars in the cluster as you can in at least two filters. You then use the CMD plotter spreadsheet, or make your own spreadsheet, to convert the values from the photometry to magnitudes. You can then subtract the magnitudes from the two colors, and plot these values against the magnitude in the longer wavelength. This plot is a color-magnitude diagram, equivalent to a H-R diagram.
