For this week we keep with the theme of giant stars and return to our auxiliary HOYS data set from the Cygnus Project. The phase folded light curve shown is of the variable star V356Cyg. It is know as a Delta-Cephei variable. These stars change their brightness due to pulsations, i.e. a change in size. More importantly, the period of these pulsations is related to their absolute magnitude. Thus, these objects are excellent standard candles to measure distances.

These type of objects can show up as background sources in our HOYS fields. Our searches for periodic variables (to measure YSO rotation periods) do sometimes find them. They are easily identifiable by their light curve. The phase folded light curve from our data has a shape that looks like a textbook Delta Cephei object.

The amplitude of the variation is 0.6033+-0.0005mag. As one can see, the individual data points scatter by about 0.015mag (or 1.5%) from the over plotted running median. This nicely represents the accuracy of the individual measurements. The huge amount of data points (~46000) allows us, however, to measure the median phase folded light curve much more accurate than this. In this case to about 0.05%, or half a milli-mag.

The period of the star is 5d 1hr 22m 23s. According to the absolute magnitude period relationship, this converts into an absolute magnitude of -3.33mag in the V-band. This shows that these objects, in particular  the longer period ones, are intrinsically very bright and can thus be observed over very large distances, even in other galaxies. For comparison the Sun has an absolute V-band magnitude of +4.83mag, which is 8.16mag or more than 1800 times fainter than V356Cyg.