SS Cygni (21h 42m 42.80s, +43d 35' 09.9") is the brightest example of a class of variable stars called dwarf novae. Dwarf novae are binary stars that orbit in close proximity to each other. One star of the pair is a white dwarf and the other a low-mass main sequence star. Due to their closeness, matter from the main sequence star is drawn towards the more massive white dwarf. This stream of matter collects around the white dwarf to form what is called an accretion disk. When the amount of matter in the disk reaches a critical point, an outburst or flare occurs, causing the system to brighten suddenly by several magnitudes. This outburst can usually last anywhere from a few days to a few weeks depending on the star. This process then starts over again with new matter being drawn into the accretion disk until another outburst occurs. Outbursts of dwarf novae usually range from 2 to 7 magnitudes, whereas a classical nova will brighten by 8 to 20 magnitudes.
SS Cygni experiences outbursts at intervals varying from 20 to 90 days, with the average being roughly 50 days. During an outburst, SS Cygni will brighten from 12th to 8th magnitude in as little as one day. The star will remain at 8th magnitude for 2 to 10 days and then fade back to 12th magnitude within a week.
I have observed SS Cygni since 1992, making roughly 1200 visual magnitude estimates of this variable. Shown below are some light curves made from these observations that illustrate the behavior of this fascinating star. Each light curve is a plot of the star's visual magnitude (Mag) vs. the Julian Date (JD).
The plot above shows the brightness of SS Cygni from June, 1992 to September, 1993. Seven distinct outbursts are seen plus a period of unusual activity in June and July, 1993.
The plot above shows the brightness of SS Cygni from June to November, 1992. There are two types of outbursts seen in this plot. The first and third outbursts are "normal outbursts", characterized by a rapid rise to peak brightness followed by an almost immediate decline. The second outburst is an "anomalous outburst", characterized by a slower rise to peak brightness and a broad maximum.
The plot above shows the brightness of SS Cygni from April to September, 1993. Both a normal and an anomalous outburst are seen in this plot. Between the two outbursts, SS Cygni had three unusual mini-outbursts.