The sudden brightening of this star is called a nova, and this one is named Nova Delphini 2013. A nova is the result when a type of star, called a white dwarf, accumulates hydrogen on it's surface. The hydrogen ignites and causes an uncontrolled nuclear chain reaction and explodes in a powerful outburst. White dwarves are stellar remnants. They are the last phase in the stellar life cycle when a star does not have sufficient mass to go supernova and become a neutron star or black hole. Most stars will become a white dwarf, including our own Sun.
Most, if not all novae, are though to happen in binary star systems that include a white dwarf and a larger companion star. If the stars are very close, the strong gravitational influence of the dwarf can rip off stellar material from the companion and deposit it on it's surface. This material becomes compressed and heated until it undergoes nuclear fusion. This reaction can become unstable and lead to a massive explosion on the dwarf's surface. This is what causes the star to become much brighter than normal.
A nova should not be confused with a supernova. A supernova results when the entire star explodes, whereas a nova is just a explosion on the surface. A white dwarf involved with a nova will still be intact and can repeat the process many times. In some cases, a white dwarf can accrete too much stellar material and explode as a type of supernova.
Currently Nova Delphini 2013 is bright enough to be seen with the naked eye in dark sky areas. It's brightness is between magnitude +4 and +5 and is steadily brightening. For more information about this nova and how to find it in the sky, follow this link:Bright New Nova In Delphinus — You can See it Tonight With Binoculars
Finally, here is an image of Delphini 2013 that I've recently taken:
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