The one depicted in the main photo of this piece is of the C-5M Super Galaxy variety. It’s the one with a wingspan of 222.8 feet (67.91 meters) and a total length of 247.8 feet (75.53 meters). It’s the most recent incarnation of the family, in that it is presently being modernized by its maker.
In the final days of the month that just ended, one of these Super Galaxies was subjected to a “defensive countermeasures test program.” As part of the program, the airplane at one point had to release its flares, and that is when the amazing photo attached to this piece was taken (click photo to enlarge).
The particular plane you see here belongs to the 436th Airlift Wing, and the image was shot at the Eglin Air Force Base in Florida. That’s the same place from where the only two F-15EX fighters jets in operation presently fly.
As for the flares released by military aircraft, most of you already know they are designed to counter heat-seeking missiles and other weapons, fooling them into not getting a lock on the plane. They do so by burning hot (flares are usually magnesium-based), much hotter than the airplane's engines, thus diverting the killer weapons to follow them instead.