As most are aware, in the Marvel Cinematic Universe Bucky Barnes, aka the Winter Soldier, was kidnapped by Hydra twice, experimented on and transformed into an assassin. However, what has remained a mystery are the specifics around how Bucky became a super-soldier.

Outside of Captain America, attempts to harness the super-soldier serum turned out to be colossal failures -- as seen by Red Skull's grotesque transformation and the Hulk's monstrous stature. So, without any further explanation, it remained unclear as to how Bucky came out more or less functional. Fortunately, the newly released book, The Wakanda Files: A Technological Exploration of the Avengers and Beyond, sheds some light on the matter.

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Although the book may not be official canon, it's presented as if it exists in-universe, serving as a collection of documents from throughout MCU history, collected by the Wakandan War Dogs for Shuri. In her research, she analyzes how the Super-Soldier Serum works, and determines there is a possibility the Winter Soldier was enhanced with the aid of an Infinity Stone.

As Shuri states, radiation is a key component to the Super-Soldier Serum. "Schmidt’s transformation failed because his serum lacked radioactive exposure," she explains. "Rogers was exposed to Vita-Rays. Banner became a massive rage monster because he was subjected to gamma radiation. It is likely that Zola utilized gamma radiation from the Tesseract to enhance Barnes. Dr. List had figured out a way to expose subjects to whatever cosmic rays the Mind Stone emits and it transformed them."

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So, in Bucky's case, it's possible that one of two things occurred: Either he is the result of Zola's experimental duplicate of the Super-Soldier Serum, combined with the energy of the Tesseract, or Bucky's powers purely come from the Space Stone, much like how the Maximoff twins developed their abilities from the Mind Stone.

To determine whether Shuri's conclusions hold up, let's look back at Captain America: The First Avenger. Captain Rogers' first real mission was to go behind enemy lines to rescue Bucky, who has already been subjected to experiments by Zola. It's later revealed those experiments contribute to Bucky being able to survive his fall on the train, although it isn't enough to necessarily give him superhuman abilities like Cap. After he falls, he is rescued by Zola, and further experimented on.

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The Tesseract in it's case in the MCU

During his first capture at the hands of Hydra, Bucky is in close proximity to the Space Stone, since Red Skull has it stored in the base he was being held, meaning Zola could have easily borrowed some of its energy to use in his experiments. While Zola didn't have access to the gem the second time around, Bucky was likely already exposed to enough radiation that little else had to be done to complete his transformation.

However, as fun as this theory is, there are some holes in it. In a similar way, Carol Danvers was exposed to the energy of the Space Stone after her plane went down in Captain Marvel. She was granted extraordinary abilities that make her one of the most powerful beings in the entire universe. So, if Bucky was exposed to the same source, it seems peculiar that it wouldn't have a somewhat similar effect, even if the circumstances were different. In the end, this portion of the book may provide more questions than answers, but at least Bucky didn't end up looking like the Hulk.

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