To answer the question of how hot a Lightsaber is, it’s first important to understand how Lightsabers actually work.
How do Lightsabers work?
The blade of a Lightsaber is a beam of plasma that is projected from its hilt. The beam is then stabilized by a containment force field, which is also projected from the same place. The force field surrounds the plasma and does not allow any of its heat to escape.
Because of this design, the blade of the Lightsaber does not actually radiate any heat. In fact, if you were to put your hand right next to a Lightsaber’s blade, you would feel absolutely no heat whatsoever. However, if you touched it, it would instantly melt your hand.
This function was described in Star Wars’ Fate of the Jedi: Outcast in the following scene:
Valin positioned the tip of his lightsaber directly beneath her chin. At the distance of half a centimetre, its containing force field kept her from feeling any heat from the blade, but a slight twitch on Valin’s part could maim or kill her instantly.
How much heat does a Lightsaber produce?
So there is no heat emitted from the blade itself, but that still leaves the question of how hot the plasma that lies within the blade is. In Star Wars, Lightsabers have been shown to cut through various types of metal.
The melting point of stainless steel is 1530 degrees Celsius (2750 degrees Fahrenheit) so a Lightsaber must reach at least this temperature. But we have also seen them cut through blast doors in The Phantom Menace. These doors are designed to withstand laser blasts at even greater temperatures than standard steel doors. Assuming that the blast doors were made from titanium, this would increase the melting point to 1668 degrees Celsius (3034 degrees Fahrenheit).
In the real world, the closest technology that exists similar to a Lightsaber is a plasma welding torch which can cut through many types of metals including carbon steel and stainless steel. A constricted plasma arc can reach temperatures up to 25000 degrees Celsius (45032°F).
Given this evidence, it can be concluded that a Lightsaber can reach temperatures of up to 25000 degrees Celsius (45032°F).