It's impossible to say definitively which star has the *absolute* lowest luminosity without a complete survey of all stars. However, we can say that red dwarfs are the least luminous type of star in the universe.

Here's why:

* Low Mass: Red dwarfs are the smallest and least massive type of star. Their mass is typically less than half that of our Sun.

* Cool Temperature: They have much lower surface temperatures than our Sun, typically around 2,500 to 3,500 Kelvin.

* Low Energy Output: Due to their small size and low temperature, red dwarfs emit very little light and energy.

Important Note: Even though red dwarfs are the least luminous stars *as a category*, individual red dwarfs can vary significantly in their luminosity. There are likely even fainter stars out there, but they are extremely difficult to detect.