Stars less massive than our Sun, which are classified as red dwarfs, primarily fuse hydrogen into helium, just like our Sun. However, due to their lower mass and cooler temperatures, they burn hydrogen much more slowly. This means they have much longer lifespans than our Sun, potentially lasting trillions of years.

Here's why they don't form heavier elements:

* Lower temperatures: Red dwarfs are cooler than our Sun. This means their core temperatures aren't high enough to initiate the fusion of helium into heavier elements like carbon and oxygen, a process called triple-alpha process.

* Lower pressures: The lower mass of red dwarfs means they have weaker gravitational pull. This results in lower pressure in their core, further inhibiting the fusion of heavier elements.

Therefore, the primary elements produced by red dwarfs are hydrogen and helium.

While they may fuse trace amounts of lithium and beryllium, the amount is insignificant compared to the hydrogen and helium they produce.