A star can form from as little as 0.05 solar masses (1029 kg) of stellar-nursery material. However, that’s mostly hydrogen, whereas bio-lego is around 14% hydrogen by mass, or 66% by molarity. The remaining mass is carbon, which will not contribute to fusion in a star of this size.
That inert mass will reduce the rate of fusion-producing hydrogen collisions at a given temperature, but for the purposes of producing a gravity well to sustain the temperature, all mass is the same. So I believe that Lego bricks will still undergo stellar ignition at around 1029 kg, but the resulting star will be cooler and shorter-lived than a normal star of this size.
