The first detection of a ^ { 36 } S-bearing molecule in interstellar space is reported .
The J = 2 - 1 and 3 - 2 transitions of C ^ { 36 } S have been observed toward eight Galactic molecular hot cores .
From a comparison with other optically thin isotopic species of CS , the abundance ratio of ^ { 34 } S/ ^ { 36 } S is 115 ( \pm 17 ) .
This is smaller than the solar system ratio of 200 and supports the idea that ^ { 36 } S is , unlike the other stable sulfur isotopes , a purely secondary nucleus that is produced by s-process nucleosynthesis in massive stars .