| 1 | "The galaxy has a very, very steep metallicity gradient, " she says. | |
| 2 | One clue comes from the metallicity of the disc' s constituent stars. | |
| 3 | The relative abundance of different nuclear isotopes in the material could reveal additional clues. | |
| 4 | Currently the cleanest probe of big bang nucleosynthesis is the abundance of deuterium. | |
| 5 | However, because few stars form on the disc's outskirts, the metallicity there stays low. |