What causes a green shooting star?

The bright green color of some meteorites is caused by the internal combustion of metals as they pass through the upper atmosphere. Many asteroids are rich in nickel and have trace amounts of copper. Both metals burn green when sufficiently heated.

As a meteor enters Earth's atmosphere, contact with the air in front of it slows its descent. As this happens, air is compressed in front of the falling object in a phenomenon known as bow shock. Air heats as it is compressed and can easily rise to the point that small quantities of copper and nickel compounds burn off. This generates a green gas that is backlit by the glowing air of the bow shock.