Final answer:
Hydrogen's unique spectral patterns, as seen in the Sun's absorption spectrum, confirm that its outer atmosphere is primarily hydrogen because only radiation matching hydrogen atoms' energy transitions gets absorbed and then re-emitted.
Step-by-step explanation:
Astronomers use hydrogen's unique spectral patterns to determine the composition of the Sun's outer atmosphere by analyzing the absorption spectrum. When light from the Sun passes through the solar corona, which consists of very hot hydrogen gas, the absorption spectrum reveals the specific pattern corresponding to hydrogen. This happens because hydrogen gas can only absorb electromagnetic (EM) radiation that has energy corresponding to a transition in the atom. The presence of these absorption lines in the solar spectrum confirms that hydrogen is a primary constituent of the Sun's atmosphere.
In contrast, when EM radiation passes through room-temperature hydrogen gas, only the Lyman series is absorbed. This is because, at lower temperatures, electrons in the hydrogen atoms are not excited to higher energy levels. Therefore, we see the specific spectral patterns of hydrogen in the solar corona's spectrum, suggesting that the corona, and by extension, the Sun's atmosphere, is primarily composed of hydrogen.