Why is 21-cm radiation a good tracer of spiral arm structure?
A) Emitted by neutral hydrogen, it traces the location of high concentrations of gas.
B) Emitted by O and B stars, it traces the location of newly formed stars.
C) Emitted by ionized hydrogen, it traces the location of H II regions.
D) Emitted by molecular hydrogen, it traces the location of clouds in the process of collapsing into stars.
E) Emitted by red supergiants, it traces the presence of massive stars that are experiencing high rates of mass loss.

Question

Quizplus · Accepted Answer

21-cm radiation is emitted by neutral hydrogen, which is the most abundant element in the universe. This radiation can penetrate through interstellar dust and can be detected from interstellar gas clouds, making it a good tracer of spiral arm structure. The radiation emitted by neutral hydrogen is also sensitive to the physical conditions of the gas, such as density and temperature, and can provide information about the dynamics and kinematics of the gas in the spiral arms.

Why Is 21-Cm Radiation a Good Tracer of Spiral Arm