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

question 1

Multiple Choice

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.