Why is 21-cm radiation a good spiral arm tracer?
A) Emitted by neutral hydrogen,it traces the location of 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.

Question

Quizplus · Accepted Answer

21-cm radiation is emitted by neutral hydrogen, which is a major constituent of the interstellar medium. It can be used to trace the location and distribution of concentrations of gas in the Milky Way, including the spiral arms.

Why Is 21-Cm Radiation a Good Spiral Arm Tracer